<?xml version="1.0" encoding="UTF-8"?>

<!--XBRL Document Created with Wdesk from Workiva-->
<!--Copyright 2022 Workiva-->
<!--r:e141d476-ae09-4ec9-a80f-baeacfb8f81a,g:a196524c-1dde-4a8b-8796-f0fa7ce72378-->
<link:linkbase xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd">
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member"/>
  <link:roleRef roleURI="http://www.iconplc.com/role/Cover" xlink:type="simple" xlink:href="iclr-20211231.xsd#Cover"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/Cover" xlink:type="extended" id="i46740e84926a4e2ab047c65bbe946662_Cover">
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentInformationLineItems_4bd9929f-5630-4f34-aa45-48aebf1de2f5" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_DocumentInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentType_5b0cc050-0218-4a3f-a5a6-c917e96a7c9e" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_DocumentType"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4bd9929f-5630-4f34-aa45-48aebf1de2f5" xlink:to="loc_dei_DocumentType_5b0cc050-0218-4a3f-a5a6-c917e96a7c9e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentRegistrationStatement_a8c2f21e-31ce-4d18-8235-ea5530877978" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_DocumentRegistrationStatement"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4bd9929f-5630-4f34-aa45-48aebf1de2f5" xlink:to="loc_dei_DocumentRegistrationStatement_a8c2f21e-31ce-4d18-8235-ea5530877978" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentAnnualReport_a85fc749-3c46-4707-ae8f-7a43362c37a7" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_DocumentAnnualReport"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4bd9929f-5630-4f34-aa45-48aebf1de2f5" xlink:to="loc_dei_DocumentAnnualReport_a85fc749-3c46-4707-ae8f-7a43362c37a7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentPeriodEndDate_ba815163-9346-4824-a4cf-0fc45e5f5b04" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_DocumentPeriodEndDate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4bd9929f-5630-4f34-aa45-48aebf1de2f5" xlink:to="loc_dei_DocumentPeriodEndDate_ba815163-9346-4824-a4cf-0fc45e5f5b04" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_CurrentFiscalYearEndDate_4e8279b4-b819-4b1b-be8b-7ed19c03e1d9" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_CurrentFiscalYearEndDate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4bd9929f-5630-4f34-aa45-48aebf1de2f5" xlink:to="loc_dei_CurrentFiscalYearEndDate_4e8279b4-b819-4b1b-be8b-7ed19c03e1d9" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentTransitionReport_8f2664d2-ee26-4197-b0bc-17e70331eb3b" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_DocumentTransitionReport"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4bd9929f-5630-4f34-aa45-48aebf1de2f5" xlink:to="loc_dei_DocumentTransitionReport_8f2664d2-ee26-4197-b0bc-17e70331eb3b" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentShellCompanyReport_3768b7de-ba37-4077-baa9-b115e576c7a4" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_DocumentShellCompanyReport"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4bd9929f-5630-4f34-aa45-48aebf1de2f5" xlink:to="loc_dei_DocumentShellCompanyReport_3768b7de-ba37-4077-baa9-b115e576c7a4" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityFileNumber_398ae245-3c10-4844-a912-61262ce63a61" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_EntityFileNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4bd9929f-5630-4f34-aa45-48aebf1de2f5" xlink:to="loc_dei_EntityFileNumber_398ae245-3c10-4844-a912-61262ce63a61" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityRegistrantName_6d6779a7-0b75-4741-a1b2-a519fa5a5e6a" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_EntityRegistrantName"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4bd9929f-5630-4f34-aa45-48aebf1de2f5" xlink:to="loc_dei_EntityRegistrantName_6d6779a7-0b75-4741-a1b2-a519fa5a5e6a" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityIncorporationStateCountryCode_603e4c7d-d615-45e8-9898-5041bdd15860" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_EntityIncorporationStateCountryCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4bd9929f-5630-4f34-aa45-48aebf1de2f5" xlink:to="loc_dei_EntityIncorporationStateCountryCode_603e4c7d-d615-45e8-9898-5041bdd15860" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressAddressLine1_788e44e1-16cd-4573-855a-12383a6a10b9" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_EntityAddressAddressLine1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4bd9929f-5630-4f34-aa45-48aebf1de2f5" xlink:to="loc_dei_EntityAddressAddressLine1_788e44e1-16cd-4573-855a-12383a6a10b9" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressAddressLine2_e0a992fe-0b3e-41d6-8b91-e96af9312ab4" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_EntityAddressAddressLine2"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4bd9929f-5630-4f34-aa45-48aebf1de2f5" xlink:to="loc_dei_EntityAddressAddressLine2_e0a992fe-0b3e-41d6-8b91-e96af9312ab4" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressCityOrTown_4ac58253-e167-4fd1-91b0-6ed958d2b5d2" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_EntityAddressCityOrTown"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4bd9929f-5630-4f34-aa45-48aebf1de2f5" xlink:to="loc_dei_EntityAddressCityOrTown_4ac58253-e167-4fd1-91b0-6ed958d2b5d2" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressPostalZipCode_07ff2094-1aab-4188-ae90-52e1069873ad" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_EntityAddressPostalZipCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4bd9929f-5630-4f34-aa45-48aebf1de2f5" xlink:to="loc_dei_EntityAddressPostalZipCode_07ff2094-1aab-4188-ae90-52e1069873ad" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressCountry_f786dc73-2cbe-46ef-a083-b5c48d5f235a" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_EntityAddressCountry"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4bd9929f-5630-4f34-aa45-48aebf1de2f5" xlink:to="loc_dei_EntityAddressCountry_f786dc73-2cbe-46ef-a083-b5c48d5f235a" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_ContactPersonnelName_405b431c-99e6-45ed-ba26-f2ba9994c6ac" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_ContactPersonnelName"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4bd9929f-5630-4f34-aa45-48aebf1de2f5" xlink:to="loc_dei_ContactPersonnelName_405b431c-99e6-45ed-ba26-f2ba9994c6ac" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_ContactPersonnelEmailAddress_c21f9fdb-f96c-4c07-8d71-4c339de4d78d" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_ContactPersonnelEmailAddress"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4bd9929f-5630-4f34-aa45-48aebf1de2f5" xlink:to="loc_dei_ContactPersonnelEmailAddress_c21f9fdb-f96c-4c07-8d71-4c339de4d78d" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_CountryRegion_72fb9290-5152-4790-b438-d100dab22cd6" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_CountryRegion"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4bd9929f-5630-4f34-aa45-48aebf1de2f5" xlink:to="loc_dei_CountryRegion_72fb9290-5152-4790-b438-d100dab22cd6" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_CityAreaCode_7acd125c-fef4-4a5b-aa2d-aa4702565c6b" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_CityAreaCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4bd9929f-5630-4f34-aa45-48aebf1de2f5" xlink:to="loc_dei_CityAreaCode_7acd125c-fef4-4a5b-aa2d-aa4702565c6b" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LocalPhoneNumber_ef4f926e-58bd-4050-83e4-2039b2cfb72c" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_LocalPhoneNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4bd9929f-5630-4f34-aa45-48aebf1de2f5" xlink:to="loc_dei_LocalPhoneNumber_ef4f926e-58bd-4050-83e4-2039b2cfb72c" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_Security12bTitle_ef40539a-04cc-4b9a-b8a3-29743037f18a" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_Security12bTitle"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4bd9929f-5630-4f34-aa45-48aebf1de2f5" xlink:to="loc_dei_Security12bTitle_ef40539a-04cc-4b9a-b8a3-29743037f18a" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_TradingSymbol_9616c03e-44d9-4be1-88b3-bcd4d5da2a13" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_TradingSymbol"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4bd9929f-5630-4f34-aa45-48aebf1de2f5" xlink:to="loc_dei_TradingSymbol_9616c03e-44d9-4be1-88b3-bcd4d5da2a13" xlink:type="arc" order="21"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_SecurityExchangeName_427d1231-a14b-42b3-a1a3-1cea2646c511" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_SecurityExchangeName"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4bd9929f-5630-4f34-aa45-48aebf1de2f5" xlink:to="loc_dei_SecurityExchangeName_427d1231-a14b-42b3-a1a3-1cea2646c511" xlink:type="arc" order="22"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCommonStockSharesOutstanding_931cba2c-aa89-49fc-813f-f264278096f1" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_EntityCommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4bd9929f-5630-4f34-aa45-48aebf1de2f5" xlink:to="loc_dei_EntityCommonStockSharesOutstanding_931cba2c-aa89-49fc-813f-f264278096f1" xlink:type="arc" order="23"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityWellKnownSeasonedIssuer_96c0767e-3452-41f1-b8eb-15dbe5eac639" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_EntityWellKnownSeasonedIssuer"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4bd9929f-5630-4f34-aa45-48aebf1de2f5" xlink:to="loc_dei_EntityWellKnownSeasonedIssuer_96c0767e-3452-41f1-b8eb-15dbe5eac639" xlink:type="arc" order="24"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityVoluntaryFilers_1d00af7f-1f7a-4233-ad58-be852439b3f7" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_EntityVoluntaryFilers"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4bd9929f-5630-4f34-aa45-48aebf1de2f5" xlink:to="loc_dei_EntityVoluntaryFilers_1d00af7f-1f7a-4233-ad58-be852439b3f7" xlink:type="arc" order="25"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCurrentReportingStatus_49078e55-d133-419f-b644-f22b75b812c4" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_EntityCurrentReportingStatus"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4bd9929f-5630-4f34-aa45-48aebf1de2f5" xlink:to="loc_dei_EntityCurrentReportingStatus_49078e55-d133-419f-b644-f22b75b812c4" xlink:type="arc" order="26"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityInteractiveDataCurrent_7a2696bb-cb65-4ca6-8974-5051439278f5" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_EntityInteractiveDataCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4bd9929f-5630-4f34-aa45-48aebf1de2f5" xlink:to="loc_dei_EntityInteractiveDataCurrent_7a2696bb-cb65-4ca6-8974-5051439278f5" xlink:type="arc" order="27"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityFilerCategory_299d2a5b-7415-4700-91ef-1a388ca1d3cb" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_EntityFilerCategory"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4bd9929f-5630-4f34-aa45-48aebf1de2f5" xlink:to="loc_dei_EntityFilerCategory_299d2a5b-7415-4700-91ef-1a388ca1d3cb" xlink:type="arc" order="28"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityEmergingGrowthCompany_ef1d34d9-e97e-401d-8410-973cf22fc079" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_EntityEmergingGrowthCompany"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4bd9929f-5630-4f34-aa45-48aebf1de2f5" xlink:to="loc_dei_EntityEmergingGrowthCompany_ef1d34d9-e97e-401d-8410-973cf22fc079" xlink:type="arc" order="29"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_IcfrAuditorAttestationFlag_1d824a4e-ad7e-4b74-a9fb-a573443066a7" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_IcfrAuditorAttestationFlag"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4bd9929f-5630-4f34-aa45-48aebf1de2f5" xlink:to="loc_dei_IcfrAuditorAttestationFlag_1d824a4e-ad7e-4b74-a9fb-a573443066a7" xlink:type="arc" order="30"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentAccountingStandard_328579fe-f4f9-4446-af47-f7777e9bff3f" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_DocumentAccountingStandard"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4bd9929f-5630-4f34-aa45-48aebf1de2f5" xlink:to="loc_dei_DocumentAccountingStandard_328579fe-f4f9-4446-af47-f7777e9bff3f" xlink:type="arc" order="31"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityShellCompany_9ac295ed-736f-4904-be04-0ffb20e4bce3" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_EntityShellCompany"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4bd9929f-5630-4f34-aa45-48aebf1de2f5" xlink:to="loc_dei_EntityShellCompany_9ac295ed-736f-4904-be04-0ffb20e4bce3" xlink:type="arc" order="32"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_AmendmentFlag_91bb6863-0450-470b-8cd7-9c68fb1d39d9" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_AmendmentFlag"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4bd9929f-5630-4f34-aa45-48aebf1de2f5" xlink:to="loc_dei_AmendmentFlag_91bb6863-0450-470b-8cd7-9c68fb1d39d9" xlink:type="arc" order="33"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentFiscalYearFocus_2e4d3c02-efc1-458c-b387-9cb383f16f08" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_DocumentFiscalYearFocus"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4bd9929f-5630-4f34-aa45-48aebf1de2f5" xlink:to="loc_dei_DocumentFiscalYearFocus_2e4d3c02-efc1-458c-b387-9cb383f16f08" xlink:type="arc" order="34"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentFiscalPeriodFocus_e030a720-1b8f-4bcc-9a9d-62f7a25fbbdd" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_DocumentFiscalPeriodFocus"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4bd9929f-5630-4f34-aa45-48aebf1de2f5" xlink:to="loc_dei_DocumentFiscalPeriodFocus_e030a720-1b8f-4bcc-9a9d-62f7a25fbbdd" xlink:type="arc" order="35"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCentralIndexKey_0af5d319-fdc4-4026-a257-98dfa0fa7db3" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_EntityCentralIndexKey"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4bd9929f-5630-4f34-aa45-48aebf1de2f5" xlink:to="loc_dei_EntityCentralIndexKey_0af5d319-fdc4-4026-a257-98dfa0fa7db3" xlink:type="arc" order="36"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentInformationTable_671dd477-2418-46e2-933a-fde83c823085" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_DocumentInformationTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_dei_DocumentInformationLineItems_4bd9929f-5630-4f34-aa45-48aebf1de2f5" xlink:to="loc_dei_DocumentInformationTable_671dd477-2418-46e2-933a-fde83c823085" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressesAddressTypeAxis_713e6f2d-85e6-41e3-a00d-8918dc21e227" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_EntityAddressesAddressTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_dei_DocumentInformationTable_671dd477-2418-46e2-933a-fde83c823085" xlink:to="loc_dei_EntityAddressesAddressTypeAxis_713e6f2d-85e6-41e3-a00d-8918dc21e227" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_AddressTypeDomain_713e6f2d-85e6-41e3-a00d-8918dc21e227_default" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_AddressTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_EntityAddressesAddressTypeAxis_713e6f2d-85e6-41e3-a00d-8918dc21e227" xlink:to="loc_dei_AddressTypeDomain_713e6f2d-85e6-41e3-a00d-8918dc21e227_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_AddressTypeDomain_71f28ab2-aab6-49f6-8e82-e80facd63240" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_AddressTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_EntityAddressesAddressTypeAxis_713e6f2d-85e6-41e3-a00d-8918dc21e227" xlink:to="loc_dei_AddressTypeDomain_71f28ab2-aab6-49f6-8e82-e80facd63240" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_BusinessContactMember_6d2d51a8-3cbf-4c05-b7fe-e6d67372c8a1" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_BusinessContactMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_AddressTypeDomain_71f28ab2-aab6-49f6-8e82-e80facd63240" xlink:to="loc_dei_BusinessContactMember_6d2d51a8-3cbf-4c05-b7fe-e6d67372c8a1" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/CONSOLIDATEDSTATEMENTSOFSHAREHOLDERSEQUITYANDCOMPREHENSIVEINCOME" xlink:type="simple" xlink:href="iclr-20211231.xsd#CONSOLIDATEDSTATEMENTSOFSHAREHOLDERSEQUITYANDCOMPREHENSIVEINCOME"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/CONSOLIDATEDSTATEMENTSOFSHAREHOLDERSEQUITYANDCOMPREHENSIVEINCOME" xlink:type="extended" id="iffbe2ff300104ff983a17a8afb44b565_CONSOLIDATEDSTATEMENTSOFSHAREHOLDERSEQUITYANDCOMPREHENSIVEINCOME">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_f79ddbaa-12b3-42ca-9242-11bdef2f4bd0" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_f46bedde-cb6b-468b-b863-a3dacc3bbb0a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_f79ddbaa-12b3-42ca-9242-11bdef2f4bd0" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_f46bedde-cb6b-468b-b863-a3dacc3bbb0a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesIssued_29b327d2-b5ca-4af0-8618-9f29dc3f9c0b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommonStockSharesIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_f46bedde-cb6b-468b-b863-a3dacc3bbb0a" xlink:to="loc_us-gaap_CommonStockSharesIssued_29b327d2-b5ca-4af0-8618-9f29dc3f9c0b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_2819df30-1484-46b4-9057-566eb39aeb13" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_f46bedde-cb6b-468b-b863-a3dacc3bbb0a" xlink:to="loc_us-gaap_StockholdersEquity_2819df30-1484-46b4-9057-566eb39aeb13" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_92cc3aca-b866-43f4-a6b1-8b17a59d812f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_f46bedde-cb6b-468b-b863-a3dacc3bbb0a" xlink:to="loc_us-gaap_NetIncomeLoss_92cc3aca-b866-43f4-a6b1-8b17a59d812f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax_6312ebfe-2c19-4227-86aa-a1dd3541be33" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_f46bedde-cb6b-468b-b863-a3dacc3bbb0a" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax_6312ebfe-2c19-4227-86aa-a1dd3541be33" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTaxLongTermFunding_fcc4571c-b3fe-4868-9d30-7c91127642ca" xlink:href="iclr-20211231.xsd#iclr_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTaxLongTermFunding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_f46bedde-cb6b-468b-b863-a3dacc3bbb0a" xlink:to="loc_iclr_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTaxLongTermFunding_fcc4571c-b3fe-4868-9d30-7c91127642ca" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax_5297f7b6-9526-4a7c-99e5-6b08cedb7d76" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_f46bedde-cb6b-468b-b863-a3dacc3bbb0a" xlink:to="loc_us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax_5297f7b6-9526-4a7c-99e5-6b08cedb7d76" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeDefinedBenefitPlansNetUnamortizedGainLossArisingDuringPeriodNetOfTax_41080f82-3133-4a2d-8a16-10467da1ae95" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OtherComprehensiveIncomeDefinedBenefitPlansNetUnamortizedGainLossArisingDuringPeriodNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_f46bedde-cb6b-468b-b863-a3dacc3bbb0a" xlink:to="loc_us-gaap_OtherComprehensiveIncomeDefinedBenefitPlansNetUnamortizedGainLossArisingDuringPeriodNetOfTax_41080f82-3133-4a2d-8a16-10467da1ae95" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossReclassificationAfterTax_00c78396-3e9c-419b-9bf0-032ee557a1a5" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossReclassificationAfterTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_f46bedde-cb6b-468b-b863-a3dacc3bbb0a" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossReclassificationAfterTax_00c78396-3e9c-419b-9bf0-032ee557a1a5" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossBeforeReclassificationAfterTax_0328e4ea-a1a5-493d-b8ea-89e23448b622" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossBeforeReclassificationAfterTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_f46bedde-cb6b-468b-b863-a3dacc3bbb0a" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossBeforeReclassificationAfterTax_0328e4ea-a1a5-493d-b8ea-89e23448b622" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ComprehensiveIncomeNetOfTax_57db4089-f785-42d6-94bc-7e03543b63ed" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ComprehensiveIncomeNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_f46bedde-cb6b-468b-b863-a3dacc3bbb0a" xlink:to="loc_us-gaap_ComprehensiveIncomeNetOfTax_57db4089-f785-42d6-94bc-7e03543b63ed" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_bdd8b3c1-0739-4517-8f8c-1f14c582d7a9" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_f46bedde-cb6b-468b-b863-a3dacc3bbb0a" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_bdd8b3c1-0739-4517-8f8c-1f14c582d7a9" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_3382e17f-be31-47a4-a8f2-6c4b7e93e80e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_f46bedde-cb6b-468b-b863-a3dacc3bbb0a" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_3382e17f-be31-47a4-a8f2-6c4b7e93e80e" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation_5435f5e4-e75a-42e0-9bd2-630d5e5c23f3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_f46bedde-cb6b-468b-b863-a3dacc3bbb0a" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation_5435f5e4-e75a-42e0-9bd2-630d5e5c23f3" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation_1865a520-cd3b-4f43-a64e-34b091449a94" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_f46bedde-cb6b-468b-b863-a3dacc3bbb0a" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation_1865a520-cd3b-4f43-a64e-34b091449a94" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_ebc8da9f-34be-458a-93a0-929cb52d1957" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_f46bedde-cb6b-468b-b863-a3dacc3bbb0a" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_ebc8da9f-34be-458a-93a0-929cb52d1957" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalStockIssuedIssuanceCosts_fda2c6d3-5c1c-4029-ba54-0eb958662e93" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalStockIssuedIssuanceCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_f46bedde-cb6b-468b-b863-a3dacc3bbb0a" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalStockIssuedIssuanceCosts_fda2c6d3-5c1c-4029-ba54-0eb958662e93" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesAcquisitions_f54e9b8f-d4b9-4a14-8eeb-0b72e6015bd0" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesAcquisitions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_f46bedde-cb6b-468b-b863-a3dacc3bbb0a" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesAcquisitions_f54e9b8f-d4b9-4a14-8eeb-0b72e6015bd0" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueAcquisitions_067bdeb8-7b8f-4eff-89a1-59b030d11290" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockIssuedDuringPeriodValueAcquisitions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_f46bedde-cb6b-468b-b863-a3dacc3bbb0a" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueAcquisitions_067bdeb8-7b8f-4eff-89a1-59b030d11290" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationOtherLongtermIncentivePlansRequisiteServicePeriodRecognition_d8b06824-baab-4779-adea-0d8e93c76256" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationOtherLongtermIncentivePlansRequisiteServicePeriodRecognition"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_f46bedde-cb6b-468b-b863-a3dacc3bbb0a" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationOtherLongtermIncentivePlansRequisiteServicePeriodRecognition_d8b06824-baab-4779-adea-0d8e93c76256" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodShares_3a0433d4-8594-482c-a24d-017b5c1acc2a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockRepurchasedAndRetiredDuringPeriodShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_f46bedde-cb6b-468b-b863-a3dacc3bbb0a" xlink:to="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodShares_3a0433d4-8594-482c-a24d-017b5c1acc2a" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodValue_f5d5a682-cbbe-41fa-8aab-7c33faf281fd" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockRepurchasedAndRetiredDuringPeriodValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_f46bedde-cb6b-468b-b863-a3dacc3bbb0a" xlink:to="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodValue_f5d5a682-cbbe-41fa-8aab-7c33faf281fd" xlink:type="arc" order="21"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_AdjustmentToRetainedEarningShareRepurchaseCosts_8072814b-4177-448e-b9ad-4a4666f36b25" xlink:href="iclr-20211231.xsd#iclr_AdjustmentToRetainedEarningShareRepurchaseCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_f46bedde-cb6b-468b-b863-a3dacc3bbb0a" xlink:to="loc_iclr_AdjustmentToRetainedEarningShareRepurchaseCosts_8072814b-4177-448e-b9ad-4a4666f36b25" xlink:type="arc" order="22"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_RedeemableNoncontrollingInterestAccretionToRedemptionValue_cb06e7de-4b25-4282-8350-7f22534f38dc" xlink:href="iclr-20211231.xsd#iclr_RedeemableNoncontrollingInterestAccretionToRedemptionValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_f46bedde-cb6b-468b-b863-a3dacc3bbb0a" xlink:to="loc_iclr_RedeemableNoncontrollingInterestAccretionToRedemptionValue_cb06e7de-4b25-4282-8350-7f22534f38dc" xlink:type="arc" order="23"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesIssued_a22faa95-ed47-49d5-b890-94d67ad2eca2" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommonStockSharesIssued"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_fa657473-bca0-4628-9464-0159768c0330" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockholdersEquity"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInTemporaryEquityRollForward_12d9d3a4-680e-482b-8e51-6f1683a0277d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncreaseDecreaseInTemporaryEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_f79ddbaa-12b3-42ca-9242-11bdef2f4bd0" xlink:to="loc_us-gaap_IncreaseDecreaseInTemporaryEquityRollForward_12d9d3a4-680e-482b-8e51-6f1683a0277d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RedeemableNoncontrollingInterestEquityCarryingAmount_e9e9d100-2a47-466c-82af-7179bd54d697" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RedeemableNoncontrollingInterestEquityCarryingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInTemporaryEquityRollForward_12d9d3a4-680e-482b-8e51-6f1683a0277d" xlink:to="loc_us-gaap_RedeemableNoncontrollingInterestEquityCarryingAmount_e9e9d100-2a47-466c-82af-7179bd54d697" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TemporaryEquityNetIncome_6e4bffac-8c96-48dc-b8a9-4f75dfa70b07" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TemporaryEquityNetIncome"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInTemporaryEquityRollForward_12d9d3a4-680e-482b-8e51-6f1683a0277d" xlink:to="loc_us-gaap_TemporaryEquityNetIncome_6e4bffac-8c96-48dc-b8a9-4f75dfa70b07" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_RedeemableNoncontrollingInterestIncreaseFromBusinessCombination_9d086b08-dab4-472f-ba19-28034be34e93" xlink:href="iclr-20211231.xsd#iclr_RedeemableNoncontrollingInterestIncreaseFromBusinessCombination"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInTemporaryEquityRollForward_12d9d3a4-680e-482b-8e51-6f1683a0277d" xlink:to="loc_iclr_RedeemableNoncontrollingInterestIncreaseFromBusinessCombination_9d086b08-dab4-472f-ba19-28034be34e93" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TemporaryEquityAccretionToRedemptionValue_3e641ed9-46a0-437d-83e5-2e599d336460" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TemporaryEquityAccretionToRedemptionValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInTemporaryEquityRollForward_12d9d3a4-680e-482b-8e51-6f1683a0277d" xlink:to="loc_us-gaap_TemporaryEquityAccretionToRedemptionValue_3e641ed9-46a0-437d-83e5-2e599d336460" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RedeemableNoncontrollingInterestEquityCarryingAmount_59485455-722f-4bf6-b1c9-1ad1b467a888" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RedeemableNoncontrollingInterestEquityCarryingAmount"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_2c2abe2e-c4c4-4b80-8f83-1ad833ddf740" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_f79ddbaa-12b3-42ca-9242-11bdef2f4bd0" xlink:to="loc_us-gaap_StatementTable_2c2abe2e-c4c4-4b80-8f83-1ad833ddf740" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_6f32ec7a-a797-447f-a147-fe5b711de14c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_2c2abe2e-c4c4-4b80-8f83-1ad833ddf740" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_6f32ec7a-a797-447f-a147-fe5b711de14c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_6f32ec7a-a797-447f-a147-fe5b711de14c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_6f32ec7a-a797-447f-a147-fe5b711de14c" xlink:to="loc_us-gaap_EquityComponentDomain_6f32ec7a-a797-447f-a147-fe5b711de14c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_e33b2fac-61e4-4544-bac5-e31adbd2de78" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_6f32ec7a-a797-447f-a147-fe5b711de14c" xlink:to="loc_us-gaap_EquityComponentDomain_e33b2fac-61e4-4544-bac5-e31adbd2de78" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_b097869a-f9fe-4a26-8173-21d784a115d6" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_e33b2fac-61e4-4544-bac5-e31adbd2de78" xlink:to="loc_us-gaap_CommonStockMember_b097869a-f9fe-4a26-8173-21d784a115d6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_01a3015c-575c-4a67-a979-14ce423ee386" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AdditionalPaidInCapitalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_e33b2fac-61e4-4544-bac5-e31adbd2de78" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_01a3015c-575c-4a67-a979-14ce423ee386" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_OtherUndenominatedCapitalMember_4dae5087-fedf-4adc-9a63-7795020e0664" xlink:href="iclr-20211231.xsd#iclr_OtherUndenominatedCapitalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_e33b2fac-61e4-4544-bac5-e31adbd2de78" xlink:to="loc_iclr_OtherUndenominatedCapitalMember_4dae5087-fedf-4adc-9a63-7795020e0664" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_49bf75cd-57dc-43de-8c09-20dcf6eda096" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_e33b2fac-61e4-4544-bac5-e31adbd2de78" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_49bf75cd-57dc-43de-8c09-20dcf6eda096" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsMember_9deca998-10a6-41f4-ba6e-27747d0ae691" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RetainedEarningsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_e33b2fac-61e4-4544-bac5-e31adbd2de78" xlink:to="loc_us-gaap_RetainedEarningsMember_9deca998-10a6-41f4-ba6e-27747d0ae691" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_RedeemableNoncontrollingInterestMember_8bfe444c-fc49-48de-b988-0ac6e4854115" xlink:href="iclr-20211231.xsd#iclr_RedeemableNoncontrollingInterestMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_e33b2fac-61e4-4544-bac5-e31adbd2de78" xlink:to="loc_iclr_RedeemableNoncontrollingInterestMember_8bfe444c-fc49-48de-b988-0ac6e4854115" xlink:type="arc" order="5"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/DescriptionofbusinessDetails" xlink:type="simple" xlink:href="iclr-20211231.xsd#DescriptionofbusinessDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/DescriptionofbusinessDetails" xlink:type="extended" id="i344cd125c7884e61a191aca092ddb4cb_DescriptionofbusinessDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProductInformationLineItems_f7c3d103-1302-481f-9d74-7124dd6d9761" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ProductInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityNumberOfEmployees_d8e855b5-8bfb-4522-b932-a991b8ae0efd" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_EntityNumberOfEmployees"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductInformationLineItems_f7c3d103-1302-481f-9d74-7124dd6d9761" xlink:to="loc_dei_EntityNumberOfEmployees_d8e855b5-8bfb-4522-b932-a991b8ae0efd" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_NumberOfLocations_24c22435-d4c3-46e9-a115-6be175a9a785" xlink:href="iclr-20211231.xsd#iclr_NumberOfLocations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductInformationLineItems_f7c3d103-1302-481f-9d74-7124dd6d9761" xlink:to="loc_iclr_NumberOfLocations_24c22435-d4c3-46e9-a115-6be175a9a785" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NumberOfCountriesInWhichEntityOperates_5651df06-05c7-48c3-b353-6bc61a8925d2" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_NumberOfCountriesInWhichEntityOperates"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductInformationLineItems_f7c3d103-1302-481f-9d74-7124dd6d9761" xlink:to="loc_us-gaap_NumberOfCountriesInWhichEntityOperates_5651df06-05c7-48c3-b353-6bc61a8925d2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskPercentage1_cc067ae3-4d4b-411b-bed0-053d9b91d87e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ConcentrationRiskPercentage1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductInformationLineItems_f7c3d103-1302-481f-9d74-7124dd6d9761" xlink:to="loc_us-gaap_ConcentrationRiskPercentage1_cc067ae3-4d4b-411b-bed0-053d9b91d87e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfProductInformationTable_da7da903-6ab0-4192-906f-7810f1b8c64d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfProductInformationTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ProductInformationLineItems_f7c3d103-1302-481f-9d74-7124dd6d9761" xlink:to="loc_us-gaap_ScheduleOfProductInformationTable_da7da903-6ab0-4192-906f-7810f1b8c64d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByTypeAxis_d03beb71-b824-4a83-94a8-0ea13339a078" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ConcentrationRiskByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfProductInformationTable_da7da903-6ab0-4192-906f-7810f1b8c64d" xlink:to="loc_us-gaap_ConcentrationRiskByTypeAxis_d03beb71-b824-4a83-94a8-0ea13339a078" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_d03beb71-b824-4a83-94a8-0ea13339a078_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_d03beb71-b824-4a83-94a8-0ea13339a078" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_d03beb71-b824-4a83-94a8-0ea13339a078_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_ce1ee798-ad31-4f40-a2b1-68a0f8507f38" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_d03beb71-b824-4a83-94a8-0ea13339a078" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_ce1ee798-ad31-4f40-a2b1-68a0f8507f38" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GeographicConcentrationRiskMember_0915046e-2926-4e09-8988-36a1b8d5b280" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_GeographicConcentrationRiskMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskTypeDomain_ce1ee798-ad31-4f40-a2b1-68a0f8507f38" xlink:to="loc_us-gaap_GeographicConcentrationRiskMember_0915046e-2926-4e09-8988-36a1b8d5b280" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementGeographicalAxis_52393636-0ec2-4eea-8e29-5d88da662fae" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_StatementGeographicalAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfProductInformationTable_da7da903-6ab0-4192-906f-7810f1b8c64d" xlink:to="loc_srt_StatementGeographicalAxis_52393636-0ec2-4eea-8e29-5d88da662fae" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_52393636-0ec2-4eea-8e29-5d88da662fae_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_52393636-0ec2-4eea-8e29-5d88da662fae" xlink:to="loc_srt_SegmentGeographicalDomain_52393636-0ec2-4eea-8e29-5d88da662fae_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_6fc9b19d-7696-4d9a-8c7c-6e4b5757ac25" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_52393636-0ec2-4eea-8e29-5d88da662fae" xlink:to="loc_srt_SegmentGeographicalDomain_6fc9b19d-7696-4d9a-8c7c-6e4b5757ac25" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_US_6c50a238-95fb-4c9e-9acc-acd1810c51c9" xlink:href="https://xbrl.sec.gov/country/2021/country-2021.xsd#country_US"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_6fc9b19d-7696-4d9a-8c7c-6e4b5757ac25" xlink:to="loc_country_US_6c50a238-95fb-4c9e-9acc-acd1810c51c9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_EuropeMember_fd9bdffc-bb15-4b2f-80c5-c47c298662b2" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_EuropeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_6fc9b19d-7696-4d9a-8c7c-6e4b5757ac25" xlink:to="loc_srt_EuropeMember_fd9bdffc-bb15-4b2f-80c5-c47c298662b2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_OtherGeographicLocationsMember_402a4331-0e80-4db0-8737-bcee8b15cc9c" xlink:href="iclr-20211231.xsd#iclr_OtherGeographicLocationsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_6fc9b19d-7696-4d9a-8c7c-6e4b5757ac25" xlink:to="loc_iclr_OtherGeographicLocationsMember_402a4331-0e80-4db0-8737-bcee8b15cc9c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_57cc8711-852f-408a-9b85-6e7f3d9ae96b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ConcentrationRiskByBenchmarkAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfProductInformationTable_da7da903-6ab0-4192-906f-7810f1b8c64d" xlink:to="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_57cc8711-852f-408a-9b85-6e7f3d9ae96b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_57cc8711-852f-408a-9b85-6e7f3d9ae96b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_57cc8711-852f-408a-9b85-6e7f3d9ae96b" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_57cc8711-852f-408a-9b85-6e7f3d9ae96b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_11e4f82f-2c0f-4c7a-9706-77ade7e790d3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_57cc8711-852f-408a-9b85-6e7f3d9ae96b" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_11e4f82f-2c0f-4c7a-9706-77ade7e790d3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SalesRevenueNetMember_a9a5e3dd-27b3-4248-bf25-3c36cf49a364" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SalesRevenueNetMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskBenchmarkDomain_11e4f82f-2c0f-4c7a-9706-77ade7e790d3" xlink:to="loc_us-gaap_SalesRevenueNetMember_a9a5e3dd-27b3-4248-bf25-3c36cf49a364" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/SignificantaccountingpoliciesNarrativeDetails" xlink:type="simple" xlink:href="iclr-20211231.xsd#SignificantaccountingpoliciesNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/SignificantaccountingpoliciesNarrativeDetails" xlink:type="extended" id="i448cbe8c9521439b82b0322b576a2ec8_SignificantaccountingpoliciesNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_ca21f24a-24fa-4cc7-84bb-2d34621db7a6" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillImpairmentLoss_6b9ae45c-d0af-4ba3-bd97-47f672119af3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_GoodwillImpairmentLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_ca21f24a-24fa-4cc7-84bb-2d34621db7a6" xlink:to="loc_us-gaap_GoodwillImpairmentLoss_6b9ae45c-d0af-4ba3-bd97-47f672119af3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InventoryNet_57257a8b-6c24-4475-8d97-aff76cd41f67" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_InventoryNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_ca21f24a-24fa-4cc7-84bb-2d34621db7a6" xlink:to="loc_us-gaap_InventoryNet_57257a8b-6c24-4475-8d97-aff76cd41f67" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationStepAcquisitionEquityInterestInAcquireeIncludingSubsequentAcquisitionPercentage_bee5a7e1-2cea-40c4-b8a6-fafdbf946c84" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationStepAcquisitionEquityInterestInAcquireeIncludingSubsequentAcquisitionPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_ca21f24a-24fa-4cc7-84bb-2d34621db7a6" xlink:to="loc_us-gaap_BusinessCombinationStepAcquisitionEquityInterestInAcquireeIncludingSubsequentAcquisitionPercentage_bee5a7e1-2cea-40c4-b8a6-fafdbf946c84" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable_8dcf05b0-418d-4544-bdde-5597ba59e6aa" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_ca21f24a-24fa-4cc7-84bb-2d34621db7a6" xlink:to="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable_8dcf05b0-418d-4544-bdde-5597ba59e6aa" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_fdeae475-c88f-4fb1-aecb-91006e92dedf" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable_8dcf05b0-418d-4544-bdde-5597ba59e6aa" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_fdeae475-c88f-4fb1-aecb-91006e92dedf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_fdeae475-c88f-4fb1-aecb-91006e92dedf_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_fdeae475-c88f-4fb1-aecb-91006e92dedf" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_fdeae475-c88f-4fb1-aecb-91006e92dedf_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_ff4faece-4b56-4ab8-9385-0b3a3a181efd" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_fdeae475-c88f-4fb1-aecb-91006e92dedf" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_ff4faece-4b56-4ab8-9385-0b3a3a181efd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_MeDiNovaResearchMember_240e3140-e910-49cc-8011-0486ad16fc61" xlink:href="iclr-20211231.xsd#iclr_MeDiNovaResearchMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_ff4faece-4b56-4ab8-9385-0b3a3a181efd" xlink:to="loc_iclr_MeDiNovaResearchMember_240e3140-e910-49cc-8011-0486ad16fc61" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/SignificantaccountingpoliciesEstimatedUsefulLivesofAssetsDetails" xlink:type="simple" xlink:href="iclr-20211231.xsd#SignificantaccountingpoliciesEstimatedUsefulLivesofAssetsDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/SignificantaccountingpoliciesEstimatedUsefulLivesofAssetsDetails" xlink:type="extended" id="ic2e60009d5a5439dad330fb8d9fdc359_SignificantaccountingpoliciesEstimatedUsefulLivesofAssetsDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentLineItems_1ec85b32-2272-4b0c-be09-12d8134247be" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PropertyPlantAndEquipmentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentUsefulLife_f3959b1b-99be-42d9-9348-b9221ed616b0" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PropertyPlantAndEquipmentUsefulLife"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_1ec85b32-2272-4b0c-be09-12d8134247be" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentUsefulLife_f3959b1b-99be-42d9-9348-b9221ed616b0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_b6654ede-7728-4042-ae83-7ebe237771b5" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_1ec85b32-2272-4b0c-be09-12d8134247be" xlink:to="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_b6654ede-7728-4042-ae83-7ebe237771b5" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_9fbc0b82-509f-4d93-99d3-ee2b8e4b0b3a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_b6654ede-7728-4042-ae83-7ebe237771b5" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_9fbc0b82-509f-4d93-99d3-ee2b8e4b0b3a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_9fbc0b82-509f-4d93-99d3-ee2b8e4b0b3a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_9fbc0b82-509f-4d93-99d3-ee2b8e4b0b3a" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_9fbc0b82-509f-4d93-99d3-ee2b8e4b0b3a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_78d0953e-6b73-44f4-9c73-e4a42ccdd4af" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_9fbc0b82-509f-4d93-99d3-ee2b8e4b0b3a" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_78d0953e-6b73-44f4-9c73-e4a42ccdd4af" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BuildingMember_6b9e9a05-8ac6-4ee4-9ac1-9a515598489e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BuildingMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_78d0953e-6b73-44f4-9c73-e4a42ccdd4af" xlink:to="loc_us-gaap_BuildingMember_6b9e9a05-8ac6-4ee4-9ac1-9a515598489e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_ComputerEquipmentAndCapitalizedSoftwareMember_c637b1cd-13f7-4714-9470-ba7f6272e259" xlink:href="iclr-20211231.xsd#iclr_ComputerEquipmentAndCapitalizedSoftwareMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_78d0953e-6b73-44f4-9c73-e4a42ccdd4af" xlink:to="loc_iclr_ComputerEquipmentAndCapitalizedSoftwareMember_c637b1cd-13f7-4714-9470-ba7f6272e259" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FurnitureAndFixturesMember_ed18a8e4-b1b5-4129-bf9a-51c51c8a3b22" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FurnitureAndFixturesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_78d0953e-6b73-44f4-9c73-e4a42ccdd4af" xlink:to="loc_us-gaap_FurnitureAndFixturesMember_ed18a8e4-b1b5-4129-bf9a-51c51c8a3b22" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquipmentMember_c1f5a205-2ae6-4bbd-94d5-9164c8def31d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EquipmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_78d0953e-6b73-44f4-9c73-e4a42ccdd4af" xlink:to="loc_us-gaap_EquipmentMember_c1f5a205-2ae6-4bbd-94d5-9164c8def31d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VehiclesMember_355c099b-d14d-48cb-9004-c5a2cf684c28" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_VehiclesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_78d0953e-6b73-44f4-9c73-e4a42ccdd4af" xlink:to="loc_us-gaap_VehiclesMember_355c099b-d14d-48cb-9004-c5a2cf684c28" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_06b87f5a-fe0b-477f-811d-0ea9cd6e4df2" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_b6654ede-7728-4042-ae83-7ebe237771b5" xlink:to="loc_srt_RangeAxis_06b87f5a-fe0b-477f-811d-0ea9cd6e4df2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_06b87f5a-fe0b-477f-811d-0ea9cd6e4df2_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_06b87f5a-fe0b-477f-811d-0ea9cd6e4df2" xlink:to="loc_srt_RangeMember_06b87f5a-fe0b-477f-811d-0ea9cd6e4df2_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_659f98ed-0e6c-4174-ac57-7ac3d97e22e7" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_06b87f5a-fe0b-477f-811d-0ea9cd6e4df2" xlink:to="loc_srt_RangeMember_659f98ed-0e6c-4174-ac57-7ac3d97e22e7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_6bd0d12d-0171-4b33-af77-2b635770e059" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_659f98ed-0e6c-4174-ac57-7ac3d97e22e7" xlink:to="loc_srt_MinimumMember_6bd0d12d-0171-4b33-af77-2b635770e059" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_4d86dc80-7376-454e-bfdc-da329e634c3b" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_659f98ed-0e6c-4174-ac57-7ac3d97e22e7" xlink:to="loc_srt_MaximumMember_4d86dc80-7376-454e-bfdc-da329e634c3b" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/InvestmentsNarrativeDetails" xlink:type="simple" xlink:href="iclr-20211231.xsd#InvestmentsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/InvestmentsNarrativeDetails" xlink:type="extended" id="i60170864b76247049bc4014d1496ab02_InvestmentsNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionLineItems_4594f19e-5e77-4b7a-a12d-8ea7bc0c7b99" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RelatedPartyTransactionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_ShortTermAvailableForSaleInvestmentsAverageRemainingMaturityPeriod_01b679a8-7da6-49e5-8157-5a790efb7590" xlink:href="iclr-20211231.xsd#iclr_ShortTermAvailableForSaleInvestmentsAverageRemainingMaturityPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_4594f19e-5e77-4b7a-a12d-8ea7bc0c7b99" xlink:to="loc_iclr_ShortTermAvailableForSaleInvestmentsAverageRemainingMaturityPeriod_01b679a8-7da6-49e5-8157-5a790efb7590" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquireLongtermInvestments_ac075432-8d27-4cd3-89d8-4243931632f7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PaymentsToAcquireLongtermInvestments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_4594f19e-5e77-4b7a-a12d-8ea7bc0c7b99" xlink:to="loc_us-gaap_PaymentsToAcquireLongtermInvestments_ac075432-8d27-4cd3-89d8-4243931632f7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesFvNiGainLoss_034777a2-3ec5-4bde-83e5-cf9bbc259c24" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EquitySecuritiesFvNiGainLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_4594f19e-5e77-4b7a-a12d-8ea7bc0c7b99" xlink:to="loc_us-gaap_EquitySecuritiesFvNiGainLoss_034777a2-3ec5-4bde-83e5-cf9bbc259c24" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesFvNi_3b8cfc64-d8fe-4721-9972-b878877c2c4d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EquitySecuritiesFvNi"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_4594f19e-5e77-4b7a-a12d-8ea7bc0c7b99" xlink:to="loc_us-gaap_EquitySecuritiesFvNi_3b8cfc64-d8fe-4721-9972-b878877c2c4d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_CommitmentToAcquireLongTermInvestments_a8f2d5b0-9a10-41c8-b4b5-ce26d367ba60" xlink:href="iclr-20211231.xsd#iclr_CommitmentToAcquireLongTermInvestments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_4594f19e-5e77-4b7a-a12d-8ea7bc0c7b99" xlink:to="loc_iclr_CommitmentToAcquireLongTermInvestments_a8f2d5b0-9a10-41c8-b4b5-ce26d367ba60" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityMethodInvestments_db4e696a-780b-46c8-ae76-e5ef9f171da2" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EquityMethodInvestments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_4594f19e-5e77-4b7a-a12d-8ea7bc0c7b99" xlink:to="loc_us-gaap_EquityMethodInvestments_db4e696a-780b-46c8-ae76-e5ef9f171da2" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityMethodInvestmentOwnershipPercentage_0a29ce61-0a69-4723-966a-fa038ac8f298" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EquityMethodInvestmentOwnershipPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_4594f19e-5e77-4b7a-a12d-8ea7bc0c7b99" xlink:to="loc_us-gaap_EquityMethodInvestmentOwnershipPercentage_0a29ce61-0a69-4723-966a-fa038ac8f298" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_EquityMethodInvestmentMajorityOwnershipPercentage_8467eafb-dd69-4cab-8b92-a22967cdb0f6" xlink:href="iclr-20211231.xsd#iclr_EquityMethodInvestmentMajorityOwnershipPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_4594f19e-5e77-4b7a-a12d-8ea7bc0c7b99" xlink:to="loc_iclr_EquityMethodInvestmentMajorityOwnershipPercentage_8467eafb-dd69-4cab-8b92-a22967cdb0f6" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_EquityMethodInvestmentPeriodMajorityVotingShareCapitalCanBeSoldToTheCompany_b8047077-a3f0-44d8-b131-213bc92245ea" xlink:href="iclr-20211231.xsd#iclr_EquityMethodInvestmentPeriodMajorityVotingShareCapitalCanBeSoldToTheCompany"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_4594f19e-5e77-4b7a-a12d-8ea7bc0c7b99" xlink:to="loc_iclr_EquityMethodInvestmentPeriodMajorityVotingShareCapitalCanBeSoldToTheCompany_b8047077-a3f0-44d8-b131-213bc92245ea" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromEquityMethodInvestments_dedc6167-1bef-4c6c-b204-77aced67afb2" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeLossFromEquityMethodInvestments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_4594f19e-5e77-4b7a-a12d-8ea7bc0c7b99" xlink:to="loc_us-gaap_IncomeLossFromEquityMethodInvestments_dedc6167-1bef-4c6c-b204-77aced67afb2" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LoansAndLeasesReceivableRelatedPartiesAdditions_996e4835-5646-4811-ad96-c4bf5352a18e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LoansAndLeasesReceivableRelatedPartiesAdditions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_4594f19e-5e77-4b7a-a12d-8ea7bc0c7b99" xlink:to="loc_us-gaap_LoansAndLeasesReceivableRelatedPartiesAdditions_996e4835-5646-4811-ad96-c4bf5352a18e" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionRate_3fc8afc0-ed63-49ec-a51e-03af3a9f7778" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RelatedPartyTransactionRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_4594f19e-5e77-4b7a-a12d-8ea7bc0c7b99" xlink:to="loc_us-gaap_RelatedPartyTransactionRate_3fc8afc0-ed63-49ec-a51e-03af3a9f7778" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_bbe38a6d-8eaa-4dcc-8d93-b0747cc61ff7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_4594f19e-5e77-4b7a-a12d-8ea7bc0c7b99" xlink:to="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_bbe38a6d-8eaa-4dcc-8d93-b0747cc61ff7" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionAxis_1fb4f11e-6b61-4adc-ac13-df79a9e56ae2" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RelatedPartyTransactionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_bbe38a6d-8eaa-4dcc-8d93-b0747cc61ff7" xlink:to="loc_us-gaap_RelatedPartyTransactionAxis_1fb4f11e-6b61-4adc-ac13-df79a9e56ae2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionDomain_1fb4f11e-6b61-4adc-ac13-df79a9e56ae2_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RelatedPartyTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RelatedPartyTransactionAxis_1fb4f11e-6b61-4adc-ac13-df79a9e56ae2" xlink:to="loc_us-gaap_RelatedPartyTransactionDomain_1fb4f11e-6b61-4adc-ac13-df79a9e56ae2_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionDomain_d6c76a7d-ba19-47d5-836e-4cdf2f7fab54" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RelatedPartyTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RelatedPartyTransactionAxis_1fb4f11e-6b61-4adc-ac13-df79a9e56ae2" xlink:to="loc_us-gaap_RelatedPartyTransactionDomain_d6c76a7d-ba19-47d5-836e-4cdf2f7fab54" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_OncacareMember_906c6cb7-955f-4432-bea5-7a1a73a6a91f" xlink:href="iclr-20211231.xsd#iclr_OncacareMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionDomain_d6c76a7d-ba19-47d5-836e-4cdf2f7fab54" xlink:to="loc_iclr_OncacareMember_906c6cb7-955f-4432-bea5-7a1a73a6a91f" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/InvestmentsAvailableForSaleShortTermInvestmentsbyMajorSecurityTypeDetails" xlink:type="simple" xlink:href="iclr-20211231.xsd#InvestmentsAvailableForSaleShortTermInvestmentsbyMajorSecurityTypeDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/InvestmentsAvailableForSaleShortTermInvestmentsbyMajorSecurityTypeDetails" xlink:type="extended" id="i2bb703a2fffe4c349b34950f66d61fa1_InvestmentsAvailableForSaleShortTermInvestmentsbyMajorSecurityTypeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_95e96b20-37dd-4883-9dfe-62386f8780dc" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis_2a67b614-a681-4864-825e-6637fe1048dc" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_95e96b20-37dd-4883-9dfe-62386f8780dc" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis_2a67b614-a681-4864-825e-6637fe1048dc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_72e96850-35c1-4f8a-b969-0cd90438ce20" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AvailableForSaleSecuritiesDebtSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_95e96b20-37dd-4883-9dfe-62386f8780dc" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_72e96850-35c1-4f8a-b969-0cd90438ce20" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue_69675350-f537-4b28-9099-b8395fe0e761" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_95e96b20-37dd-4883-9dfe-62386f8780dc" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue_69675350-f537-4b28-9099-b8395fe0e761" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue_086c14ab-9020-4cb1-8f0f-acdf2d0f45d6" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_95e96b20-37dd-4883-9dfe-62386f8780dc" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue_086c14ab-9020-4cb1-8f0f-acdf2d0f45d6" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesTable_494f6a7a-9143-4b62-93b1-908fe0cb34b5" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfAvailableForSaleSecuritiesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_95e96b20-37dd-4883-9dfe-62386f8780dc" xlink:to="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesTable_494f6a7a-9143-4b62-93b1-908fe0cb34b5" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_c2563fae-2d70-46f0-b31e-a8408b068296" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesTable_494f6a7a-9143-4b62-93b1-908fe0cb34b5" xlink:to="loc_us-gaap_FinancialInstrumentAxis_c2563fae-2d70-46f0-b31e-a8408b068296" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_c2563fae-2d70-46f0-b31e-a8408b068296_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_c2563fae-2d70-46f0-b31e-a8408b068296" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_c2563fae-2d70-46f0-b31e-a8408b068296_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_b0b6a8e0-e044-48e1-b869-d3a87b27b365" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_c2563fae-2d70-46f0-b31e-a8408b068296" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_b0b6a8e0-e044-48e1-b869-d3a87b27b365" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_TermDepositsMember_4b4d98a1-a87f-4a3f-a8ee-788d0e4df7e6" xlink:href="iclr-20211231.xsd#iclr_TermDepositsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_b0b6a8e0-e044-48e1-b869-d3a87b27b365" xlink:to="loc_iclr_TermDepositsMember_4b4d98a1-a87f-4a3f-a8ee-788d0e4df7e6" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/IntangibleAssetsSummaryofIntangibleAssetsDetails" xlink:type="simple" xlink:href="iclr-20211231.xsd#IntangibleAssetsSummaryofIntangibleAssetsDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/IntangibleAssetsSummaryofIntangibleAssetsDetails" xlink:type="extended" id="i74a01bdf9dca4b1485fff19ffc720a2f_IntangibleAssetsSummaryofIntangibleAssetsDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsLineItems_28fba519-0e52-4207-81ec-6c6d27b72a2f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AcquiredFiniteLivedIntangibleAssetsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsGross_78d67eaf-e3e0-4fbf-9573-3fb744f1b282" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsLineItems_28fba519-0e52-4207-81ec-6c6d27b72a2f" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsGross_78d67eaf-e3e0-4fbf-9573-3fb744f1b282" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_de86b3bc-d7f0-4189-9d31-ba1de1f3fa46" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsLineItems_28fba519-0e52-4207-81ec-6c6d27b72a2f" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_de86b3bc-d7f0-4189-9d31-ba1de1f3fa46" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsNet_ea9a5f61-766a-4823-9578-023117b5a8a0" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsLineItems_28fba519-0e52-4207-81ec-6c6d27b72a2f" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsNet_ea9a5f61-766a-4823-9578-023117b5a8a0" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAcquiredFiniteLivedIntangibleAssetByMajorClassTable_711c3279-5675-4178-ad6f-7e83d327cda5" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfAcquiredFiniteLivedIntangibleAssetByMajorClassTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsLineItems_28fba519-0e52-4207-81ec-6c6d27b72a2f" xlink:to="loc_us-gaap_ScheduleOfAcquiredFiniteLivedIntangibleAssetByMajorClassTable_711c3279-5675-4178-ad6f-7e83d327cda5" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_bf6c0a83-093e-403c-a8a7-50fc97c19180" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAcquiredFiniteLivedIntangibleAssetByMajorClassTable_711c3279-5675-4178-ad6f-7e83d327cda5" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_bf6c0a83-093e-403c-a8a7-50fc97c19180" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_bf6c0a83-093e-403c-a8a7-50fc97c19180_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_bf6c0a83-093e-403c-a8a7-50fc97c19180" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_bf6c0a83-093e-403c-a8a7-50fc97c19180_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_ac2195b8-0f21-4af5-9516-ba995878c846" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_bf6c0a83-093e-403c-a8a7-50fc97c19180" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_ac2195b8-0f21-4af5-9516-ba995878c846" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerRelationshipsMember_556923be-c532-4898-886b-1de4eff5533d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CustomerRelationshipsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_ac2195b8-0f21-4af5-9516-ba995878c846" xlink:to="loc_us-gaap_CustomerRelationshipsMember_556923be-c532-4898-886b-1de4eff5533d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OrderOrProductionBacklogMember_21fed44e-475a-4050-b60b-2cf989d5a0a2" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OrderOrProductionBacklogMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_ac2195b8-0f21-4af5-9516-ba995878c846" xlink:to="loc_us-gaap_OrderOrProductionBacklogMember_21fed44e-475a-4050-b60b-2cf989d5a0a2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TradeNamesMember_ad0bc0cc-5e68-4cbe-851c-b855ca59a07d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TradeNamesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_ac2195b8-0f21-4af5-9516-ba995878c846" xlink:to="loc_us-gaap_TradeNamesMember_ad0bc0cc-5e68-4cbe-851c-b855ca59a07d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DatabasesMember_cc882de7-884a-4949-a610-10b38a822dc4" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DatabasesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_ac2195b8-0f21-4af5-9516-ba995878c846" xlink:to="loc_us-gaap_DatabasesMember_cc882de7-884a-4949-a610-10b38a822dc4" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TechnologyBasedIntangibleAssetsMember_f45681bb-4b54-4a5a-9da4-737dd2985c94" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TechnologyBasedIntangibleAssetsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_ac2195b8-0f21-4af5-9516-ba995878c846" xlink:to="loc_us-gaap_TechnologyBasedIntangibleAssetsMember_f45681bb-4b54-4a5a-9da4-737dd2985c94" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/IntangibleAssetsNarrativeDetails" xlink:type="simple" xlink:href="iclr-20211231.xsd#IntangibleAssetsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/IntangibleAssetsNarrativeDetails" xlink:type="extended" id="i0cc5786283d34265bd97952547d647be_IntangibleAssetsNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassLineItems_8ddc74b5-2a96-420d-a661-f65bffcf9335" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles_ef20c3d8-acf4-4991-b628-388c8265d7d1" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassLineItems_8ddc74b5-2a96-420d-a661-f65bffcf9335" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles_ef20c3d8-acf4-4991-b628-388c8265d7d1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetUsefulLife_6ca34707-7a7b-4a8e-a4c7-e7198c4e71ac" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetUsefulLife"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassLineItems_8ddc74b5-2a96-420d-a661-f65bffcf9335" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetUsefulLife_6ca34707-7a7b-4a8e-a4c7-e7198c4e71ac" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AmortizationOfIntangibleAssets_9a709bac-da1b-4c58-8138-ab4166509070" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AmortizationOfIntangibleAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassLineItems_8ddc74b5-2a96-420d-a661-f65bffcf9335" xlink:to="loc_us-gaap_AmortizationOfIntangibleAssets_9a709bac-da1b-4c58-8138-ab4166509070" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionPercentageOfVotingInterestsAcquired_64cc3df3-611f-4a58-a216-5371bdedd782" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionPercentageOfVotingInterestsAcquired"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassLineItems_8ddc74b5-2a96-420d-a661-f65bffcf9335" xlink:to="loc_us-gaap_BusinessAcquisitionPercentageOfVotingInterestsAcquired_64cc3df3-611f-4a58-a216-5371bdedd782" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfIndefiniteLivedIntangibleAssetsTable_9df358c7-07b0-4595-b9eb-3bb3dd44a77c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfIndefiniteLivedIntangibleAssetsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassLineItems_8ddc74b5-2a96-420d-a661-f65bffcf9335" xlink:to="loc_us-gaap_ScheduleOfIndefiniteLivedIntangibleAssetsTable_9df358c7-07b0-4595-b9eb-3bb3dd44a77c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_e19d31fc-1a6c-4870-9e65-44d22cbc292d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfIndefiniteLivedIntangibleAssetsTable_9df358c7-07b0-4595-b9eb-3bb3dd44a77c" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_e19d31fc-1a6c-4870-9e65-44d22cbc292d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_e19d31fc-1a6c-4870-9e65-44d22cbc292d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_e19d31fc-1a6c-4870-9e65-44d22cbc292d" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_e19d31fc-1a6c-4870-9e65-44d22cbc292d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_53203016-8fa6-4e7a-a869-53ab7ea41d95" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_e19d31fc-1a6c-4870-9e65-44d22cbc292d" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_53203016-8fa6-4e7a-a869-53ab7ea41d95" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_PRAHealthSciencesIncMember_8096ff42-62af-4241-8c1e-1b60d64f2b55" xlink:href="iclr-20211231.xsd#iclr_PRAHealthSciencesIncMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_53203016-8fa6-4e7a-a869-53ab7ea41d95" xlink:to="loc_iclr_PRAHealthSciencesIncMember_8096ff42-62af-4241-8c1e-1b60d64f2b55" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_MedpassGroupLimitedMember_411b0520-221b-4e80-a640-bae00badfd58" xlink:href="iclr-20211231.xsd#iclr_MedpassGroupLimitedMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_53203016-8fa6-4e7a-a869-53ab7ea41d95" xlink:to="loc_iclr_MedpassGroupLimitedMember_411b0520-221b-4e80-a640-bae00badfd58" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_0ea78864-b6fc-4bbe-ae95-50b2ce830afa" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfIndefiniteLivedIntangibleAssetsTable_9df358c7-07b0-4595-b9eb-3bb3dd44a77c" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_0ea78864-b6fc-4bbe-ae95-50b2ce830afa" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_0ea78864-b6fc-4bbe-ae95-50b2ce830afa_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_0ea78864-b6fc-4bbe-ae95-50b2ce830afa" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_0ea78864-b6fc-4bbe-ae95-50b2ce830afa_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_a2b54d45-b4bc-41e1-938f-bca1d821df0c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_0ea78864-b6fc-4bbe-ae95-50b2ce830afa" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_a2b54d45-b4bc-41e1-938f-bca1d821df0c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerRelationshipsMember_2d7fc7bc-0651-414c-9fff-d9a26b9cb408" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CustomerRelationshipsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_a2b54d45-b4bc-41e1-938f-bca1d821df0c" xlink:to="loc_us-gaap_CustomerRelationshipsMember_2d7fc7bc-0651-414c-9fff-d9a26b9cb408" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OrderOrProductionBacklogMember_998ab98f-5739-4ec7-b07b-1d7d68b1f94d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OrderOrProductionBacklogMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_a2b54d45-b4bc-41e1-938f-bca1d821df0c" xlink:to="loc_us-gaap_OrderOrProductionBacklogMember_998ab98f-5739-4ec7-b07b-1d7d68b1f94d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TradeNamesMember_a465a660-15fa-4ad3-b3d2-95bb26fd7074" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TradeNamesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_a2b54d45-b4bc-41e1-938f-bca1d821df0c" xlink:to="loc_us-gaap_TradeNamesMember_a465a660-15fa-4ad3-b3d2-95bb26fd7074" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DatabasesMember_97706b51-dae7-4ccf-ac8d-10b6f469a4c3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DatabasesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_a2b54d45-b4bc-41e1-938f-bca1d821df0c" xlink:to="loc_us-gaap_DatabasesMember_97706b51-dae7-4ccf-ac8d-10b6f469a4c3" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TechnologyBasedIntangibleAssetsMember_640e4b1a-a251-43a4-b1bf-252cba02f7dc" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TechnologyBasedIntangibleAssetsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_a2b54d45-b4bc-41e1-938f-bca1d821df0c" xlink:to="loc_us-gaap_TechnologyBasedIntangibleAssetsMember_640e4b1a-a251-43a4-b1bf-252cba02f7dc" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_08db1676-e831-4679-b9f5-de44f79d3a9d" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfIndefiniteLivedIntangibleAssetsTable_9df358c7-07b0-4595-b9eb-3bb3dd44a77c" xlink:to="loc_srt_RangeAxis_08db1676-e831-4679-b9f5-de44f79d3a9d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_08db1676-e831-4679-b9f5-de44f79d3a9d_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_08db1676-e831-4679-b9f5-de44f79d3a9d" xlink:to="loc_srt_RangeMember_08db1676-e831-4679-b9f5-de44f79d3a9d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_098a27f3-701a-4f9a-9f05-925efd6ed0d7" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_08db1676-e831-4679-b9f5-de44f79d3a9d" xlink:to="loc_srt_RangeMember_098a27f3-701a-4f9a-9f05-925efd6ed0d7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_73a59527-43c1-47d6-8764-8297e6f711ef" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_098a27f3-701a-4f9a-9f05-925efd6ed0d7" xlink:to="loc_srt_MinimumMember_73a59527-43c1-47d6-8764-8297e6f711ef" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_32d2d5ca-c2da-4863-8434-52bfccb6909d" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_098a27f3-701a-4f9a-9f05-925efd6ed0d7" xlink:to="loc_srt_MaximumMember_32d2d5ca-c2da-4863-8434-52bfccb6909d" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/BusinesscombinationsNarrativePRAAcquisitionDetails" xlink:type="simple" xlink:href="iclr-20211231.xsd#BusinesscombinationsNarrativePRAAcquisitionDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/BusinesscombinationsNarrativePRAAcquisitionDetails" xlink:type="extended" id="ie0cc94662195415cbd2574fd6e34d38d_BusinesscombinationsNarrativePRAAcquisitionDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_13a4a12e-7791-45b6-a73f-130536550a97" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityNumberOfEmployees_228e3629-0fc2-46f1-837d-4be5ec260179" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_EntityNumberOfEmployees"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_13a4a12e-7791-45b6-a73f-130536550a97" xlink:to="loc_dei_EntityNumberOfEmployees_228e3629-0fc2-46f1-837d-4be5ec260179" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BusinessCombinationConsiderationTransferredCashPaymentToShareholdersPerShareHeld_67dd07aa-59cf-4121-ae7e-338a2bcfe2a3" xlink:href="iclr-20211231.xsd#iclr_BusinessCombinationConsiderationTransferredCashPaymentToShareholdersPerShareHeld"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_13a4a12e-7791-45b6-a73f-130536550a97" xlink:to="loc_iclr_BusinessCombinationConsiderationTransferredCashPaymentToShareholdersPerShareHeld_67dd07aa-59cf-4121-ae7e-338a2bcfe2a3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BusinessCombinationConsiderationTransferredSharesIssuedToShareholdersPerShareHeld_5ee1702e-64f2-4fe4-b215-d9572f44a952" xlink:href="iclr-20211231.xsd#iclr_BusinessCombinationConsiderationTransferredSharesIssuedToShareholdersPerShareHeld"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_13a4a12e-7791-45b6-a73f-130536550a97" xlink:to="loc_iclr_BusinessCombinationConsiderationTransferredSharesIssuedToShareholdersPerShareHeld_5ee1702e-64f2-4fe4-b215-d9572f44a952" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationIntegrationRelatedCosts_fe40abe2-4cca-45ba-b45f-ef996dc1584c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationIntegrationRelatedCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_13a4a12e-7791-45b6-a73f-130536550a97" xlink:to="loc_us-gaap_BusinessCombinationIntegrationRelatedCosts_fe40abe2-4cca-45ba-b45f-ef996dc1584c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationAcquisitionRelatedCosts_29cd9144-6905-4da0-8f4f-1d02d25665db" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationAcquisitionRelatedCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_13a4a12e-7791-45b6-a73f-130536550a97" xlink:to="loc_us-gaap_BusinessCombinationAcquisitionRelatedCosts_29cd9144-6905-4da0-8f4f-1d02d25665db" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpense_9c27ef57-a58d-4070-a12d-f7893525f7f3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_InterestExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_13a4a12e-7791-45b6-a73f-130536550a97" xlink:to="loc_us-gaap_InterestExpense_9c27ef57-a58d-4070-a12d-f7893525f7f3" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredFinanceCostsGross_accf99b4-6c52-4697-be68-32b5ca86b486" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DeferredFinanceCostsGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_13a4a12e-7791-45b6-a73f-130536550a97" xlink:to="loc_us-gaap_DeferredFinanceCostsGross_accf99b4-6c52-4697-be68-32b5ca86b486" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationProFormaInformationRevenueOfAcquireeSinceAcquisitionDateActual_5b7af423-f0c3-49d2-8832-b589e1588265" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationProFormaInformationRevenueOfAcquireeSinceAcquisitionDateActual"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_13a4a12e-7791-45b6-a73f-130536550a97" xlink:to="loc_us-gaap_BusinessCombinationProFormaInformationRevenueOfAcquireeSinceAcquisitionDateActual_5b7af423-f0c3-49d2-8832-b589e1588265" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BusinessCombinationProFormaInformationEarningsOrLossBeforeTaxOfAcquireeSinceAcquisitionDateActual_a909130f-62fc-4bb9-9ecf-c53505a1db4c" xlink:href="iclr-20211231.xsd#iclr_BusinessCombinationProFormaInformationEarningsOrLossBeforeTaxOfAcquireeSinceAcquisitionDateActual"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_13a4a12e-7791-45b6-a73f-130536550a97" xlink:to="loc_iclr_BusinessCombinationProFormaInformationEarningsOrLossBeforeTaxOfAcquireeSinceAcquisitionDateActual_a909130f-62fc-4bb9-9ecf-c53505a1db4c" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_TransactionAndOneTimeFinancingCosts_2bc53d40-832b-414b-bb40-f7f1f79d3b79" xlink:href="iclr-20211231.xsd#iclr_TransactionAndOneTimeFinancingCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_13a4a12e-7791-45b6-a73f-130536550a97" xlink:to="loc_iclr_TransactionAndOneTimeFinancingCosts_2bc53d40-832b-414b-bb40-f7f1f79d3b79" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_908c1006-f6ef-4a28-903d-dc45dc7726dd" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_13a4a12e-7791-45b6-a73f-130536550a97" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_908c1006-f6ef-4a28-903d-dc45dc7726dd" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_0f0cfa75-23b6-4846-a842-e1809e3da56c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_908c1006-f6ef-4a28-903d-dc45dc7726dd" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_0f0cfa75-23b6-4846-a842-e1809e3da56c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_0f0cfa75-23b6-4846-a842-e1809e3da56c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_0f0cfa75-23b6-4846-a842-e1809e3da56c" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_0f0cfa75-23b6-4846-a842-e1809e3da56c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_b4afdf00-0551-4c89-9607-f8b3cb50c187" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_0f0cfa75-23b6-4846-a842-e1809e3da56c" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_b4afdf00-0551-4c89-9607-f8b3cb50c187" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_PRAHealthSciencesIncMember_cddaa2a8-3563-49a8-a269-708eb99b0493" xlink:href="iclr-20211231.xsd#iclr_PRAHealthSciencesIncMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_b4afdf00-0551-4c89-9607-f8b3cb50c187" xlink:to="loc_iclr_PRAHealthSciencesIncMember_cddaa2a8-3563-49a8-a269-708eb99b0493" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/BusinesscombinationsFairValueofConsiderationTransferredPRAAcquisitionDetails" xlink:type="simple" xlink:href="iclr-20211231.xsd#BusinesscombinationsFairValueofConsiderationTransferredPRAAcquisitionDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/BusinesscombinationsFairValueofConsiderationTransferredPRAAcquisitionDetails" xlink:type="extended" id="i5fcc89f6785545f2832791fb3305d596_BusinesscombinationsFairValueofConsiderationTransferredPRAAcquisitionDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_198f527d-52b2-4abe-8575-68bf23f46c9d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquireBusinessesGross_9eac9072-29a0-45a4-aad8-61c58e80138d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PaymentsToAcquireBusinessesGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_198f527d-52b2-4abe-8575-68bf23f46c9d" xlink:to="loc_us-gaap_PaymentsToAcquireBusinessesGross_9eac9072-29a0-45a4-aad8-61c58e80138d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationConsiderationTransferredEquityInterestsIssuedAndIssuable_acdcfa18-3998-4fa3-be68-9373dc5b7388" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationConsiderationTransferredEquityInterestsIssuedAndIssuable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_198f527d-52b2-4abe-8575-68bf23f46c9d" xlink:to="loc_us-gaap_BusinessCombinationConsiderationTransferredEquityInterestsIssuedAndIssuable_acdcfa18-3998-4fa3-be68-9373dc5b7388" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationConsiderationTransferredOther1_afd93d58-9c32-4f84-a9f4-bc98a2e52906" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationConsiderationTransferredOther1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_198f527d-52b2-4abe-8575-68bf23f46c9d" xlink:to="loc_us-gaap_BusinessCombinationConsiderationTransferredOther1_afd93d58-9c32-4f84-a9f4-bc98a2e52906" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationConsiderationTransferredLiabilitiesIncurred_01b39f0d-6204-4d4a-83a4-b470a50eb1ae" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationConsiderationTransferredLiabilitiesIncurred"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_198f527d-52b2-4abe-8575-68bf23f46c9d" xlink:to="loc_us-gaap_BusinessCombinationConsiderationTransferredLiabilitiesIncurred_01b39f0d-6204-4d4a-83a4-b470a50eb1ae" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationConsiderationTransferred1_3d6a16ce-867d-42ae-b263-571c65a6dc89" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationConsiderationTransferred1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_198f527d-52b2-4abe-8575-68bf23f46c9d" xlink:to="loc_us-gaap_BusinessCombinationConsiderationTransferred1_3d6a16ce-867d-42ae-b263-571c65a6dc89" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BusinessCombinationConsiderationTransferredPaymentsToSettleDebtObligationsThatExistedPriorToAcquisitionDate_f34575c0-9ecf-4c4b-8e31-3fb40aa0f91c" xlink:href="iclr-20211231.xsd#iclr_BusinessCombinationConsiderationTransferredPaymentsToSettleDebtObligationsThatExistedPriorToAcquisitionDate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_198f527d-52b2-4abe-8575-68bf23f46c9d" xlink:to="loc_iclr_BusinessCombinationConsiderationTransferredPaymentsToSettleDebtObligationsThatExistedPriorToAcquisitionDate_f34575c0-9ecf-4c4b-8e31-3fb40aa0f91c" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_93c62576-1a0b-445f-b64a-3a7c49916bb2" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_198f527d-52b2-4abe-8575-68bf23f46c9d" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_93c62576-1a0b-445f-b64a-3a7c49916bb2" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_be0cd952-cfb6-4a4c-9fd5-74d5935d637f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_93c62576-1a0b-445f-b64a-3a7c49916bb2" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_be0cd952-cfb6-4a4c-9fd5-74d5935d637f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_be0cd952-cfb6-4a4c-9fd5-74d5935d637f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_be0cd952-cfb6-4a4c-9fd5-74d5935d637f" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_be0cd952-cfb6-4a4c-9fd5-74d5935d637f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_1fb3c026-a30b-44ab-bd58-5c20a2def684" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_be0cd952-cfb6-4a4c-9fd5-74d5935d637f" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_1fb3c026-a30b-44ab-bd58-5c20a2def684" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_PRAHealthSciencesIncMember_25518024-4f60-4f09-84b5-bc8cb509b48c" xlink:href="iclr-20211231.xsd#iclr_PRAHealthSciencesIncMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_1fb3c026-a30b-44ab-bd58-5c20a2def684" xlink:to="loc_iclr_PRAHealthSciencesIncMember_25518024-4f60-4f09-84b5-bc8cb509b48c" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/BusinesscombinationsScheduleofAcquisitionsPRAAcquisitionDetails" xlink:type="simple" xlink:href="iclr-20211231.xsd#BusinesscombinationsScheduleofAcquisitionsPRAAcquisitionDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/BusinesscombinationsScheduleofAcquisitionsPRAAcquisitionDetails" xlink:type="extended" id="ia289d219900045499ff38bc0dd8b8bdf_BusinesscombinationsScheduleofAcquisitionsPRAAcquisitionDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_29862e60-7764-41d2-9d27-f392c80b9a3f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents_ba16ee24-1bfe-4778-8180-6cf5f8f1d18e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_29862e60-7764-41d2-9d27-f392c80b9a3f" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents_ba16ee24-1bfe-4778-8180-6cf5f8f1d18e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsReceivables_465491fe-9aa5-4a14-be28-34aed3d81d14" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsReceivables"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_29862e60-7764-41d2-9d27-f392c80b9a3f" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsReceivables_465491fe-9aa5-4a14-be28-34aed3d81d14" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsOther_eee07fb4-75f7-4bb9-9f4b-fddfd3c8f813" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsOther"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_29862e60-7764-41d2-9d27-f392c80b9a3f" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsOther_eee07fb4-75f7-4bb9-9f4b-fddfd3c8f813" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment_e897bdd6-5791-4359-9bb4-1192b939dac3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_29862e60-7764-41d2-9d27-f392c80b9a3f" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment_e897bdd6-5791-4359-9bb4-1192b939dac3" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOperatingLeaseRightOfUseAsset_ea10577b-150b-497e-8c06-868caad5d5cd" xlink:href="iclr-20211231.xsd#iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOperatingLeaseRightOfUseAsset"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_29862e60-7764-41d2-9d27-f392c80b9a3f" xlink:to="loc_iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOperatingLeaseRightOfUseAsset_ea10577b-150b-497e-8c06-868caad5d5cd" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_c069c62c-6760-46d9-a992-91b98b9b2d66" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_Goodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_29862e60-7764-41d2-9d27-f392c80b9a3f" xlink:to="loc_us-gaap_Goodwill_c069c62c-6760-46d9-a992-91b98b9b2d66" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles_2e55bc01-54ac-42bc-aa63-ff0b5dcf6ef0" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_29862e60-7764-41d2-9d27-f392c80b9a3f" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles_2e55bc01-54ac-42bc-aa63-ff0b5dcf6ef0" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxAssets_e41c0c5c-e973-4bc1-af2c-375af99619b3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_29862e60-7764-41d2-9d27-f392c80b9a3f" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxAssets_e41c0c5c-e973-4bc1-af2c-375af99619b3" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOtherNoncurrentAssets_f40f01e2-b8f9-4da2-ab20-7787811193f6" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOtherNoncurrentAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_29862e60-7764-41d2-9d27-f392c80b9a3f" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOtherNoncurrentAssets_f40f01e2-b8f9-4da2-ab20-7787811193f6" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesAccountsPayable_a5731e9b-c502-42a3-88f5-020a6bf3d180" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesAccountsPayable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_29862e60-7764-41d2-9d27-f392c80b9a3f" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesAccountsPayable_a5731e9b-c502-42a3-88f5-020a6bf3d180" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesOther_8fd86fc7-d092-4863-adb1-d581653533a3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesOther"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_29862e60-7764-41d2-9d27-f392c80b9a3f" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesOther_8fd86fc7-d092-4863-adb1-d581653533a3" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedCurrentOperatingLeaseLiability_8e90529b-1480-4465-884c-ef833daffb1b" xlink:href="iclr-20211231.xsd#iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedCurrentOperatingLeaseLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_29862e60-7764-41d2-9d27-f392c80b9a3f" xlink:to="loc_iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedCurrentOperatingLeaseLiability_8e90529b-1480-4465-884c-ef833daffb1b" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedContractWithCustomerLiabilityCurrent_8f11d881-4aa5-4103-adba-3264e0f962c5" xlink:href="iclr-20211231.xsd#iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedContractWithCustomerLiabilityCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_29862e60-7764-41d2-9d27-f392c80b9a3f" xlink:to="loc_iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedContractWithCustomerLiabilityCurrent_8f11d881-4aa5-4103-adba-3264e0f962c5" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedNoncurrentOperatingLeaseLiability_a14528c7-dca7-49dc-9580-0fd0280e4119" xlink:href="iclr-20211231.xsd#iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedNoncurrentOperatingLeaseLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_29862e60-7764-41d2-9d27-f392c80b9a3f" xlink:to="loc_iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedNoncurrentOperatingLeaseLiability_a14528c7-dca7-49dc-9580-0fd0280e4119" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxLiabilities_0444f6f6-19be-4699-9a9d-52636ff2afc0" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_29862e60-7764-41d2-9d27-f392c80b9a3f" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxLiabilities_0444f6f6-19be-4699-9a9d-52636ff2afc0" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilitiesOther_096e183d-9c2b-4db2-921b-2a21cc79320c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilitiesOther"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_29862e60-7764-41d2-9d27-f392c80b9a3f" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilitiesOther_096e183d-9c2b-4db2-921b-2a21cc79320c" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet_bbdba907-7ce4-4621-b0be-14a28ffcd777" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_29862e60-7764-41d2-9d27-f392c80b9a3f" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet_bbdba907-7ce4-4621-b0be-14a28ffcd777" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionPurchasePriceAllocationGoodwillExpectedTaxDeductibleAmount_2bbac4e6-a041-4497-8274-d395d967f016" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionPurchasePriceAllocationGoodwillExpectedTaxDeductibleAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_29862e60-7764-41d2-9d27-f392c80b9a3f" xlink:to="loc_us-gaap_BusinessAcquisitionPurchasePriceAllocationGoodwillExpectedTaxDeductibleAmount_2bbac4e6-a041-4497-8274-d395d967f016" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_88631924-f421-4a49-9712-6e8a7c438c08" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_29862e60-7764-41d2-9d27-f392c80b9a3f" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_88631924-f421-4a49-9712-6e8a7c438c08" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_2262b55c-c76b-4471-8884-47c91b0725c8" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_88631924-f421-4a49-9712-6e8a7c438c08" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_2262b55c-c76b-4471-8884-47c91b0725c8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_2262b55c-c76b-4471-8884-47c91b0725c8_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_2262b55c-c76b-4471-8884-47c91b0725c8" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_2262b55c-c76b-4471-8884-47c91b0725c8_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_a4601519-6a10-4cc9-9ccc-517c9cc24e7c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_2262b55c-c76b-4471-8884-47c91b0725c8" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_a4601519-6a10-4cc9-9ccc-517c9cc24e7c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_PRAHealthSciencesIncMember_4bbfe7f6-ea02-4e91-9223-53bdfe7960b0" xlink:href="iclr-20211231.xsd#iclr_PRAHealthSciencesIncMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_a4601519-6a10-4cc9-9ccc-517c9cc24e7c" xlink:to="loc_iclr_PRAHealthSciencesIncMember_4bbfe7f6-ea02-4e91-9223-53bdfe7960b0" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/BusinesscombinationsFairValueofIdentifiedIntangibleAssetsAcquiredPRAAcquisitionDetails" xlink:type="simple" xlink:href="iclr-20211231.xsd#BusinesscombinationsFairValueofIdentifiedIntangibleAssetsAcquiredPRAAcquisitionDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/BusinesscombinationsFairValueofIdentifiedIntangibleAssetsAcquiredPRAAcquisitionDetails" xlink:type="extended" id="if8c7515bf74a457792d6d1d82abffd86_BusinesscombinationsFairValueofIdentifiedIntangibleAssetsAcquiredPRAAcquisitionDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_d36b3910-bb01-4e1c-b8d3-dae45f31d099" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles_41cf105a-a7a8-4613-bb29-39b33c469f81" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_d36b3910-bb01-4e1c-b8d3-dae45f31d099" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles_41cf105a-a7a8-4613-bb29-39b33c469f81" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife_0be27567-7640-4cba-af51-9faeb6241817" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_d36b3910-bb01-4e1c-b8d3-dae45f31d099" xlink:to="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife_0be27567-7640-4cba-af51-9faeb6241817" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_a7544d6b-3735-40c3-a8a1-c9e62fc90906" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_d36b3910-bb01-4e1c-b8d3-dae45f31d099" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_a7544d6b-3735-40c3-a8a1-c9e62fc90906" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_bd9297a5-23a6-4859-b7b6-9634292017a4" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_a7544d6b-3735-40c3-a8a1-c9e62fc90906" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_bd9297a5-23a6-4859-b7b6-9634292017a4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_bd9297a5-23a6-4859-b7b6-9634292017a4_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_bd9297a5-23a6-4859-b7b6-9634292017a4" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_bd9297a5-23a6-4859-b7b6-9634292017a4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_ed802be7-3b04-4df5-ae51-4aa166cd8344" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_bd9297a5-23a6-4859-b7b6-9634292017a4" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_ed802be7-3b04-4df5-ae51-4aa166cd8344" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_PRAHealthSciencesIncMember_a2f4ad46-20b9-42d8-8306-9d0106a1561e" xlink:href="iclr-20211231.xsd#iclr_PRAHealthSciencesIncMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_ed802be7-3b04-4df5-ae51-4aa166cd8344" xlink:to="loc_iclr_PRAHealthSciencesIncMember_a2f4ad46-20b9-42d8-8306-9d0106a1561e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_7badaa7a-0cc2-4183-b4dd-cf1a1ab02eeb" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_a7544d6b-3735-40c3-a8a1-c9e62fc90906" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_7badaa7a-0cc2-4183-b4dd-cf1a1ab02eeb" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_7badaa7a-0cc2-4183-b4dd-cf1a1ab02eeb_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_7badaa7a-0cc2-4183-b4dd-cf1a1ab02eeb" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_7badaa7a-0cc2-4183-b4dd-cf1a1ab02eeb_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_318aa6f2-ad04-4b91-be55-ad74d61a0679" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_7badaa7a-0cc2-4183-b4dd-cf1a1ab02eeb" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_318aa6f2-ad04-4b91-be55-ad74d61a0679" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerRelationshipsMember_ee7b4f7c-98ba-4e14-ba9b-64498e8aa77b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CustomerRelationshipsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_318aa6f2-ad04-4b91-be55-ad74d61a0679" xlink:to="loc_us-gaap_CustomerRelationshipsMember_ee7b4f7c-98ba-4e14-ba9b-64498e8aa77b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OrderOrProductionBacklogMember_3fb6af56-4436-4708-a735-ea0a2596c580" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OrderOrProductionBacklogMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_318aa6f2-ad04-4b91-be55-ad74d61a0679" xlink:to="loc_us-gaap_OrderOrProductionBacklogMember_3fb6af56-4436-4708-a735-ea0a2596c580" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TradeNamesMember_92118354-468b-47d5-8c15-1a4b5e4fcb9b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TradeNamesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_318aa6f2-ad04-4b91-be55-ad74d61a0679" xlink:to="loc_us-gaap_TradeNamesMember_92118354-468b-47d5-8c15-1a4b5e4fcb9b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DatabasesMember_3e8ff900-ffaa-4b97-bbe7-07d4fd2d5320" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DatabasesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_318aa6f2-ad04-4b91-be55-ad74d61a0679" xlink:to="loc_us-gaap_DatabasesMember_3e8ff900-ffaa-4b97-bbe7-07d4fd2d5320" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TechnologyBasedIntangibleAssetsMember_9141322c-f935-4990-8558-ad4fce7c5fdc" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TechnologyBasedIntangibleAssetsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_318aa6f2-ad04-4b91-be55-ad74d61a0679" xlink:to="loc_us-gaap_TechnologyBasedIntangibleAssetsMember_9141322c-f935-4990-8558-ad4fce7c5fdc" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/BusinesscombinationsScheduleofProFormaInformationPRAAcquisitionDetails" xlink:type="simple" xlink:href="iclr-20211231.xsd#BusinesscombinationsScheduleofProFormaInformationPRAAcquisitionDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/BusinesscombinationsScheduleofProFormaInformationPRAAcquisitionDetails" xlink:type="extended" id="i7621a63fe1e543348bb46a5a40e24bac_BusinesscombinationsScheduleofProFormaInformationPRAAcquisitionDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_80c18041-4905-4784-870b-44b713e61395" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionsProFormaRevenue_b78f9343-fa3a-46c8-a0ff-fa5742fa4aad" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionsProFormaRevenue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_80c18041-4905-4784-870b-44b713e61395" xlink:to="loc_us-gaap_BusinessAcquisitionsProFormaRevenue_b78f9343-fa3a-46c8-a0ff-fa5742fa4aad" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionsProFormaNetIncomeLoss_f7b58412-f4c1-427b-a096-ed258455711c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionsProFormaNetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_80c18041-4905-4784-870b-44b713e61395" xlink:to="loc_us-gaap_BusinessAcquisitionsProFormaNetIncomeLoss_f7b58412-f4c1-427b-a096-ed258455711c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_cb506d8b-cc56-4019-a32b-49dcab1db96e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_80c18041-4905-4784-870b-44b713e61395" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_cb506d8b-cc56-4019-a32b-49dcab1db96e" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_a1b986eb-08a1-41ef-aa0e-72fc7d7cb100" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_cb506d8b-cc56-4019-a32b-49dcab1db96e" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_a1b986eb-08a1-41ef-aa0e-72fc7d7cb100" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_a1b986eb-08a1-41ef-aa0e-72fc7d7cb100_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_a1b986eb-08a1-41ef-aa0e-72fc7d7cb100" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_a1b986eb-08a1-41ef-aa0e-72fc7d7cb100_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_6a882123-b3b2-45b2-9cf6-2995e465e50d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_a1b986eb-08a1-41ef-aa0e-72fc7d7cb100" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_6a882123-b3b2-45b2-9cf6-2995e465e50d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_PRAHealthSciencesIncMember_3f38cd46-5f2e-4342-80c7-9a3a76c75a2a" xlink:href="iclr-20211231.xsd#iclr_PRAHealthSciencesIncMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_6a882123-b3b2-45b2-9cf6-2995e465e50d" xlink:to="loc_iclr_PRAHealthSciencesIncMember_3f38cd46-5f2e-4342-80c7-9a3a76c75a2a" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/BusinesscombinationsNarrativeMedPassGroupAcquisitionDetails" xlink:type="simple" xlink:href="iclr-20211231.xsd#BusinesscombinationsNarrativeMedPassGroupAcquisitionDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/BusinesscombinationsNarrativeMedPassGroupAcquisitionDetails" xlink:type="extended" id="i13bedc9393fc40ab8729d84f0846f04b_BusinesscombinationsNarrativeMedPassGroupAcquisitionDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_9a75dfa0-91f9-40de-9a4c-4b1b566b1e12" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionPercentageOfVotingInterestsAcquired_268e4aaf-1f62-42a5-9e3c-3aa37cc531cf" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionPercentageOfVotingInterestsAcquired"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_9a75dfa0-91f9-40de-9a4c-4b1b566b1e12" xlink:to="loc_us-gaap_BusinessAcquisitionPercentageOfVotingInterestsAcquired_268e4aaf-1f62-42a5-9e3c-3aa37cc531cf" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentReceivables_2319eca6-5756-4c41-b192-796f25437652" xlink:href="iclr-20211231.xsd#iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentReceivables"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_9a75dfa0-91f9-40de-9a4c-4b1b566b1e12" xlink:to="loc_iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentReceivables_2319eca6-5756-4c41-b192-796f25437652" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentDeferredRevenue_698e41d5-f597-44da-ab87-63e9875bb073" xlink:href="iclr-20211231.xsd#iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentDeferredRevenue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_9a75dfa0-91f9-40de-9a4c-4b1b566b1e12" xlink:to="loc_iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentDeferredRevenue_698e41d5-f597-44da-ab87-63e9875bb073" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentOperatingRightToUseAssets_af7bf7aa-095f-4271-988a-499d603f1a06" xlink:href="iclr-20211231.xsd#iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentOperatingRightToUseAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_9a75dfa0-91f9-40de-9a4c-4b1b566b1e12" xlink:to="loc_iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentOperatingRightToUseAssets_af7bf7aa-095f-4271-988a-499d603f1a06" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentOtherLiabilities_4028c884-fc8b-4c79-b63f-06c5da7cdd8f" xlink:href="iclr-20211231.xsd#iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentOtherLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_9a75dfa0-91f9-40de-9a4c-4b1b566b1e12" xlink:to="loc_iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentOtherLiabilities_4028c884-fc8b-4c79-b63f-06c5da7cdd8f" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentCurrentLeaseLiabilities_dec487e3-4284-416d-b27c-b1eeddbd37b1" xlink:href="iclr-20211231.xsd#iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentCurrentLeaseLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_9a75dfa0-91f9-40de-9a4c-4b1b566b1e12" xlink:to="loc_iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentCurrentLeaseLiabilities_dec487e3-4284-416d-b27c-b1eeddbd37b1" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentNonCurrentLeaseLiabilities_24514bdb-d030-4a7d-9ef6-0d84d84e43bf" xlink:href="iclr-20211231.xsd#iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentNonCurrentLeaseLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_9a75dfa0-91f9-40de-9a4c-4b1b566b1e12" xlink:to="loc_iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentNonCurrentLeaseLiabilities_24514bdb-d030-4a7d-9ef6-0d84d84e43bf" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentNonCurrentDeferredTaxLiability_3b7175c0-4387-4d65-91a2-9c0aa20d1ffb" xlink:href="iclr-20211231.xsd#iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentNonCurrentDeferredTaxLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_9a75dfa0-91f9-40de-9a4c-4b1b566b1e12" xlink:to="loc_iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentNonCurrentDeferredTaxLiability_3b7175c0-4387-4d65-91a2-9c0aa20d1ffb" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationProFormaInformationRevenueOfAcquireeSinceAcquisitionDateActual_d0d2d47a-1682-4105-8628-3f466a0b90a8" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationProFormaInformationRevenueOfAcquireeSinceAcquisitionDateActual"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_9a75dfa0-91f9-40de-9a4c-4b1b566b1e12" xlink:to="loc_us-gaap_BusinessCombinationProFormaInformationRevenueOfAcquireeSinceAcquisitionDateActual_d0d2d47a-1682-4105-8628-3f466a0b90a8" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationProFormaInformationEarningsOrLossOfAcquireeSinceAcquisitionDateActual_10303fb6-0b8e-433c-8130-73abdc981081" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationProFormaInformationEarningsOrLossOfAcquireeSinceAcquisitionDateActual"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_9a75dfa0-91f9-40de-9a4c-4b1b566b1e12" xlink:to="loc_us-gaap_BusinessCombinationProFormaInformationEarningsOrLossOfAcquireeSinceAcquisitionDateActual_10303fb6-0b8e-433c-8130-73abdc981081" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_c97d96de-b45b-464e-ad4b-e02ba2fc4203" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_9a75dfa0-91f9-40de-9a4c-4b1b566b1e12" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_c97d96de-b45b-464e-ad4b-e02ba2fc4203" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_547151cf-60a2-462d-b4d9-716a7d025dd0" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_c97d96de-b45b-464e-ad4b-e02ba2fc4203" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_547151cf-60a2-462d-b4d9-716a7d025dd0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_547151cf-60a2-462d-b4d9-716a7d025dd0_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_547151cf-60a2-462d-b4d9-716a7d025dd0" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_547151cf-60a2-462d-b4d9-716a7d025dd0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_6ae8991b-4acf-45bd-8c7e-1d11406706ae" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_547151cf-60a2-462d-b4d9-716a7d025dd0" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_6ae8991b-4acf-45bd-8c7e-1d11406706ae" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_MedpassGroupLimitedMember_cf959184-276e-41a8-af9d-bc66cba9ec1a" xlink:href="iclr-20211231.xsd#iclr_MedpassGroupLimitedMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_6ae8991b-4acf-45bd-8c7e-1d11406706ae" xlink:to="loc_iclr_MedpassGroupLimitedMember_cf959184-276e-41a8-af9d-bc66cba9ec1a" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/BusinesscombinationsScheduleofAcquisitionsMedPassGroupAcquisitionDetails" xlink:type="simple" xlink:href="iclr-20211231.xsd#BusinesscombinationsScheduleofAcquisitionsMedPassGroupAcquisitionDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/BusinesscombinationsScheduleofAcquisitionsMedPassGroupAcquisitionDetails" xlink:type="extended" id="i6a8d62ddf13c4bbc95a95412be7a7949_BusinesscombinationsScheduleofAcquisitionsMedPassGroupAcquisitionDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_b21f6406-a990-4982-9ca2-a2b1ddddf123" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents_b502e4d9-3069-4fde-84e7-7f7f46e9a5e5" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_b21f6406-a990-4982-9ca2-a2b1ddddf123" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents_b502e4d9-3069-4fde-84e7-7f7f46e9a5e5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment_61b46693-e1d9-43d8-8345-55d9816c113b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_b21f6406-a990-4982-9ca2-a2b1ddddf123" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment_61b46693-e1d9-43d8-8345-55d9816c113b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOperatingLeaseRightOfUseAsset_66737245-9870-483a-9f0d-fd438eec2a50" xlink:href="iclr-20211231.xsd#iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOperatingLeaseRightOfUseAsset"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_b21f6406-a990-4982-9ca2-a2b1ddddf123" xlink:to="loc_iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOperatingLeaseRightOfUseAsset_66737245-9870-483a-9f0d-fd438eec2a50" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_d849c45e-032c-4e8b-8a97-64671b30e3f0" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_Goodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_b21f6406-a990-4982-9ca2-a2b1ddddf123" xlink:to="loc_us-gaap_Goodwill_d849c45e-032c-4e8b-8a97-64671b30e3f0" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles_1a676dae-4579-480c-ad98-5f236df2dc96" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_b21f6406-a990-4982-9ca2-a2b1ddddf123" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles_1a676dae-4579-480c-ad98-5f236df2dc96" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsReceivables_e703a54d-b644-485a-a42b-cca601a066ed" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsReceivables"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_b21f6406-a990-4982-9ca2-a2b1ddddf123" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsReceivables_e703a54d-b644-485a-a42b-cca601a066ed" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsPrepaidExpenseAndOtherAssets_329c77e2-e6b1-4640-8b0b-bdcbbaeb764a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsPrepaidExpenseAndOtherAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_b21f6406-a990-4982-9ca2-a2b1ddddf123" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsPrepaidExpenseAndOtherAssets_329c77e2-e6b1-4640-8b0b-bdcbbaeb764a" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesAccountsPayable_71e64a53-070f-46c8-a89a-bb8e44e0a55e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesAccountsPayable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_b21f6406-a990-4982-9ca2-a2b1ddddf123" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesAccountsPayable_71e64a53-070f-46c8-a89a-bb8e44e0a55e" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedContractWithCustomerLiabilityCurrent_ccb72c73-fc73-4863-be93-3240825824b9" xlink:href="iclr-20211231.xsd#iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedContractWithCustomerLiabilityCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_b21f6406-a990-4982-9ca2-a2b1ddddf123" xlink:to="loc_iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedContractWithCustomerLiabilityCurrent_ccb72c73-fc73-4863-be93-3240825824b9" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesOther_b370e26d-be19-4258-a361-4419140d6eac" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesOther"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_b21f6406-a990-4982-9ca2-a2b1ddddf123" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesOther_b370e26d-be19-4258-a361-4419140d6eac" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedCurrentOperatingLeaseLiability_26b77698-362c-4e55-be2d-c2b1216a1d16" xlink:href="iclr-20211231.xsd#iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedCurrentOperatingLeaseLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_b21f6406-a990-4982-9ca2-a2b1ddddf123" xlink:to="loc_iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedCurrentOperatingLeaseLiability_26b77698-362c-4e55-be2d-c2b1216a1d16" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedNoncurrentOperatingLeaseLiability_7229512b-73b6-48c3-9d32-dca0aafe54d3" xlink:href="iclr-20211231.xsd#iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedNoncurrentOperatingLeaseLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_b21f6406-a990-4982-9ca2-a2b1ddddf123" xlink:to="loc_iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedNoncurrentOperatingLeaseLiability_7229512b-73b6-48c3-9d32-dca0aafe54d3" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxLiabilities_4e8c59a5-579d-4be9-b35c-37b8e3a614ca" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_b21f6406-a990-4982-9ca2-a2b1ddddf123" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxLiabilities_4e8c59a5-579d-4be9-b35c-37b8e3a614ca" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet_9641a136-0c6c-4f83-9530-fe9044c290de" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_b21f6406-a990-4982-9ca2-a2b1ddddf123" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet_9641a136-0c6c-4f83-9530-fe9044c290de" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquireBusinessesGross_60e1234f-5102-47f6-97df-e91b931223bc" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PaymentsToAcquireBusinessesGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_b21f6406-a990-4982-9ca2-a2b1ddddf123" xlink:to="loc_us-gaap_PaymentsToAcquireBusinessesGross_60e1234f-5102-47f6-97df-e91b931223bc" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BusinessCombinationConsiderationTransferredWorkingCapitalAdjustments_8964f283-eb3b-4c0d-a10a-421ce0a617f7" xlink:href="iclr-20211231.xsd#iclr_BusinessCombinationConsiderationTransferredWorkingCapitalAdjustments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_b21f6406-a990-4982-9ca2-a2b1ddddf123" xlink:to="loc_iclr_BusinessCombinationConsiderationTransferredWorkingCapitalAdjustments_8964f283-eb3b-4c0d-a10a-421ce0a617f7" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationContingentConsiderationLiability_87e9e5ba-c4d6-4344-9b16-526f9f80f4da" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationContingentConsiderationLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_b21f6406-a990-4982-9ca2-a2b1ddddf123" xlink:to="loc_us-gaap_BusinessCombinationContingentConsiderationLiability_87e9e5ba-c4d6-4344-9b16-526f9f80f4da" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationConsiderationTransferred1_06b98fce-77f6-4b06-844b-55791c2dd113" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationConsiderationTransferred1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_b21f6406-a990-4982-9ca2-a2b1ddddf123" xlink:to="loc_us-gaap_BusinessCombinationConsiderationTransferred1_06b98fce-77f6-4b06-844b-55791c2dd113" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionPurchasePriceAllocationGoodwillExpectedTaxDeductibleAmount_109709b2-c49d-4a69-affd-79c00ba3d496" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionPurchasePriceAllocationGoodwillExpectedTaxDeductibleAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_b21f6406-a990-4982-9ca2-a2b1ddddf123" xlink:to="loc_us-gaap_BusinessAcquisitionPurchasePriceAllocationGoodwillExpectedTaxDeductibleAmount_109709b2-c49d-4a69-affd-79c00ba3d496" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationContingentConsiderationArrangementsRangeOfOutcomesValueHigh_c5905b95-b0f3-4f73-825f-c2d723d8d6bd" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationContingentConsiderationArrangementsRangeOfOutcomesValueHigh"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_b21f6406-a990-4982-9ca2-a2b1ddddf123" xlink:to="loc_us-gaap_BusinessCombinationContingentConsiderationArrangementsRangeOfOutcomesValueHigh_c5905b95-b0f3-4f73-825f-c2d723d8d6bd" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_2d857a5e-1732-4b94-92ff-73291d260a2f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_b21f6406-a990-4982-9ca2-a2b1ddddf123" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_2d857a5e-1732-4b94-92ff-73291d260a2f" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_14d8e4ee-5f19-4710-a7aa-34e8839afae7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_2d857a5e-1732-4b94-92ff-73291d260a2f" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_14d8e4ee-5f19-4710-a7aa-34e8839afae7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_14d8e4ee-5f19-4710-a7aa-34e8839afae7_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_14d8e4ee-5f19-4710-a7aa-34e8839afae7" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_14d8e4ee-5f19-4710-a7aa-34e8839afae7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_efdc8568-79c8-40a7-aef6-93f20d04409b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_14d8e4ee-5f19-4710-a7aa-34e8839afae7" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_efdc8568-79c8-40a7-aef6-93f20d04409b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_MedpassGroupLimitedMember_8dd3680d-0cc8-45e5-838a-e11c602b9094" xlink:href="iclr-20211231.xsd#iclr_MedpassGroupLimitedMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_efdc8568-79c8-40a7-aef6-93f20d04409b" xlink:to="loc_iclr_MedpassGroupLimitedMember_8dd3680d-0cc8-45e5-838a-e11c602b9094" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_b556b770-9336-4600-9180-77c117924a7a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_2d857a5e-1732-4b94-92ff-73291d260a2f" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_b556b770-9336-4600-9180-77c117924a7a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_b556b770-9336-4600-9180-77c117924a7a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_b556b770-9336-4600-9180-77c117924a7a" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_b556b770-9336-4600-9180-77c117924a7a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_38588fd8-821a-4ac0-853b-43236425c77b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_b556b770-9336-4600-9180-77c117924a7a" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_38588fd8-821a-4ac0-853b-43236425c77b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerRelationshipsMember_83703873-071e-4fb2-92dc-07827a4ecb4e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CustomerRelationshipsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_38588fd8-821a-4ac0-853b-43236425c77b" xlink:to="loc_us-gaap_CustomerRelationshipsMember_83703873-071e-4fb2-92dc-07827a4ecb4e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OrderOrProductionBacklogMember_2f609f2f-0041-4fc9-884a-ab0b6bdff850" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OrderOrProductionBacklogMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_38588fd8-821a-4ac0-853b-43236425c77b" xlink:to="loc_us-gaap_OrderOrProductionBacklogMember_2f609f2f-0041-4fc9-884a-ab0b6bdff850" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/BusinesscombinationsScheduleofProFormaInformationMedPassGroupAcquisitionDetails" xlink:type="simple" xlink:href="iclr-20211231.xsd#BusinesscombinationsScheduleofProFormaInformationMedPassGroupAcquisitionDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/BusinesscombinationsScheduleofProFormaInformationMedPassGroupAcquisitionDetails" xlink:type="extended" id="i7b89bbf766344e028e9eaf5a40c4f131_BusinesscombinationsScheduleofProFormaInformationMedPassGroupAcquisitionDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_b3c4eb45-87a4-4513-ba41-bd9266a08049" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionsProFormaRevenue_bb9ca3a2-157d-48d2-849a-c0ad9c517595" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionsProFormaRevenue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_b3c4eb45-87a4-4513-ba41-bd9266a08049" xlink:to="loc_us-gaap_BusinessAcquisitionsProFormaRevenue_bb9ca3a2-157d-48d2-849a-c0ad9c517595" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionsProFormaNetIncomeLoss_56855dd8-fe6d-4da4-ac64-eb9c05f3d494" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionsProFormaNetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_b3c4eb45-87a4-4513-ba41-bd9266a08049" xlink:to="loc_us-gaap_BusinessAcquisitionsProFormaNetIncomeLoss_56855dd8-fe6d-4da4-ac64-eb9c05f3d494" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_563cde77-1309-48db-b81e-23e2deef8b93" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_b3c4eb45-87a4-4513-ba41-bd9266a08049" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_563cde77-1309-48db-b81e-23e2deef8b93" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_867d3d3c-026c-4eea-a334-e16f8670e884" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_563cde77-1309-48db-b81e-23e2deef8b93" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_867d3d3c-026c-4eea-a334-e16f8670e884" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_867d3d3c-026c-4eea-a334-e16f8670e884_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_867d3d3c-026c-4eea-a334-e16f8670e884" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_867d3d3c-026c-4eea-a334-e16f8670e884_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_c60def2b-2cbc-4526-af76-e9916b2b5724" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_867d3d3c-026c-4eea-a334-e16f8670e884" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_c60def2b-2cbc-4526-af76-e9916b2b5724" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_MedpassGroupLimitedMember_a0a1c7d7-7940-48e1-9542-29c0a16d2461" xlink:href="iclr-20211231.xsd#iclr_MedpassGroupLimitedMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_c60def2b-2cbc-4526-af76-e9916b2b5724" xlink:to="loc_iclr_MedpassGroupLimitedMember_a0a1c7d7-7940-48e1-9542-29c0a16d2461" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/EmployeeBenefitsNarrativeDetails" xlink:type="simple" xlink:href="iclr-20211231.xsd#EmployeeBenefitsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/EmployeeBenefitsNarrativeDetails" xlink:type="extended" id="i3289e9884b2646a9b1b52c48d91c725b_EmployeeBenefitsNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_b40e5333-c02f-41f5-8b6e-90f3d13e5a04" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanDisclosureLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedContributionPlanMaximumAnnualContributionsPerEmployeePercent_9a3c7a25-7e47-4bd9-85b9-876c316040a8" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedContributionPlanMaximumAnnualContributionsPerEmployeePercent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_b40e5333-c02f-41f5-8b6e-90f3d13e5a04" xlink:to="loc_us-gaap_DefinedContributionPlanMaximumAnnualContributionsPerEmployeePercent_9a3c7a25-7e47-4bd9-85b9-876c316040a8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedContributionPlanCostRecognized_db40baf6-bf17-4795-9a4d-57142bd0dbd6" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedContributionPlanCostRecognized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_b40e5333-c02f-41f5-8b6e-90f3d13e5a04" xlink:to="loc_us-gaap_DefinedContributionPlanCostRecognized_db40baf6-bf17-4795-9a4d-57142bd0dbd6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanBenefitObligation_52ed2698-3f5a-4de6-94e0-4fb32ed210b3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanBenefitObligation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_b40e5333-c02f-41f5-8b6e-90f3d13e5a04" xlink:to="loc_us-gaap_DefinedBenefitPlanBenefitObligation_52ed2698-3f5a-4de6-94e0-4fb32ed210b3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanExpectedAmortizationOfGainLossNextFiscalYear_fc525cf0-b1a7-41d2-8172-8f16cd840069" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanExpectedAmortizationOfGainLossNextFiscalYear"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_b40e5333-c02f-41f5-8b6e-90f3d13e5a04" xlink:to="loc_us-gaap_DefinedBenefitPlanExpectedAmortizationOfGainLossNextFiscalYear_fc525cf0-b1a7-41d2-8172-8f16cd840069" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanExpectedAmortizationOfPriorServiceCostCreditNextFiscalYear_26c99a65-3e47-411a-89ef-b473e544af6a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanExpectedAmortizationOfPriorServiceCostCreditNextFiscalYear"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_b40e5333-c02f-41f5-8b6e-90f3d13e5a04" xlink:to="loc_us-gaap_DefinedBenefitPlanExpectedAmortizationOfPriorServiceCostCreditNextFiscalYear_26c99a65-3e47-411a-89ef-b473e544af6a" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationPlanPeriod_2fbb0e3f-88b2-4604-ac79-0bc429d7a3bb" xlink:href="iclr-20211231.xsd#iclr_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationPlanPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_b40e5333-c02f-41f5-8b6e-90f3d13e5a04" xlink:to="loc_iclr_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationPlanPeriod_2fbb0e3f-88b2-4604-ac79-0bc429d7a3bb" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanPlanAssetsTargetAllocationPercentage_62b24467-e5b1-4ef1-afa2-85bc93771093" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanPlanAssetsTargetAllocationPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_b40e5333-c02f-41f5-8b6e-90f3d13e5a04" xlink:to="loc_us-gaap_DefinedBenefitPlanPlanAssetsTargetAllocationPercentage_62b24467-e5b1-4ef1-afa2-85bc93771093" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets_089d2543-d286-4701-8152-02a5fe677c83" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_b40e5333-c02f-41f5-8b6e-90f3d13e5a04" xlink:to="loc_us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets_089d2543-d286-4701-8152-02a5fe677c83" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanExpectedFutureEmployerContributionsNextFiscalYear_4e31ef75-f7e7-4b59-877f-952ad6c5728c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanExpectedFutureEmployerContributionsNextFiscalYear"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_b40e5333-c02f-41f5-8b6e-90f3d13e5a04" xlink:to="loc_us-gaap_DefinedBenefitPlanExpectedFutureEmployerContributionsNextFiscalYear_4e31ef75-f7e7-4b59-877f-952ad6c5728c" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_ExpectedRetirementPeriod_5eb1d401-03e7-4d32-a1a1-bb8ffeba8048" xlink:href="iclr-20211231.xsd#iclr_ExpectedRetirementPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_b40e5333-c02f-41f5-8b6e-90f3d13e5a04" xlink:to="loc_iclr_ExpectedRetirementPeriod_5eb1d401-03e7-4d32-a1a1-bb8ffeba8048" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionPercentageOfVotingInterestsAcquired_e0ffe56c-c92d-47a5-96b4-5c253462d7ff" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionPercentageOfVotingInterestsAcquired"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_b40e5333-c02f-41f5-8b6e-90f3d13e5a04" xlink:to="loc_us-gaap_BusinessAcquisitionPercentageOfVotingInterestsAcquired_e0ffe56c-c92d-47a5-96b4-5c253462d7ff" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_d9f7d6a7-fe3f-42d6-9c2f-297e00c62d3c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_b40e5333-c02f-41f5-8b6e-90f3d13e5a04" xlink:to="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_d9f7d6a7-fe3f-42d6-9c2f-297e00c62d3c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationAxis_583aac41-a86c-4bc5-aad9-d6477587a4a8" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RetirementPlanSponsorLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_d9f7d6a7-fe3f-42d6-9c2f-297e00c62d3c" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationAxis_583aac41-a86c-4bc5-aad9-d6477587a4a8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationDomain_583aac41-a86c-4bc5-aad9-d6477587a4a8_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RetirementPlanSponsorLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationAxis_583aac41-a86c-4bc5-aad9-d6477587a4a8" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationDomain_583aac41-a86c-4bc5-aad9-d6477587a4a8_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationDomain_ca999cdd-674b-45fe-aca6-f8837b61cdb2" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RetirementPlanSponsorLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationAxis_583aac41-a86c-4bc5-aad9-d6477587a4a8" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationDomain_ca999cdd-674b-45fe-aca6-f8837b61cdb2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForeignPlanMember_b35ef169-b571-4537-8131-7a6817cc0bca" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ForeignPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationDomain_ca999cdd-674b-45fe-aca6-f8837b61cdb2" xlink:to="loc_us-gaap_ForeignPlanMember_b35ef169-b571-4537-8131-7a6817cc0bca" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_US_62c74ec6-2d87-4edc-ad84-49e2caca3044" xlink:href="https://xbrl.sec.gov/country/2021/country-2021.xsd#country_US"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ForeignPlanMember_b35ef169-b571-4537-8131-7a6817cc0bca" xlink:to="loc_country_US_62c74ec6-2d87-4edc-ad84-49e2caca3044" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_GB_4a0aac0f-6942-48a1-8b69-3eee9a90b170" xlink:href="https://xbrl.sec.gov/country/2021/country-2021.xsd#country_GB"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ForeignPlanMember_b35ef169-b571-4537-8131-7a6817cc0bca" xlink:to="loc_country_GB_4a0aac0f-6942-48a1-8b69-3eee9a90b170" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_CH_5e965077-26ee-4937-adb5-d83557da30a9" xlink:href="https://xbrl.sec.gov/country/2021/country-2021.xsd#country_CH"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ForeignPlanMember_b35ef169-b571-4537-8131-7a6817cc0bca" xlink:to="loc_country_CH_5e965077-26ee-4937-adb5-d83557da30a9" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanNameAxis_a35e880e-79ee-4896-9536-15eb89701fb7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RetirementPlanNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_d9f7d6a7-fe3f-42d6-9c2f-297e00c62d3c" xlink:to="loc_us-gaap_RetirementPlanNameAxis_a35e880e-79ee-4896-9536-15eb89701fb7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanNameDomain_a35e880e-79ee-4896-9536-15eb89701fb7_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RetirementPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RetirementPlanNameAxis_a35e880e-79ee-4896-9536-15eb89701fb7" xlink:to="loc_us-gaap_RetirementPlanNameDomain_a35e880e-79ee-4896-9536-15eb89701fb7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanNameDomain_1a60c155-b944-4433-b29f-f5153b157492" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RetirementPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RetirementPlanNameAxis_a35e880e-79ee-4896-9536-15eb89701fb7" xlink:to="loc_us-gaap_RetirementPlanNameDomain_1a60c155-b944-4433-b29f-f5153b157492" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_OtherRetirementPlansMember_80bfb5e9-cb9e-4ca5-ae50-44f6016bfe22" xlink:href="iclr-20211231.xsd#iclr_OtherRetirementPlansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanNameDomain_1a60c155-b944-4433-b29f-f5153b157492" xlink:to="loc_iclr_OtherRetirementPlansMember_80bfb5e9-cb9e-4ca5-ae50-44f6016bfe22" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_ICONDevelopmentSolutionsLimitedPensionPlanMember_492e3a11-168a-486c-a57e-01791d310860" xlink:href="iclr-20211231.xsd#iclr_ICONDevelopmentSolutionsLimitedPensionPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanNameDomain_1a60c155-b944-4433-b29f-f5153b157492" xlink:to="loc_iclr_ICONDevelopmentSolutionsLimitedPensionPlanMember_492e3a11-168a-486c-a57e-01791d310860" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_AptivSolutionsPensionPlanMember_b0443d92-775e-45b0-b2f5-1e944140b93a" xlink:href="iclr-20211231.xsd#iclr_AptivSolutionsPensionPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanNameDomain_1a60c155-b944-4433-b29f-f5153b157492" xlink:to="loc_iclr_AptivSolutionsPensionPlanMember_b0443d92-775e-45b0-b2f5-1e944140b93a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis_9646777e-7fe8-46fd-87dc-03ea45f81121" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_d9f7d6a7-fe3f-42d6-9c2f-297e00c62d3c" xlink:to="loc_us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis_9646777e-7fe8-46fd-87dc-03ea45f81121" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanAssetCategoriesDomain_9646777e-7fe8-46fd-87dc-03ea45f81121_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PlanAssetCategoriesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis_9646777e-7fe8-46fd-87dc-03ea45f81121" xlink:to="loc_us-gaap_PlanAssetCategoriesDomain_9646777e-7fe8-46fd-87dc-03ea45f81121_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanAssetCategoriesDomain_d04bef1d-0cd6-4685-88e1-c737de8ebb39" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PlanAssetCategoriesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis_9646777e-7fe8-46fd-87dc-03ea45f81121" xlink:to="loc_us-gaap_PlanAssetCategoriesDomain_d04bef1d-0cd6-4685-88e1-c737de8ebb39" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember_4a4ccbb8-6489-4460-8d50-0e723b11a70d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanAssetCategoriesDomain_d04bef1d-0cd6-4685-88e1-c737de8ebb39" xlink:to="loc_us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember_4a4ccbb8-6489-4460-8d50-0e723b11a70d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateDebtSecuritiesMember_d3b4473c-6210-498e-a997-0114afea2821" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CorporateDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanAssetCategoriesDomain_d04bef1d-0cd6-4685-88e1-c737de8ebb39" xlink:to="loc_us-gaap_CorporateDebtSecuritiesMember_d3b4473c-6210-498e-a997-0114afea2821" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_HighYieldBondsMember_72e1a7c5-a27e-405b-bbbd-3839b65d8e77" xlink:href="iclr-20211231.xsd#iclr_HighYieldBondsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanAssetCategoriesDomain_d04bef1d-0cd6-4685-88e1-c737de8ebb39" xlink:to="loc_iclr_HighYieldBondsMember_72e1a7c5-a27e-405b-bbbd-3839b65d8e77" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanEquitySecuritiesMember_e6bc20c9-de53-4e85-b28d-464d389976bc" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanEquitySecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanAssetCategoriesDomain_d04bef1d-0cd6-4685-88e1-c737de8ebb39" xlink:to="loc_us-gaap_DefinedBenefitPlanEquitySecuritiesMember_e6bc20c9-de53-4e85-b28d-464d389976bc" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_5fc51a97-dfdc-4435-8810-f40dd97d2f05" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_d9f7d6a7-fe3f-42d6-9c2f-297e00c62d3c" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_5fc51a97-dfdc-4435-8810-f40dd97d2f05" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_5fc51a97-dfdc-4435-8810-f40dd97d2f05_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_5fc51a97-dfdc-4435-8810-f40dd97d2f05" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_5fc51a97-dfdc-4435-8810-f40dd97d2f05_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_999297f0-bffa-40c1-a124-64060401de83" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_5fc51a97-dfdc-4435-8810-f40dd97d2f05" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_999297f0-bffa-40c1-a124-64060401de83" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_AptivSolutionsMember_dc41908d-314d-45fa-9b3c-2b9419aa4e92" xlink:href="iclr-20211231.xsd#iclr_AptivSolutionsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_999297f0-bffa-40c1-a124-64060401de83" xlink:to="loc_iclr_AptivSolutionsMember_dc41908d-314d-45fa-9b3c-2b9419aa4e92" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/EmployeeBenefitsFundedStatusDetails" xlink:type="simple" xlink:href="iclr-20211231.xsd#EmployeeBenefitsFundedStatusDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/EmployeeBenefitsFundedStatusDetails" xlink:type="extended" id="if7b808a40b6544a1ae960be46fe01130_EmployeeBenefitsFundedStatusDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_15374cc2-0416-48d3-8e80-3074c631ea8e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanDisclosureLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanPensionPlansWithAccumulatedBenefitObligationsInExcessOfPlanAssetsAggregateProjectedBenefitObligation_09910e38-d795-4308-bbee-8423cca0a90c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanPensionPlansWithAccumulatedBenefitObligationsInExcessOfPlanAssetsAggregateProjectedBenefitObligation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_15374cc2-0416-48d3-8e80-3074c631ea8e" xlink:to="loc_us-gaap_DefinedBenefitPlanPensionPlansWithAccumulatedBenefitObligationsInExcessOfPlanAssetsAggregateProjectedBenefitObligation_09910e38-d795-4308-bbee-8423cca0a90c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanFairValueOfPlanAssets_80eaffcb-854e-403a-a157-dcd1f8a3ffd2" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanFairValueOfPlanAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_15374cc2-0416-48d3-8e80-3074c631ea8e" xlink:to="loc_us-gaap_DefinedBenefitPlanFairValueOfPlanAssets_80eaffcb-854e-403a-a157-dcd1f8a3ffd2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanFundedStatusOfPlan_8502cd13-5d44-4647-9bf0-117767d3de30" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanFundedStatusOfPlan"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_15374cc2-0416-48d3-8e80-3074c631ea8e" xlink:to="loc_us-gaap_DefinedBenefitPlanFundedStatusOfPlan_8502cd13-5d44-4647-9bf0-117767d3de30" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanAmountsRecognizedInBalanceSheet_048dad94-b83f-4c6d-9fcc-c69e828b234b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanAmountsRecognizedInBalanceSheet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_15374cc2-0416-48d3-8e80-3074c631ea8e" xlink:to="loc_us-gaap_DefinedBenefitPlanAmountsRecognizedInBalanceSheet_048dad94-b83f-4c6d-9fcc-c69e828b234b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_36df239c-a1cb-4b57-a4c3-96402ab57db7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_15374cc2-0416-48d3-8e80-3074c631ea8e" xlink:to="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_36df239c-a1cb-4b57-a4c3-96402ab57db7" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationAxis_cdb64ddd-8abe-4b0f-905f-4a14b9747e50" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RetirementPlanSponsorLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_36df239c-a1cb-4b57-a4c3-96402ab57db7" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationAxis_cdb64ddd-8abe-4b0f-905f-4a14b9747e50" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationDomain_cdb64ddd-8abe-4b0f-905f-4a14b9747e50_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RetirementPlanSponsorLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationAxis_cdb64ddd-8abe-4b0f-905f-4a14b9747e50" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationDomain_cdb64ddd-8abe-4b0f-905f-4a14b9747e50_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationDomain_ce5b5fda-3109-4f65-a765-e9b5a0bbf79d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RetirementPlanSponsorLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationAxis_cdb64ddd-8abe-4b0f-905f-4a14b9747e50" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationDomain_ce5b5fda-3109-4f65-a765-e9b5a0bbf79d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForeignPlanMember_5f0aeb54-8dc4-4117-a7a1-ec84b35284eb" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ForeignPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationDomain_ce5b5fda-3109-4f65-a765-e9b5a0bbf79d" xlink:to="loc_us-gaap_ForeignPlanMember_5f0aeb54-8dc4-4117-a7a1-ec84b35284eb" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_GB_19ca4e47-724c-4535-a514-99dae3605db3" xlink:href="https://xbrl.sec.gov/country/2021/country-2021.xsd#country_GB"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ForeignPlanMember_5f0aeb54-8dc4-4117-a7a1-ec84b35284eb" xlink:to="loc_country_GB_19ca4e47-724c-4535-a514-99dae3605db3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_CH_914862d9-a098-4584-953e-6ff0ef0b0c91" xlink:href="https://xbrl.sec.gov/country/2021/country-2021.xsd#country_CH"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ForeignPlanMember_5f0aeb54-8dc4-4117-a7a1-ec84b35284eb" xlink:to="loc_country_CH_914862d9-a098-4584-953e-6ff0ef0b0c91" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanNameAxis_581abc8f-c259-415e-8a47-4102d6cbc896" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RetirementPlanNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_36df239c-a1cb-4b57-a4c3-96402ab57db7" xlink:to="loc_us-gaap_RetirementPlanNameAxis_581abc8f-c259-415e-8a47-4102d6cbc896" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanNameDomain_581abc8f-c259-415e-8a47-4102d6cbc896_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RetirementPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RetirementPlanNameAxis_581abc8f-c259-415e-8a47-4102d6cbc896" xlink:to="loc_us-gaap_RetirementPlanNameDomain_581abc8f-c259-415e-8a47-4102d6cbc896_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanNameDomain_3ac2ee7f-108e-4f80-b925-c676767852fd" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RetirementPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RetirementPlanNameAxis_581abc8f-c259-415e-8a47-4102d6cbc896" xlink:to="loc_us-gaap_RetirementPlanNameDomain_3ac2ee7f-108e-4f80-b925-c676767852fd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_ICONDevelopmentSolutionsLimitedPensionPlanMember_5ecf7122-4eaa-44c3-8896-ca435fa3d6da" xlink:href="iclr-20211231.xsd#iclr_ICONDevelopmentSolutionsLimitedPensionPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanNameDomain_3ac2ee7f-108e-4f80-b925-c676767852fd" xlink:to="loc_iclr_ICONDevelopmentSolutionsLimitedPensionPlanMember_5ecf7122-4eaa-44c3-8896-ca435fa3d6da" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_AptivSolutionsPensionPlanMember_446709f4-c382-4138-ad42-46058ff6b22c" xlink:href="iclr-20211231.xsd#iclr_AptivSolutionsPensionPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanNameDomain_3ac2ee7f-108e-4f80-b925-c676767852fd" xlink:to="loc_iclr_AptivSolutionsPensionPlanMember_446709f4-c382-4138-ad42-46058ff6b22c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_PRASwitzerlandAGPensionPlanMember_d973f14a-6cb2-4fce-a67d-f168c428b5a2" xlink:href="iclr-20211231.xsd#iclr_PRASwitzerlandAGPensionPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanNameDomain_3ac2ee7f-108e-4f80-b925-c676767852fd" xlink:to="loc_iclr_PRASwitzerlandAGPensionPlanMember_d973f14a-6cb2-4fce-a67d-f168c428b5a2" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/EmployeeBenefitsChangeinBenefitObligationsDetails" xlink:type="simple" xlink:href="iclr-20211231.xsd#EmployeeBenefitsChangeinBenefitObligationsDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/EmployeeBenefitsChangeinBenefitObligationsDetails" xlink:type="extended" id="ia83f9e8f2bc2477aa3c6969358bf4d13_EmployeeBenefitsChangeinBenefitObligationsDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_179676b4-6fd2-47a9-a526-1763857390d0" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanDisclosureLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanChangeInBenefitObligationRollForward_34579916-71e2-4a99-8cdb-0d897b803474" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanChangeInBenefitObligationRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_179676b4-6fd2-47a9-a526-1763857390d0" xlink:to="loc_us-gaap_DefinedBenefitPlanChangeInBenefitObligationRollForward_34579916-71e2-4a99-8cdb-0d897b803474" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanBenefitObligation_0085676b-5331-41f1-be5a-d5e520b780a7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanBenefitObligation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanChangeInBenefitObligationRollForward_34579916-71e2-4a99-8cdb-0d897b803474" xlink:to="loc_us-gaap_DefinedBenefitPlanBenefitObligation_0085676b-5331-41f1-be5a-d5e520b780a7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanServiceCost_00cc7d2e-2892-4f2a-9e90-8dde8b1d1166" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanServiceCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanChangeInBenefitObligationRollForward_34579916-71e2-4a99-8cdb-0d897b803474" xlink:to="loc_us-gaap_DefinedBenefitPlanServiceCost_00cc7d2e-2892-4f2a-9e90-8dde8b1d1166" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanInterestCost_ad23de1d-4478-4c75-a3b7-29aef54364c5" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanInterestCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanChangeInBenefitObligationRollForward_34579916-71e2-4a99-8cdb-0d897b803474" xlink:to="loc_us-gaap_DefinedBenefitPlanInterestCost_ad23de1d-4478-4c75-a3b7-29aef54364c5" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanBenefitObligationContributionsByPlanParticipant_83c29e47-82a1-4ec1-ba6e-a3c93f982f9d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanBenefitObligationContributionsByPlanParticipant"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanChangeInBenefitObligationRollForward_34579916-71e2-4a99-8cdb-0d897b803474" xlink:to="loc_us-gaap_DefinedBenefitPlanBenefitObligationContributionsByPlanParticipant_83c29e47-82a1-4ec1-ba6e-a3c93f982f9d" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanBenefitObligationBenefitsPaid_677ef52f-d997-4f9d-b97b-d02c57b4a0be" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanBenefitObligationBenefitsPaid"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanChangeInBenefitObligationRollForward_34579916-71e2-4a99-8cdb-0d897b803474" xlink:to="loc_us-gaap_DefinedBenefitPlanBenefitObligationBenefitsPaid_677ef52f-d997-4f9d-b97b-d02c57b4a0be" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanSettlementsBenefitObligation_255a53f2-c437-4b57-9893-ead2ad71493c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanSettlementsBenefitObligation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanChangeInBenefitObligationRollForward_34579916-71e2-4a99-8cdb-0d897b803474" xlink:to="loc_us-gaap_DefinedBenefitPlanSettlementsBenefitObligation_255a53f2-c437-4b57-9893-ead2ad71493c" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit_51bc2370-aeb6-441c-9b95-204153d35c46" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanChangeInBenefitObligationRollForward_34579916-71e2-4a99-8cdb-0d897b803474" xlink:to="loc_us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit_51bc2370-aeb6-441c-9b95-204153d35c46" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanOtherChanges_d0e66b7f-6122-461d-86d5-9a6994a35ba2" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanOtherChanges"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanChangeInBenefitObligationRollForward_34579916-71e2-4a99-8cdb-0d897b803474" xlink:to="loc_us-gaap_DefinedBenefitPlanOtherChanges_d0e66b7f-6122-461d-86d5-9a6994a35ba2" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanActuarialGainLoss_f0ce360a-f909-4b12-9ec3-4c2f12cf9658" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanActuarialGainLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanChangeInBenefitObligationRollForward_34579916-71e2-4a99-8cdb-0d897b803474" xlink:to="loc_us-gaap_DefinedBenefitPlanActuarialGainLoss_f0ce360a-f909-4b12-9ec3-4c2f12cf9658" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanForeignCurrencyExchangeRateChangesBenefitObligation_bd79ed77-c769-4cae-be83-357b3c01f406" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanForeignCurrencyExchangeRateChangesBenefitObligation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanChangeInBenefitObligationRollForward_34579916-71e2-4a99-8cdb-0d897b803474" xlink:to="loc_us-gaap_DefinedBenefitPlanForeignCurrencyExchangeRateChangesBenefitObligation_bd79ed77-c769-4cae-be83-357b3c01f406" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanBenefitObligation_655fa9c3-0e3f-4dfc-9025-7bb3ddbf8cfd" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanBenefitObligation"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_6a694cf4-9b13-41e4-80a9-fecfe5116ef4" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_179676b4-6fd2-47a9-a526-1763857390d0" xlink:to="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_6a694cf4-9b13-41e4-80a9-fecfe5116ef4" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationAxis_4cb3d3a8-7810-4beb-ac61-756b3c0fce13" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RetirementPlanSponsorLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_6a694cf4-9b13-41e4-80a9-fecfe5116ef4" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationAxis_4cb3d3a8-7810-4beb-ac61-756b3c0fce13" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationDomain_4cb3d3a8-7810-4beb-ac61-756b3c0fce13_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RetirementPlanSponsorLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationAxis_4cb3d3a8-7810-4beb-ac61-756b3c0fce13" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationDomain_4cb3d3a8-7810-4beb-ac61-756b3c0fce13_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationDomain_5a7ee8ab-aba5-4929-ae01-274ee208f8f2" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RetirementPlanSponsorLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationAxis_4cb3d3a8-7810-4beb-ac61-756b3c0fce13" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationDomain_5a7ee8ab-aba5-4929-ae01-274ee208f8f2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForeignPlanMember_efaad376-7f65-474c-84fb-5a8c7e737ac7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ForeignPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationDomain_5a7ee8ab-aba5-4929-ae01-274ee208f8f2" xlink:to="loc_us-gaap_ForeignPlanMember_efaad376-7f65-474c-84fb-5a8c7e737ac7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_GB_5da3c5e2-69d0-4806-b4ac-3b39affda054" xlink:href="https://xbrl.sec.gov/country/2021/country-2021.xsd#country_GB"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ForeignPlanMember_efaad376-7f65-474c-84fb-5a8c7e737ac7" xlink:to="loc_country_GB_5da3c5e2-69d0-4806-b4ac-3b39affda054" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_CH_c7baff6d-0374-4026-8710-2a534927a6bd" xlink:href="https://xbrl.sec.gov/country/2021/country-2021.xsd#country_CH"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ForeignPlanMember_efaad376-7f65-474c-84fb-5a8c7e737ac7" xlink:to="loc_country_CH_c7baff6d-0374-4026-8710-2a534927a6bd" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanNameAxis_3ce4919c-fe1f-47c4-89f0-ab36b3f69547" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RetirementPlanNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_6a694cf4-9b13-41e4-80a9-fecfe5116ef4" xlink:to="loc_us-gaap_RetirementPlanNameAxis_3ce4919c-fe1f-47c4-89f0-ab36b3f69547" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanNameDomain_3ce4919c-fe1f-47c4-89f0-ab36b3f69547_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RetirementPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RetirementPlanNameAxis_3ce4919c-fe1f-47c4-89f0-ab36b3f69547" xlink:to="loc_us-gaap_RetirementPlanNameDomain_3ce4919c-fe1f-47c4-89f0-ab36b3f69547_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanNameDomain_8567b49e-427d-41a1-9515-ff76c2b8fb16" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RetirementPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RetirementPlanNameAxis_3ce4919c-fe1f-47c4-89f0-ab36b3f69547" xlink:to="loc_us-gaap_RetirementPlanNameDomain_8567b49e-427d-41a1-9515-ff76c2b8fb16" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_ICONDevelopmentSolutionsLimitedPensionPlanMember_6a762c7f-399c-4e1f-a811-77abb2a330e5" xlink:href="iclr-20211231.xsd#iclr_ICONDevelopmentSolutionsLimitedPensionPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanNameDomain_8567b49e-427d-41a1-9515-ff76c2b8fb16" xlink:to="loc_iclr_ICONDevelopmentSolutionsLimitedPensionPlanMember_6a762c7f-399c-4e1f-a811-77abb2a330e5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_AptivSolutionsPensionPlanMember_5dcfef6b-9ec3-4313-89e4-660b7d30b4f6" xlink:href="iclr-20211231.xsd#iclr_AptivSolutionsPensionPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanNameDomain_8567b49e-427d-41a1-9515-ff76c2b8fb16" xlink:to="loc_iclr_AptivSolutionsPensionPlanMember_5dcfef6b-9ec3-4313-89e4-660b7d30b4f6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_PRASwitzerlandAGPensionPlanMember_82d782b8-a769-4e4b-8e2d-5c63dd35583e" xlink:href="iclr-20211231.xsd#iclr_PRASwitzerlandAGPensionPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanNameDomain_8567b49e-427d-41a1-9515-ff76c2b8fb16" xlink:to="loc_iclr_PRASwitzerlandAGPensionPlanMember_82d782b8-a769-4e4b-8e2d-5c63dd35583e" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/EmployeeBenefitsChangeinPlanAssetsDetails" xlink:type="simple" xlink:href="iclr-20211231.xsd#EmployeeBenefitsChangeinPlanAssetsDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/EmployeeBenefitsChangeinPlanAssetsDetails" xlink:type="extended" id="i231c4a52e902410eaccee44e8b2480fb_EmployeeBenefitsChangeinPlanAssetsDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_d2c13637-0970-4e6a-96c2-7fdba83bd6cd" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanDisclosureLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanChangeInFairValueOfPlanAssetsRollForward_81c4aaad-af30-48da-a9d9-db6fb5bc4149" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanChangeInFairValueOfPlanAssetsRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_d2c13637-0970-4e6a-96c2-7fdba83bd6cd" xlink:to="loc_us-gaap_DefinedBenefitPlanChangeInFairValueOfPlanAssetsRollForward_81c4aaad-af30-48da-a9d9-db6fb5bc4149" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanFairValueOfPlanAssets_0043d89f-0d59-4215-8214-f26b02e2b542" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanFairValueOfPlanAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanChangeInFairValueOfPlanAssetsRollForward_81c4aaad-af30-48da-a9d9-db6fb5bc4149" xlink:to="loc_us-gaap_DefinedBenefitPlanFairValueOfPlanAssets_0043d89f-0d59-4215-8214-f26b02e2b542" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets_e31e825c-f0f1-4016-82e8-f6519ba07258" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanChangeInFairValueOfPlanAssetsRollForward_81c4aaad-af30-48da-a9d9-db6fb5bc4149" xlink:to="loc_us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets_e31e825c-f0f1-4016-82e8-f6519ba07258" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanActualReturnOnPlanAssets_2a3c8085-87f2-4d1a-a409-a1e8e9de247e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanActualReturnOnPlanAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanChangeInFairValueOfPlanAssetsRollForward_81c4aaad-af30-48da-a9d9-db6fb5bc4149" xlink:to="loc_us-gaap_DefinedBenefitPlanActualReturnOnPlanAssets_2a3c8085-87f2-4d1a-a409-a1e8e9de247e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanContributionsByEmployer_959442e2-070b-49a6-a916-e1e50186ab00" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanContributionsByEmployer"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanChangeInFairValueOfPlanAssetsRollForward_81c4aaad-af30-48da-a9d9-db6fb5bc4149" xlink:to="loc_us-gaap_DefinedBenefitPlanContributionsByEmployer_959442e2-070b-49a6-a916-e1e50186ab00" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_DefinedBenefitPlanSchemeContributions_01c8628b-ac8b-4870-8182-3b1ff99dbcce" xlink:href="iclr-20211231.xsd#iclr_DefinedBenefitPlanSchemeContributions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanChangeInFairValueOfPlanAssetsRollForward_81c4aaad-af30-48da-a9d9-db6fb5bc4149" xlink:to="loc_iclr_DefinedBenefitPlanSchemeContributions_01c8628b-ac8b-4870-8182-3b1ff99dbcce" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanPlanAssetsContributionsByPlanParticipant_f28fb0bc-251b-4fdc-95ce-e0410970ec40" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanPlanAssetsContributionsByPlanParticipant"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanChangeInFairValueOfPlanAssetsRollForward_81c4aaad-af30-48da-a9d9-db6fb5bc4149" xlink:to="loc_us-gaap_DefinedBenefitPlanPlanAssetsContributionsByPlanParticipant_f28fb0bc-251b-4fdc-95ce-e0410970ec40" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanPlanAssetsBenefitsPaid_c786b007-4da6-4bd0-bb71-c77eb01dca6a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanPlanAssetsBenefitsPaid"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanChangeInFairValueOfPlanAssetsRollForward_81c4aaad-af30-48da-a9d9-db6fb5bc4149" xlink:to="loc_us-gaap_DefinedBenefitPlanPlanAssetsBenefitsPaid_c786b007-4da6-4bd0-bb71-c77eb01dca6a" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanAssetsTransferredToFromPlan_b507191a-9746-456a-8790-fa0cf20b63dd" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanAssetsTransferredToFromPlan"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanChangeInFairValueOfPlanAssetsRollForward_81c4aaad-af30-48da-a9d9-db6fb5bc4149" xlink:to="loc_us-gaap_DefinedBenefitPlanAssetsTransferredToFromPlan_b507191a-9746-456a-8790-fa0cf20b63dd" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanSettlementsPlanAssets_c207ad95-1885-486d-b896-2f5845ac631c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanSettlementsPlanAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanChangeInFairValueOfPlanAssetsRollForward_81c4aaad-af30-48da-a9d9-db6fb5bc4149" xlink:to="loc_us-gaap_DefinedBenefitPlanSettlementsPlanAssets_c207ad95-1885-486d-b896-2f5845ac631c" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanPlanAssetsForeignCurrencyTranslationGainLoss_1e70f7eb-573c-49e2-a4fa-20f1265af643" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanPlanAssetsForeignCurrencyTranslationGainLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanChangeInFairValueOfPlanAssetsRollForward_81c4aaad-af30-48da-a9d9-db6fb5bc4149" xlink:to="loc_us-gaap_DefinedBenefitPlanPlanAssetsForeignCurrencyTranslationGainLoss_1e70f7eb-573c-49e2-a4fa-20f1265af643" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanFairValueOfPlanAssets_cf6dd498-c459-4f77-847b-8fdbeff174fe" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanFairValueOfPlanAssets"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_547316a2-e366-48b9-895e-11144cce00cf" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_d2c13637-0970-4e6a-96c2-7fdba83bd6cd" xlink:to="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_547316a2-e366-48b9-895e-11144cce00cf" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationAxis_00f69008-4446-4a77-93f3-2e615dc597b4" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RetirementPlanSponsorLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_547316a2-e366-48b9-895e-11144cce00cf" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationAxis_00f69008-4446-4a77-93f3-2e615dc597b4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationDomain_00f69008-4446-4a77-93f3-2e615dc597b4_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RetirementPlanSponsorLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationAxis_00f69008-4446-4a77-93f3-2e615dc597b4" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationDomain_00f69008-4446-4a77-93f3-2e615dc597b4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationDomain_c5cf67b5-2e0f-413e-99e8-7e7213d56110" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RetirementPlanSponsorLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationAxis_00f69008-4446-4a77-93f3-2e615dc597b4" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationDomain_c5cf67b5-2e0f-413e-99e8-7e7213d56110" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForeignPlanMember_2a897f65-f937-48cf-b913-769718b7d124" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ForeignPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationDomain_c5cf67b5-2e0f-413e-99e8-7e7213d56110" xlink:to="loc_us-gaap_ForeignPlanMember_2a897f65-f937-48cf-b913-769718b7d124" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_GB_daf06edb-11b0-4e74-9c93-1649ab086a82" xlink:href="https://xbrl.sec.gov/country/2021/country-2021.xsd#country_GB"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ForeignPlanMember_2a897f65-f937-48cf-b913-769718b7d124" xlink:to="loc_country_GB_daf06edb-11b0-4e74-9c93-1649ab086a82" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_CH_3923cf78-c5af-4dd9-9c29-22d97696a4b9" xlink:href="https://xbrl.sec.gov/country/2021/country-2021.xsd#country_CH"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ForeignPlanMember_2a897f65-f937-48cf-b913-769718b7d124" xlink:to="loc_country_CH_3923cf78-c5af-4dd9-9c29-22d97696a4b9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanNameAxis_da9a8ade-91ff-4423-8b82-84473d3d4cf5" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RetirementPlanNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_547316a2-e366-48b9-895e-11144cce00cf" xlink:to="loc_us-gaap_RetirementPlanNameAxis_da9a8ade-91ff-4423-8b82-84473d3d4cf5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanNameDomain_da9a8ade-91ff-4423-8b82-84473d3d4cf5_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RetirementPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RetirementPlanNameAxis_da9a8ade-91ff-4423-8b82-84473d3d4cf5" xlink:to="loc_us-gaap_RetirementPlanNameDomain_da9a8ade-91ff-4423-8b82-84473d3d4cf5_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanNameDomain_6a4bf314-276d-4fac-a535-95f96f055b90" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RetirementPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RetirementPlanNameAxis_da9a8ade-91ff-4423-8b82-84473d3d4cf5" xlink:to="loc_us-gaap_RetirementPlanNameDomain_6a4bf314-276d-4fac-a535-95f96f055b90" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_ICONDevelopmentSolutionsLimitedPensionPlanMember_2762432e-f745-4d79-9d23-0444b2f8740c" xlink:href="iclr-20211231.xsd#iclr_ICONDevelopmentSolutionsLimitedPensionPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanNameDomain_6a4bf314-276d-4fac-a535-95f96f055b90" xlink:to="loc_iclr_ICONDevelopmentSolutionsLimitedPensionPlanMember_2762432e-f745-4d79-9d23-0444b2f8740c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_AptivSolutionsPensionPlanMember_6d3b70c6-77f0-4d38-bb84-25f69b6d110d" xlink:href="iclr-20211231.xsd#iclr_AptivSolutionsPensionPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanNameDomain_6a4bf314-276d-4fac-a535-95f96f055b90" xlink:to="loc_iclr_AptivSolutionsPensionPlanMember_6d3b70c6-77f0-4d38-bb84-25f69b6d110d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_PRASwitzerlandAGPensionPlanMember_296327fb-fd35-497d-ae17-942ef9447ce1" xlink:href="iclr-20211231.xsd#iclr_PRASwitzerlandAGPensionPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanNameDomain_6a4bf314-276d-4fac-a535-95f96f055b90" xlink:to="loc_iclr_PRASwitzerlandAGPensionPlanMember_296327fb-fd35-497d-ae17-942ef9447ce1" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/EmployeeBenefitsComponentsofNetPeriodicBenefitCostDetails" xlink:type="simple" xlink:href="iclr-20211231.xsd#EmployeeBenefitsComponentsofNetPeriodicBenefitCostDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/EmployeeBenefitsComponentsofNetPeriodicBenefitCostDetails" xlink:type="extended" id="idd44f8c4dcfe4eef892e728051c6f634_EmployeeBenefitsComponentsofNetPeriodicBenefitCostDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_ea13de00-435d-41aa-a937-693b0729f981" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanDisclosureLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanServiceCost_7419ffb4-037e-456e-acf4-ee0a421f53f5" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanServiceCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_ea13de00-435d-41aa-a937-693b0729f981" xlink:to="loc_us-gaap_DefinedBenefitPlanServiceCost_7419ffb4-037e-456e-acf4-ee0a421f53f5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanInterestCost_a50155dc-ba6c-4f06-915b-d6965882498a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanInterestCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_ea13de00-435d-41aa-a937-693b0729f981" xlink:to="loc_us-gaap_DefinedBenefitPlanInterestCost_a50155dc-ba6c-4f06-915b-d6965882498a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets_d5d52a96-2447-4a29-9a1d-eea774d5294c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_ea13de00-435d-41aa-a937-693b0729f981" xlink:to="loc_us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets_d5d52a96-2447-4a29-9a1d-eea774d5294c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses_d88f9185-d9c3-4777-bb2a-7b48a6816920" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_ea13de00-435d-41aa-a937-693b0729f981" xlink:to="loc_us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses_d88f9185-d9c3-4777-bb2a-7b48a6816920" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost_26a0d526-ab22-43f3-8c75-be3126b005e2" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_ea13de00-435d-41aa-a937-693b0729f981" xlink:to="loc_us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost_26a0d526-ab22-43f3-8c75-be3126b005e2" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_3abf14e0-c9d7-41cf-9520-0db1993d47a7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_ea13de00-435d-41aa-a937-693b0729f981" xlink:to="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_3abf14e0-c9d7-41cf-9520-0db1993d47a7" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationAxis_d1b1d3ea-31b5-4cb4-929f-6656d3404c50" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RetirementPlanSponsorLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_3abf14e0-c9d7-41cf-9520-0db1993d47a7" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationAxis_d1b1d3ea-31b5-4cb4-929f-6656d3404c50" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationDomain_d1b1d3ea-31b5-4cb4-929f-6656d3404c50_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RetirementPlanSponsorLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationAxis_d1b1d3ea-31b5-4cb4-929f-6656d3404c50" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationDomain_d1b1d3ea-31b5-4cb4-929f-6656d3404c50_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationDomain_eddd0644-e849-45c5-8f3c-bd38431531d1" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RetirementPlanSponsorLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationAxis_d1b1d3ea-31b5-4cb4-929f-6656d3404c50" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationDomain_eddd0644-e849-45c5-8f3c-bd38431531d1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForeignPlanMember_033d43d4-2822-45cb-b58c-a0ce57342106" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ForeignPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationDomain_eddd0644-e849-45c5-8f3c-bd38431531d1" xlink:to="loc_us-gaap_ForeignPlanMember_033d43d4-2822-45cb-b58c-a0ce57342106" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_GB_dc46f4fb-8160-45fa-80f4-9c2a8097dfa7" xlink:href="https://xbrl.sec.gov/country/2021/country-2021.xsd#country_GB"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ForeignPlanMember_033d43d4-2822-45cb-b58c-a0ce57342106" xlink:to="loc_country_GB_dc46f4fb-8160-45fa-80f4-9c2a8097dfa7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_CH_b23d9677-93d1-4a54-98a7-5c9f5966329f" xlink:href="https://xbrl.sec.gov/country/2021/country-2021.xsd#country_CH"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ForeignPlanMember_033d43d4-2822-45cb-b58c-a0ce57342106" xlink:to="loc_country_CH_b23d9677-93d1-4a54-98a7-5c9f5966329f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanNameAxis_d7a615f2-d615-46d7-bd8d-d2d0c1ae6a27" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RetirementPlanNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_3abf14e0-c9d7-41cf-9520-0db1993d47a7" xlink:to="loc_us-gaap_RetirementPlanNameAxis_d7a615f2-d615-46d7-bd8d-d2d0c1ae6a27" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanNameDomain_d7a615f2-d615-46d7-bd8d-d2d0c1ae6a27_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RetirementPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RetirementPlanNameAxis_d7a615f2-d615-46d7-bd8d-d2d0c1ae6a27" xlink:to="loc_us-gaap_RetirementPlanNameDomain_d7a615f2-d615-46d7-bd8d-d2d0c1ae6a27_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanNameDomain_5bb0d0b8-fc79-4bcf-9334-1396a34fa04c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RetirementPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RetirementPlanNameAxis_d7a615f2-d615-46d7-bd8d-d2d0c1ae6a27" xlink:to="loc_us-gaap_RetirementPlanNameDomain_5bb0d0b8-fc79-4bcf-9334-1396a34fa04c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_ICONDevelopmentSolutionsLimitedPensionPlanMember_3d370bdc-1c80-4b6a-844e-906442d5aeef" xlink:href="iclr-20211231.xsd#iclr_ICONDevelopmentSolutionsLimitedPensionPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanNameDomain_5bb0d0b8-fc79-4bcf-9334-1396a34fa04c" xlink:to="loc_iclr_ICONDevelopmentSolutionsLimitedPensionPlanMember_3d370bdc-1c80-4b6a-844e-906442d5aeef" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/EmployeeBenefitsNetPeriodicPensionBenefitCostAssumptionsDetails" xlink:type="simple" xlink:href="iclr-20211231.xsd#EmployeeBenefitsNetPeriodicPensionBenefitCostAssumptionsDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/EmployeeBenefitsNetPeriodicPensionBenefitCostAssumptionsDetails" xlink:type="extended" id="if27750f2c7ed4aea8fd4d304df900e1e_EmployeeBenefitsNetPeriodicPensionBenefitCostAssumptionsDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_0b7054d5-04e8-44da-b5c7-7391593d61eb" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanDisclosureLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate_7ce520a9-0170-41f0-afb5-73a84282c8a8" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_0b7054d5-04e8-44da-b5c7-7391593d61eb" xlink:to="loc_us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate_7ce520a9-0170-41f0-afb5-73a84282c8a8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostRateOfCompensationIncrease_32191da1-87f4-4722-82de-811a99349232" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostRateOfCompensationIncrease"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_0b7054d5-04e8-44da-b5c7-7391593d61eb" xlink:to="loc_us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostRateOfCompensationIncrease_32191da1-87f4-4722-82de-811a99349232" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets_011a71d7-4898-4a60-a8f7-b60a4d0d7d72" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_0b7054d5-04e8-44da-b5c7-7391593d61eb" xlink:to="loc_us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets_011a71d7-4898-4a60-a8f7-b60a4d0d7d72" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPeriodIncreaseDecreaseAbstract_67b96ace-e38e-4382-8972-5ad4880c1526" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTaxPeriodIncreaseDecreaseAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_0b7054d5-04e8-44da-b5c7-7391593d61eb" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPeriodIncreaseDecreaseAbstract_67b96ace-e38e-4382-8972-5ad4880c1526" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_DefinedBenefitPlanActuarialLossesToBeAmortizedAndReclassifiedFromAccumulatedOtherComprehensiveIncomeLossToNetPensionCost_f9359c0f-f169-404a-9650-7bf6ce884f63" xlink:href="iclr-20211231.xsd#iclr_DefinedBenefitPlanActuarialLossesToBeAmortizedAndReclassifiedFromAccumulatedOtherComprehensiveIncomeLossToNetPensionCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPeriodIncreaseDecreaseAbstract_67b96ace-e38e-4382-8972-5ad4880c1526" xlink:to="loc_iclr_DefinedBenefitPlanActuarialLossesToBeAmortizedAndReclassifiedFromAccumulatedOtherComprehensiveIncomeLossToNetPensionCost_f9359c0f-f169-404a-9650-7bf6ce884f63" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossNetOfTax_32f774d1-4ad3-4335-a907-db74a0ee2110" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPeriodIncreaseDecreaseAbstract_67b96ace-e38e-4382-8972-5ad4880c1526" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossNetOfTax_32f774d1-4ad3-4335-a907-db74a0ee2110" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_OtherComprehensiveIncomeAmortizationOfNetActuarialGainLossNetOfTax_0e9927ad-0d8a-46bf-9267-5f39b1caabef" xlink:href="iclr-20211231.xsd#iclr_OtherComprehensiveIncomeAmortizationOfNetActuarialGainLossNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPeriodIncreaseDecreaseAbstract_67b96ace-e38e-4382-8972-5ad4880c1526" xlink:to="loc_iclr_OtherComprehensiveIncomeAmortizationOfNetActuarialGainLossNetOfTax_0e9927ad-0d8a-46bf-9267-5f39b1caabef" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetTransitionAssetObligationNetOfTax_9e5704dc-d052-41c6-a5b9-9f95a33def35" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetTransitionAssetObligationNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPeriodIncreaseDecreaseAbstract_67b96ace-e38e-4382-8972-5ad4880c1526" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetTransitionAssetObligationNetOfTax_9e5704dc-d052-41c6-a5b9-9f95a33def35" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_d6de8551-d0a2-41f5-a3c5-c4574325b4b9" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_0b7054d5-04e8-44da-b5c7-7391593d61eb" xlink:to="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_d6de8551-d0a2-41f5-a3c5-c4574325b4b9" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationAxis_9acd140a-4768-4df8-b476-e5e1ca5c80ce" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RetirementPlanSponsorLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_d6de8551-d0a2-41f5-a3c5-c4574325b4b9" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationAxis_9acd140a-4768-4df8-b476-e5e1ca5c80ce" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationDomain_9acd140a-4768-4df8-b476-e5e1ca5c80ce_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RetirementPlanSponsorLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationAxis_9acd140a-4768-4df8-b476-e5e1ca5c80ce" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationDomain_9acd140a-4768-4df8-b476-e5e1ca5c80ce_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationDomain_6cdc3625-21cc-41fc-82a6-09576cd843f8" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RetirementPlanSponsorLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationAxis_9acd140a-4768-4df8-b476-e5e1ca5c80ce" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationDomain_6cdc3625-21cc-41fc-82a6-09576cd843f8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForeignPlanMember_b729a014-d3e8-4600-a80f-2ad49dada12c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ForeignPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationDomain_6cdc3625-21cc-41fc-82a6-09576cd843f8" xlink:to="loc_us-gaap_ForeignPlanMember_b729a014-d3e8-4600-a80f-2ad49dada12c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_GB_ca24e4bf-207a-43f6-a4c5-f983c7eea5f7" xlink:href="https://xbrl.sec.gov/country/2021/country-2021.xsd#country_GB"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ForeignPlanMember_b729a014-d3e8-4600-a80f-2ad49dada12c" xlink:to="loc_country_GB_ca24e4bf-207a-43f6-a4c5-f983c7eea5f7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_CH_4d5da15a-f98e-4f2b-a1e8-e230cc13bffc" xlink:href="https://xbrl.sec.gov/country/2021/country-2021.xsd#country_CH"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ForeignPlanMember_b729a014-d3e8-4600-a80f-2ad49dada12c" xlink:to="loc_country_CH_4d5da15a-f98e-4f2b-a1e8-e230cc13bffc" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanNameAxis_a4c84ed2-ede8-4ff9-bf64-7d31b968afb0" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RetirementPlanNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_d6de8551-d0a2-41f5-a3c5-c4574325b4b9" xlink:to="loc_us-gaap_RetirementPlanNameAxis_a4c84ed2-ede8-4ff9-bf64-7d31b968afb0" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanNameDomain_a4c84ed2-ede8-4ff9-bf64-7d31b968afb0_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RetirementPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RetirementPlanNameAxis_a4c84ed2-ede8-4ff9-bf64-7d31b968afb0" xlink:to="loc_us-gaap_RetirementPlanNameDomain_a4c84ed2-ede8-4ff9-bf64-7d31b968afb0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanNameDomain_326e88b4-1be5-4da4-ba36-1bece4d88c69" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RetirementPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RetirementPlanNameAxis_a4c84ed2-ede8-4ff9-bf64-7d31b968afb0" xlink:to="loc_us-gaap_RetirementPlanNameDomain_326e88b4-1be5-4da4-ba36-1bece4d88c69" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_ICONDevelopmentSolutionsLimitedPensionPlanMember_44e159a7-d0e6-4b83-bda9-2803acd1dbd0" xlink:href="iclr-20211231.xsd#iclr_ICONDevelopmentSolutionsLimitedPensionPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanNameDomain_326e88b4-1be5-4da4-ba36-1bece4d88c69" xlink:to="loc_iclr_ICONDevelopmentSolutionsLimitedPensionPlanMember_44e159a7-d0e6-4b83-bda9-2803acd1dbd0" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/EmployeeBenefitsAssumptionsUsedinDeterminingBenefitObligationDetails" xlink:type="simple" xlink:href="iclr-20211231.xsd#EmployeeBenefitsAssumptionsUsedinDeterminingBenefitObligationDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/EmployeeBenefitsAssumptionsUsedinDeterminingBenefitObligationDetails" xlink:type="extended" id="i2b0c42e21b6c47bf8d582ce3bd56c841_EmployeeBenefitsAssumptionsUsedinDeterminingBenefitObligationDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_c31d486e-4423-4a9a-a087-86bc28466c8f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanDisclosureLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate_1ee32ee5-7084-4bc1-a0f1-efdbcf4613aa" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_c31d486e-4423-4a9a-a087-86bc28466c8f" xlink:to="loc_us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate_1ee32ee5-7084-4bc1-a0f1-efdbcf4613aa" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationRateOfCompensationIncrease_07a01ed5-6c88-46f3-909d-6b1499c46a53" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationRateOfCompensationIncrease"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_c31d486e-4423-4a9a-a087-86bc28466c8f" xlink:to="loc_us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationRateOfCompensationIncrease_07a01ed5-6c88-46f3-909d-6b1499c46a53" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_f1b1f229-74ec-4656-81b7-f4e68c125fb4" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_c31d486e-4423-4a9a-a087-86bc28466c8f" xlink:to="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_f1b1f229-74ec-4656-81b7-f4e68c125fb4" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationAxis_47ba6d09-3ca1-451d-aa8c-58d8b6adf3e0" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RetirementPlanSponsorLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_f1b1f229-74ec-4656-81b7-f4e68c125fb4" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationAxis_47ba6d09-3ca1-451d-aa8c-58d8b6adf3e0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationDomain_47ba6d09-3ca1-451d-aa8c-58d8b6adf3e0_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RetirementPlanSponsorLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationAxis_47ba6d09-3ca1-451d-aa8c-58d8b6adf3e0" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationDomain_47ba6d09-3ca1-451d-aa8c-58d8b6adf3e0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationDomain_8a864d36-0c1c-46c0-8d1e-15e928fe0b79" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RetirementPlanSponsorLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationAxis_47ba6d09-3ca1-451d-aa8c-58d8b6adf3e0" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationDomain_8a864d36-0c1c-46c0-8d1e-15e928fe0b79" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForeignPlanMember_e40447db-ae64-4019-8616-a1b15992da48" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ForeignPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationDomain_8a864d36-0c1c-46c0-8d1e-15e928fe0b79" xlink:to="loc_us-gaap_ForeignPlanMember_e40447db-ae64-4019-8616-a1b15992da48" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_GB_812e4724-1b06-4f26-8c52-288a13df51c4" xlink:href="https://xbrl.sec.gov/country/2021/country-2021.xsd#country_GB"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ForeignPlanMember_e40447db-ae64-4019-8616-a1b15992da48" xlink:to="loc_country_GB_812e4724-1b06-4f26-8c52-288a13df51c4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_CH_21ced01e-a2de-473e-ad21-cad3d4f5a543" xlink:href="https://xbrl.sec.gov/country/2021/country-2021.xsd#country_CH"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ForeignPlanMember_e40447db-ae64-4019-8616-a1b15992da48" xlink:to="loc_country_CH_21ced01e-a2de-473e-ad21-cad3d4f5a543" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanNameAxis_d64fc927-cd2d-418c-b7f2-a8809e3a193a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RetirementPlanNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_f1b1f229-74ec-4656-81b7-f4e68c125fb4" xlink:to="loc_us-gaap_RetirementPlanNameAxis_d64fc927-cd2d-418c-b7f2-a8809e3a193a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanNameDomain_d64fc927-cd2d-418c-b7f2-a8809e3a193a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RetirementPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RetirementPlanNameAxis_d64fc927-cd2d-418c-b7f2-a8809e3a193a" xlink:to="loc_us-gaap_RetirementPlanNameDomain_d64fc927-cd2d-418c-b7f2-a8809e3a193a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanNameDomain_658526c8-7638-4850-aa74-91e64462507c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RetirementPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RetirementPlanNameAxis_d64fc927-cd2d-418c-b7f2-a8809e3a193a" xlink:to="loc_us-gaap_RetirementPlanNameDomain_658526c8-7638-4850-aa74-91e64462507c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_ICONDevelopmentSolutionsLimitedPensionPlanMember_8b458405-8169-4a79-9125-887dc3309549" xlink:href="iclr-20211231.xsd#iclr_ICONDevelopmentSolutionsLimitedPensionPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanNameDomain_658526c8-7638-4850-aa74-91e64462507c" xlink:to="loc_iclr_ICONDevelopmentSolutionsLimitedPensionPlanMember_8b458405-8169-4a79-9125-887dc3309549" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/EmployeeBenefitsExpectedRateofReturnandActualPlanAssetAllocationDetails" xlink:type="simple" xlink:href="iclr-20211231.xsd#EmployeeBenefitsExpectedRateofReturnandActualPlanAssetAllocationDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/EmployeeBenefitsExpectedRateofReturnandActualPlanAssetAllocationDetails" xlink:type="extended" id="iec89d5f3f8fb403d993116e7c6164dde_EmployeeBenefitsExpectedRateofReturnandActualPlanAssetAllocationDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_459c4258-7b8d-4302-b8bb-5e25805e7840" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanDisclosureLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_DefinedBenefitPlanPlanAssetsCorporateBondsTargetAllocationHighYieldPercentage_16747a71-b976-41bd-a488-38a257439f65" xlink:href="iclr-20211231.xsd#iclr_DefinedBenefitPlanPlanAssetsCorporateBondsTargetAllocationHighYieldPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_459c4258-7b8d-4302-b8bb-5e25805e7840" xlink:to="loc_iclr_DefinedBenefitPlanPlanAssetsCorporateBondsTargetAllocationHighYieldPercentage_16747a71-b976-41bd-a488-38a257439f65" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets_d42519c7-b1f0-4586-8aab-aa8dd907e0ae" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_459c4258-7b8d-4302-b8bb-5e25805e7840" xlink:to="loc_us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets_d42519c7-b1f0-4586-8aab-aa8dd907e0ae" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations_fa281c6a-3f97-4015-9819-30a27dd5bd8d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_459c4258-7b8d-4302-b8bb-5e25805e7840" xlink:to="loc_us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations_fa281c6a-3f97-4015-9819-30a27dd5bd8d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_730c9868-ba7e-49ca-bcab-4b7441d234ad" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_459c4258-7b8d-4302-b8bb-5e25805e7840" xlink:to="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_730c9868-ba7e-49ca-bcab-4b7441d234ad" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationAxis_1567b934-2aaa-40aa-ae6f-7731c780bec8" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RetirementPlanSponsorLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_730c9868-ba7e-49ca-bcab-4b7441d234ad" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationAxis_1567b934-2aaa-40aa-ae6f-7731c780bec8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationDomain_1567b934-2aaa-40aa-ae6f-7731c780bec8_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RetirementPlanSponsorLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationAxis_1567b934-2aaa-40aa-ae6f-7731c780bec8" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationDomain_1567b934-2aaa-40aa-ae6f-7731c780bec8_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationDomain_3f060dd3-93d1-4cab-9fe7-2b14dacd0e24" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RetirementPlanSponsorLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationAxis_1567b934-2aaa-40aa-ae6f-7731c780bec8" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationDomain_3f060dd3-93d1-4cab-9fe7-2b14dacd0e24" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForeignPlanMember_5db0c54d-a8e3-43a2-adbe-1e574720c3b0" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ForeignPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationDomain_3f060dd3-93d1-4cab-9fe7-2b14dacd0e24" xlink:to="loc_us-gaap_ForeignPlanMember_5db0c54d-a8e3-43a2-adbe-1e574720c3b0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_GB_72450bf5-9b98-4764-adeb-70e276a01471" xlink:href="https://xbrl.sec.gov/country/2021/country-2021.xsd#country_GB"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ForeignPlanMember_5db0c54d-a8e3-43a2-adbe-1e574720c3b0" xlink:to="loc_country_GB_72450bf5-9b98-4764-adeb-70e276a01471" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis_e13e9368-a8a3-4297-b4a3-51ddcb38f5a3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_730c9868-ba7e-49ca-bcab-4b7441d234ad" xlink:to="loc_us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis_e13e9368-a8a3-4297-b4a3-51ddcb38f5a3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanAssetCategoriesDomain_e13e9368-a8a3-4297-b4a3-51ddcb38f5a3_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PlanAssetCategoriesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis_e13e9368-a8a3-4297-b4a3-51ddcb38f5a3" xlink:to="loc_us-gaap_PlanAssetCategoriesDomain_e13e9368-a8a3-4297-b4a3-51ddcb38f5a3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanAssetCategoriesDomain_29323668-1e77-4631-886f-4e1992695b05" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PlanAssetCategoriesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis_e13e9368-a8a3-4297-b4a3-51ddcb38f5a3" xlink:to="loc_us-gaap_PlanAssetCategoriesDomain_29323668-1e77-4631-886f-4e1992695b05" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateDebtSecuritiesMember_dfffb3db-2481-4d0e-b7b6-c15c1fdc4a67" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CorporateDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanAssetCategoriesDomain_29323668-1e77-4631-886f-4e1992695b05" xlink:to="loc_us-gaap_CorporateDebtSecuritiesMember_dfffb3db-2481-4d0e-b7b6-c15c1fdc4a67" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanEquitySecuritiesMember_d236b892-43bf-4084-bcf5-5b38cee833fd" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanEquitySecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanAssetCategoriesDomain_29323668-1e77-4631-886f-4e1992695b05" xlink:to="loc_us-gaap_DefinedBenefitPlanEquitySecuritiesMember_d236b892-43bf-4084-bcf5-5b38cee833fd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember_898796fd-6cc7-4ef0-92d7-7368b7be3533" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanAssetCategoriesDomain_29323668-1e77-4631-886f-4e1992695b05" xlink:to="loc_us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember_898796fd-6cc7-4ef0-92d7-7368b7be3533" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanNameAxis_b7267244-9c0f-4d00-8afa-a530c96df2a4" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RetirementPlanNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_730c9868-ba7e-49ca-bcab-4b7441d234ad" xlink:to="loc_us-gaap_RetirementPlanNameAxis_b7267244-9c0f-4d00-8afa-a530c96df2a4" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanNameDomain_b7267244-9c0f-4d00-8afa-a530c96df2a4_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RetirementPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RetirementPlanNameAxis_b7267244-9c0f-4d00-8afa-a530c96df2a4" xlink:to="loc_us-gaap_RetirementPlanNameDomain_b7267244-9c0f-4d00-8afa-a530c96df2a4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanNameDomain_300baf8f-d96f-45c9-9fce-2658a475b870" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RetirementPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RetirementPlanNameAxis_b7267244-9c0f-4d00-8afa-a530c96df2a4" xlink:to="loc_us-gaap_RetirementPlanNameDomain_300baf8f-d96f-45c9-9fce-2658a475b870" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_ICONDevelopmentSolutionsLimitedPensionPlanMember_3b930aba-3c5b-4b99-a752-32252ab6569f" xlink:href="iclr-20211231.xsd#iclr_ICONDevelopmentSolutionsLimitedPensionPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanNameDomain_300baf8f-d96f-45c9-9fce-2658a475b870" xlink:to="loc_iclr_ICONDevelopmentSolutionsLimitedPensionPlanMember_3b930aba-3c5b-4b99-a752-32252ab6569f" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/EmployeeBenefitsPlanAssetFairValueMeasurementsDetails" xlink:type="simple" xlink:href="iclr-20211231.xsd#EmployeeBenefitsPlanAssetFairValueMeasurementsDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/EmployeeBenefitsPlanAssetFairValueMeasurementsDetails" xlink:type="extended" id="i48f5b6080c194eeb8a86b42fef6ffa0a_EmployeeBenefitsPlanAssetFairValueMeasurementsDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_5ed7aec1-cea7-4f0a-89ef-dd682b082a43" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanDisclosureLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanFairValueOfPlanAssets_c83a9a10-1a68-472d-b2d9-afa245ba2b7a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanFairValueOfPlanAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_5ed7aec1-cea7-4f0a-89ef-dd682b082a43" xlink:to="loc_us-gaap_DefinedBenefitPlanFairValueOfPlanAssets_c83a9a10-1a68-472d-b2d9-afa245ba2b7a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_DefinedBenefitPlanPlanAssetsCorporateBondsTargetAllocationHighYieldPercentage_7cc575a2-6331-4568-a08a-e57703e21134" xlink:href="iclr-20211231.xsd#iclr_DefinedBenefitPlanPlanAssetsCorporateBondsTargetAllocationHighYieldPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_5ed7aec1-cea7-4f0a-89ef-dd682b082a43" xlink:to="loc_iclr_DefinedBenefitPlanPlanAssetsCorporateBondsTargetAllocationHighYieldPercentage_7cc575a2-6331-4568-a08a-e57703e21134" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_6ced0559-d7a7-4e0f-9b88-9888d538563b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_5ed7aec1-cea7-4f0a-89ef-dd682b082a43" xlink:to="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_6ced0559-d7a7-4e0f-9b88-9888d538563b" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_cb833189-217f-444a-9b62-27a47caa54a6" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_6ced0559-d7a7-4e0f-9b88-9888d538563b" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_cb833189-217f-444a-9b62-27a47caa54a6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_cb833189-217f-444a-9b62-27a47caa54a6_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_cb833189-217f-444a-9b62-27a47caa54a6" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_cb833189-217f-444a-9b62-27a47caa54a6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_8fcb757c-72bc-4f8a-8eb6-06638824d0d7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_cb833189-217f-444a-9b62-27a47caa54a6" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_8fcb757c-72bc-4f8a-8eb6-06638824d0d7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel1Member_eed78bab-4d0e-42e2-b906-81ae581bf5fa" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueInputsLevel1Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_8fcb757c-72bc-4f8a-8eb6-06638824d0d7" xlink:to="loc_us-gaap_FairValueInputsLevel1Member_eed78bab-4d0e-42e2-b906-81ae581bf5fa" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis_1158a486-1aa3-4a3d-b5ee-37f788dcc015" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_6ced0559-d7a7-4e0f-9b88-9888d538563b" xlink:to="loc_us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis_1158a486-1aa3-4a3d-b5ee-37f788dcc015" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanAssetCategoriesDomain_1158a486-1aa3-4a3d-b5ee-37f788dcc015_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PlanAssetCategoriesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis_1158a486-1aa3-4a3d-b5ee-37f788dcc015" xlink:to="loc_us-gaap_PlanAssetCategoriesDomain_1158a486-1aa3-4a3d-b5ee-37f788dcc015_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanAssetCategoriesDomain_d76ecf48-5412-4df9-b30f-ce2c8baf3466" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PlanAssetCategoriesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis_1158a486-1aa3-4a3d-b5ee-37f788dcc015" xlink:to="loc_us-gaap_PlanAssetCategoriesDomain_d76ecf48-5412-4df9-b30f-ce2c8baf3466" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanCashMember_55868277-5b96-4408-aa42-22499a641d3c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanCashMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanAssetCategoriesDomain_d76ecf48-5412-4df9-b30f-ce2c8baf3466" xlink:to="loc_us-gaap_DefinedBenefitPlanCashMember_55868277-5b96-4408-aa42-22499a641d3c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_LAndGLifeGPBFAllWorldEquityIndexFundMember_ea4b4373-03a8-48cd-a9cf-4911b5206e4c" xlink:href="iclr-20211231.xsd#iclr_LAndGLifeGPBFAllWorldEquityIndexFundMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanAssetCategoriesDomain_d76ecf48-5412-4df9-b30f-ce2c8baf3466" xlink:to="loc_iclr_LAndGLifeGPBFAllWorldEquityIndexFundMember_ea4b4373-03a8-48cd-a9cf-4911b5206e4c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_LAndGLifeDCActiveCorporateBondMember_6d269c32-43b1-453e-b477-cf2635873c5f" xlink:href="iclr-20211231.xsd#iclr_LAndGLifeDCActiveCorporateBondMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanAssetCategoriesDomain_d76ecf48-5412-4df9-b30f-ce2c8baf3466" xlink:to="loc_iclr_LAndGLifeDCActiveCorporateBondMember_6d269c32-43b1-453e-b477-cf2635873c5f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_StoneHarborHighYieldBondFundMember_9a6c9c1f-98fa-47d6-b344-e6f555a0042d" xlink:href="iclr-20211231.xsd#iclr_StoneHarborHighYieldBondFundMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanAssetCategoriesDomain_d76ecf48-5412-4df9-b30f-ce2c8baf3466" xlink:to="loc_iclr_StoneHarborHighYieldBondFundMember_9a6c9c1f-98fa-47d6-b344-e6f555a0042d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_NinetyOneGlobalTotalReturnCreditMember_ce6cf277-045d-4516-92fe-82e01c893094" xlink:href="iclr-20211231.xsd#iclr_NinetyOneGlobalTotalReturnCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanAssetCategoriesDomain_d76ecf48-5412-4df9-b30f-ce2c8baf3466" xlink:to="loc_iclr_NinetyOneGlobalTotalReturnCreditMember_ce6cf277-045d-4516-92fe-82e01c893094" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_StoneHarborMultiAssetCreditPortfolioMember_20302033-e497-486b-9d46-3603baa559bb" xlink:href="iclr-20211231.xsd#iclr_StoneHarborMultiAssetCreditPortfolioMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanAssetCategoriesDomain_d76ecf48-5412-4df9-b30f-ce2c8baf3466" xlink:to="loc_iclr_StoneHarborMultiAssetCreditPortfolioMember_20302033-e497-486b-9d46-3603baa559bb" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BaringsEuropeanLoanFundBuyAndHoldMember_bc56848d-11fc-4076-aa38-c3e63a9d4920" xlink:href="iclr-20211231.xsd#iclr_BaringsEuropeanLoanFundBuyAndHoldMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanAssetCategoriesDomain_d76ecf48-5412-4df9-b30f-ce2c8baf3466" xlink:to="loc_iclr_BaringsEuropeanLoanFundBuyAndHoldMember_bc56848d-11fc-4076-aa38-c3e63a9d4920" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanNameAxis_645c8224-f0eb-4657-b684-d55f60bbdaa7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RetirementPlanNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_6ced0559-d7a7-4e0f-9b88-9888d538563b" xlink:to="loc_us-gaap_RetirementPlanNameAxis_645c8224-f0eb-4657-b684-d55f60bbdaa7" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanNameDomain_645c8224-f0eb-4657-b684-d55f60bbdaa7_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RetirementPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RetirementPlanNameAxis_645c8224-f0eb-4657-b684-d55f60bbdaa7" xlink:to="loc_us-gaap_RetirementPlanNameDomain_645c8224-f0eb-4657-b684-d55f60bbdaa7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanNameDomain_e1b673ae-77a3-4f89-bea7-b2b5f2381ece" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RetirementPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RetirementPlanNameAxis_645c8224-f0eb-4657-b684-d55f60bbdaa7" xlink:to="loc_us-gaap_RetirementPlanNameDomain_e1b673ae-77a3-4f89-bea7-b2b5f2381ece" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_ICONDevelopmentSolutionsLimitedPensionPlanMember_f556ec98-4277-4207-a481-5d1f8d89076c" xlink:href="iclr-20211231.xsd#iclr_ICONDevelopmentSolutionsLimitedPensionPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanNameDomain_e1b673ae-77a3-4f89-bea7-b2b5f2381ece" xlink:to="loc_iclr_ICONDevelopmentSolutionsLimitedPensionPlanMember_f556ec98-4277-4207-a481-5d1f8d89076c" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/EmployeeBenefitsAnnualBenefitPaymentsDetails" xlink:type="simple" xlink:href="iclr-20211231.xsd#EmployeeBenefitsAnnualBenefitPaymentsDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/EmployeeBenefitsAnnualBenefitPaymentsDetails" xlink:type="extended" id="i78b2aebc075747158b1cb0bc828f9b9d_EmployeeBenefitsAnnualBenefitPaymentsDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_3dc5ef08-3010-41d5-91db-0c2eb1923fdf" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanDisclosureLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsNextTwelveMonths_27dc3be6-befb-4c0b-9001-28586057e027" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsNextTwelveMonths"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_3dc5ef08-3010-41d5-91db-0c2eb1923fdf" xlink:to="loc_us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsNextTwelveMonths_27dc3be6-befb-4c0b-9001-28586057e027" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearTwo_58145d01-9a06-43a2-baab-b9670f65eebd" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearTwo"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_3dc5ef08-3010-41d5-91db-0c2eb1923fdf" xlink:to="loc_us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearTwo_58145d01-9a06-43a2-baab-b9670f65eebd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearThree_4ba28c5b-3ac7-49dc-9ce1-8258f603a3f5" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearThree"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_3dc5ef08-3010-41d5-91db-0c2eb1923fdf" xlink:to="loc_us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearThree_4ba28c5b-3ac7-49dc-9ce1-8258f603a3f5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearFour_7e3dbfe1-e897-4ac1-b8f7-dbcdcc7c43b3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearFour"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_3dc5ef08-3010-41d5-91db-0c2eb1923fdf" xlink:to="loc_us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearFour_7e3dbfe1-e897-4ac1-b8f7-dbcdcc7c43b3" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearFive_aedffdc9-9156-41a8-be37-9945eb72edf3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearFive"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_3dc5ef08-3010-41d5-91db-0c2eb1923fdf" xlink:to="loc_us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearFive_aedffdc9-9156-41a8-be37-9945eb72edf3" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsFiveFiscalYearsThereafter_ce5f5d68-bf0d-4d55-8e86-c5282a80c91f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsFiveFiscalYearsThereafter"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_3dc5ef08-3010-41d5-91db-0c2eb1923fdf" xlink:to="loc_us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsFiveFiscalYearsThereafter_ce5f5d68-bf0d-4d55-8e86-c5282a80c91f" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_129ea23f-eb2a-4113-b39c-e83e17784505" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_3dc5ef08-3010-41d5-91db-0c2eb1923fdf" xlink:to="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_129ea23f-eb2a-4113-b39c-e83e17784505" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationAxis_6dfc5a20-3248-4251-81a9-8c13718a5381" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RetirementPlanSponsorLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_129ea23f-eb2a-4113-b39c-e83e17784505" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationAxis_6dfc5a20-3248-4251-81a9-8c13718a5381" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationDomain_6dfc5a20-3248-4251-81a9-8c13718a5381_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RetirementPlanSponsorLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationAxis_6dfc5a20-3248-4251-81a9-8c13718a5381" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationDomain_6dfc5a20-3248-4251-81a9-8c13718a5381_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationDomain_6e64242f-b4bc-4b58-a995-2a4c94aeddad" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RetirementPlanSponsorLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationAxis_6dfc5a20-3248-4251-81a9-8c13718a5381" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationDomain_6e64242f-b4bc-4b58-a995-2a4c94aeddad" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForeignPlanMember_1c3ec20f-6e9a-4eb0-941f-10d851735862" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ForeignPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationDomain_6e64242f-b4bc-4b58-a995-2a4c94aeddad" xlink:to="loc_us-gaap_ForeignPlanMember_1c3ec20f-6e9a-4eb0-941f-10d851735862" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_GB_294cb106-718a-495f-9127-e1f919172768" xlink:href="https://xbrl.sec.gov/country/2021/country-2021.xsd#country_GB"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ForeignPlanMember_1c3ec20f-6e9a-4eb0-941f-10d851735862" xlink:to="loc_country_GB_294cb106-718a-495f-9127-e1f919172768" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_CH_5244c4f5-285c-4aec-9d89-19f7950b8ea7" xlink:href="https://xbrl.sec.gov/country/2021/country-2021.xsd#country_CH"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ForeignPlanMember_1c3ec20f-6e9a-4eb0-941f-10d851735862" xlink:to="loc_country_CH_5244c4f5-285c-4aec-9d89-19f7950b8ea7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameAxis_2d38e17d-5aee-424b-993e-2965eabb07fd" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PlanNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_129ea23f-eb2a-4113-b39c-e83e17784505" xlink:to="loc_us-gaap_PlanNameAxis_2d38e17d-5aee-424b-993e-2965eabb07fd" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_2d38e17d-5aee-424b-993e-2965eabb07fd_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PlanNameAxis_2d38e17d-5aee-424b-993e-2965eabb07fd" xlink:to="loc_us-gaap_PlanNameDomain_2d38e17d-5aee-424b-993e-2965eabb07fd_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_9b038e41-100c-49e6-ad79-5813146b3b5d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PlanNameAxis_2d38e17d-5aee-424b-993e-2965eabb07fd" xlink:to="loc_us-gaap_PlanNameDomain_9b038e41-100c-49e6-ad79-5813146b3b5d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_ICONDevelopmentSolutionsLimitedPensionPlanMember_fc9da55b-b868-462e-9121-f53435466a00" xlink:href="iclr-20211231.xsd#iclr_ICONDevelopmentSolutionsLimitedPensionPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_9b038e41-100c-49e6-ad79-5813146b3b5d" xlink:to="loc_iclr_ICONDevelopmentSolutionsLimitedPensionPlanMember_fc9da55b-b868-462e-9121-f53435466a00" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/EquityIncentiveSchemesandStockCompensationChargesNarrativeDetails" xlink:type="simple" xlink:href="iclr-20211231.xsd#EquityIncentiveSchemesandStockCompensationChargesNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/EquityIncentiveSchemesandStockCompensationChargesNarrativeDetails" xlink:type="extended" id="i38b381d659bd46698028365be8a3b170_EquityIncentiveSchemesandStockCompensationChargesNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureLineItems_b2a4de1c-b202-435c-8eab-1d7bb3149260" xlink:href="iclr-20211231.xsd#iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized_c45f8e63-68fb-4c26-b626-618d0e565785" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureLineItems_b2a4de1c-b202-435c-8eab-1d7bb3149260" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized_c45f8e63-68fb-4c26-b626-618d0e565785" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_LimitOfSharesIssuedOrToBeIssuedPursuantToOptionsGranted_cf3ce8e5-8947-4cd4-8fa9-d0875fb10b22" xlink:href="iclr-20211231.xsd#iclr_LimitOfSharesIssuedOrToBeIssuedPursuantToOptionsGranted"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureLineItems_b2a4de1c-b202-435c-8eab-1d7bb3149260" xlink:to="loc_iclr_LimitOfSharesIssuedOrToBeIssuedPursuantToOptionsGranted_cf3ce8e5-8947-4cd4-8fa9-d0875fb10b22" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardMaximumNumberOfSharesPerEmployee_1d06d554-b560-4d3d-bf4c-7a368e707569" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardMaximumNumberOfSharesPerEmployee"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureLineItems_b2a4de1c-b202-435c-8eab-1d7bb3149260" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardMaximumNumberOfSharesPerEmployee_1d06d554-b560-4d3d-bf4c-7a368e707569" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardPurchasePriceOfCommonStockPercent_5d7339a5-c2a1-4bf0-b34c-f65aa48673c6" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardPurchasePriceOfCommonStockPercent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureLineItems_b2a4de1c-b202-435c-8eab-1d7bb3149260" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardPurchasePriceOfCommonStockPercent_5d7339a5-c2a1-4bf0-b34c-f65aa48673c6" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardPercentageOfOutstandingStockMaximum_cb077f20-9255-4ed8-82ac-5d161a3fa143" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardPercentageOfOutstandingStockMaximum"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureLineItems_b2a4de1c-b202-435c-8eab-1d7bb3149260" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardPercentageOfOutstandingStockMaximum_cb077f20-9255-4ed8-82ac-5d161a3fa143" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_7a2d8add-9ed7-476a-8461-904fd2deb919" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureLineItems_b2a4de1c-b202-435c-8eab-1d7bb3149260" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_7a2d8add-9ed7-476a-8461-904fd2deb919" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod_78e06422-bb10-463a-a781-e5fb3fd24e50" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureLineItems_b2a4de1c-b202-435c-8eab-1d7bb3149260" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod_78e06422-bb10-463a-a781-e5fb3fd24e50" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_ShareBasedCompensationArrangementByShareBasedPaymentAwardPercentageOfOutstandingSharesAuthorized_82f68d38-cb52-4f9b-a1f5-f34317f9bfe6" xlink:href="iclr-20211231.xsd#iclr_ShareBasedCompensationArrangementByShareBasedPaymentAwardPercentageOfOutstandingSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureLineItems_b2a4de1c-b202-435c-8eab-1d7bb3149260" xlink:to="loc_iclr_ShareBasedCompensationArrangementByShareBasedPaymentAwardPercentageOfOutstandingSharesAuthorized_82f68d38-cb52-4f9b-a1f5-f34317f9bfe6" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2_4fa2be42-9030-4ad0-962a-b7198ac59658" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureLineItems_b2a4de1c-b202-435c-8eab-1d7bb3149260" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2_4fa2be42-9030-4ad0-962a-b7198ac59658" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedAndExpectedToVestExercisableWeightedAverageRemainingContractualTerm1_38d8e686-6d42-4b23-bfbd-c69fa7fff529" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedAndExpectedToVestExercisableWeightedAverageRemainingContractualTerm1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureLineItems_b2a4de1c-b202-435c-8eab-1d7bb3149260" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedAndExpectedToVestExercisableWeightedAverageRemainingContractualTerm1_38d8e686-6d42-4b23-bfbd-c69fa7fff529" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized_16ce410b-21d0-4edd-a007-403fa1a27f3b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureLineItems_b2a4de1c-b202-435c-8eab-1d7bb3149260" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized_16ce410b-21d0-4edd-a007-403fa1a27f3b" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue_7b6ce88a-e718-4376-a780-50a790a65ee4" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureLineItems_b2a4de1c-b202-435c-8eab-1d7bb3149260" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue_7b6ce88a-e718-4376-a780-50a790a65ee4" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodFairValuePerShare_d813dbd5-9392-4572-a64e-e1c5672c7d04" xlink:href="iclr-20211231.xsd#iclr_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodFairValuePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureLineItems_b2a4de1c-b202-435c-8eab-1d7bb3149260" xlink:to="loc_iclr_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodFairValuePerShare_d813dbd5-9392-4572-a64e-e1c5672c7d04" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_5e89b440-943b-40a2-8bc8-70f1d6fd16a1" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureLineItems_b2a4de1c-b202-435c-8eab-1d7bb3149260" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_5e89b440-943b-40a2-8bc8-70f1d6fd16a1" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensation_a7cd7919-cd44-4986-ac4f-8fa64a2658ad" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureLineItems_b2a4de1c-b202-435c-8eab-1d7bb3149260" xlink:to="loc_us-gaap_ShareBasedCompensation_a7cd7919-cd44-4986-ac4f-8fa64a2658ad" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense_23a32870-d6a1-4b9e-89a8-65d45bf98490" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureLineItems_b2a4de1c-b202-435c-8eab-1d7bb3149260" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense_23a32870-d6a1-4b9e-89a8-65d45bf98490" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromExerciseOfStockOptions_d07b33e4-a13f-4685-9547-6a3c4463e1e1" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromExerciseOfStockOptions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureLineItems_b2a4de1c-b202-435c-8eab-1d7bb3149260" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromExerciseOfStockOptions_d07b33e4-a13f-4685-9547-6a3c4463e1e1" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureTable_fa69d466-a334-4a58-83d8-c44b739572cd" xlink:href="iclr-20211231.xsd#iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureLineItems_b2a4de1c-b202-435c-8eab-1d7bb3149260" xlink:to="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureTable_fa69d466-a334-4a58-83d8-c44b739572cd" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_f01c7be2-2876-46cc-9270-014840934c92" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureTable_fa69d466-a334-4a58-83d8-c44b739572cd" xlink:to="loc_us-gaap_AwardTypeAxis_f01c7be2-2876-46cc-9270-014840934c92" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_f01c7be2-2876-46cc-9270-014840934c92_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_f01c7be2-2876-46cc-9270-014840934c92" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_f01c7be2-2876-46cc-9270-014840934c92_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_7abdaaeb-6b5e-4c0e-aba0-ef67ef197556" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_f01c7be2-2876-46cc-9270-014840934c92" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_7abdaaeb-6b5e-4c0e-aba0-ef67ef197556" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_ConsultantsStockPlan2008PlanMember_d8f749e5-6fda-4d80-8f24-ee9e324d72e7" xlink:href="iclr-20211231.xsd#iclr_ConsultantsStockPlan2008PlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_7abdaaeb-6b5e-4c0e-aba0-ef67ef197556" xlink:to="loc_iclr_ConsultantsStockPlan2008PlanMember_d8f749e5-6fda-4d80-8f24-ee9e324d72e7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_EmployeeStockPlanTwentyZeroEightPlanMember_335bdb48-75d6-4423-9bec-056b6be3ded9" xlink:href="iclr-20211231.xsd#iclr_EmployeeStockPlanTwentyZeroEightPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_7abdaaeb-6b5e-4c0e-aba0-ef67ef197556" xlink:to="loc_iclr_EmployeeStockPlanTwentyZeroEightPlanMember_335bdb48-75d6-4423-9bec-056b6be3ded9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_OptionPlans2008Member_fc58bd2f-8dc5-4f6e-8cac-ae48c7429466" xlink:href="iclr-20211231.xsd#iclr_OptionPlans2008Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_7abdaaeb-6b5e-4c0e-aba0-ef67ef197556" xlink:to="loc_iclr_OptionPlans2008Member_fc58bd2f-8dc5-4f6e-8cac-ae48c7429466" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_EmployeeStockPlanTwentyZeroThreePlanMember_088b0ff8-273a-4e06-a06f-7f64490c136d" xlink:href="iclr-20211231.xsd#iclr_EmployeeStockPlanTwentyZeroThreePlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_7abdaaeb-6b5e-4c0e-aba0-ef67ef197556" xlink:to="loc_iclr_EmployeeStockPlanTwentyZeroThreePlanMember_088b0ff8-273a-4e06-a06f-7f64490c136d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_f8f07be1-0a67-4451-97b2-bfdb9f206a3d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_7abdaaeb-6b5e-4c0e-aba0-ef67ef197556" xlink:to="loc_us-gaap_EmployeeStockOptionMember_f8f07be1-0a67-4451-97b2-bfdb9f206a3d" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_RestrictedStockUnitsTwentyThirteenMember_19adab04-dabf-4463-a779-3263b1660023" xlink:href="iclr-20211231.xsd#iclr_RestrictedStockUnitsTwentyThirteenMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_7abdaaeb-6b5e-4c0e-aba0-ef67ef197556" xlink:to="loc_iclr_RestrictedStockUnitsTwentyThirteenMember_19adab04-dabf-4463-a779-3263b1660023" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_ConsultantsRestrictedStockUnitsTwentyNineteenMember_c5f204b0-c750-4139-8b4d-782665fc931f" xlink:href="iclr-20211231.xsd#iclr_ConsultantsRestrictedStockUnitsTwentyNineteenMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_7abdaaeb-6b5e-4c0e-aba0-ef67ef197556" xlink:to="loc_iclr_ConsultantsRestrictedStockUnitsTwentyNineteenMember_c5f204b0-c750-4139-8b4d-782665fc931f" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_c63fa4ee-4ce0-4c4b-8417-ff7a14fc20d6" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_7abdaaeb-6b5e-4c0e-aba0-ef67ef197556" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_c63fa4ee-4ce0-4c4b-8417-ff7a14fc20d6" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_PerformanceShareUnitMember_148ab804-3981-4568-9a6d-48e9b971d4c5" xlink:href="iclr-20211231.xsd#iclr_PerformanceShareUnitMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_7abdaaeb-6b5e-4c0e-aba0-ef67ef197556" xlink:to="loc_iclr_PerformanceShareUnitMember_148ab804-3981-4568-9a6d-48e9b971d4c5" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_PerformanceBasedGrantsMember_404cd73b-82a9-4b22-9ad1-19ef2a166bb9" xlink:href="iclr-20211231.xsd#iclr_PerformanceBasedGrantsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_7abdaaeb-6b5e-4c0e-aba0-ef67ef197556" xlink:to="loc_iclr_PerformanceBasedGrantsMember_404cd73b-82a9-4b22-9ad1-19ef2a166bb9" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardDateAxis_0af9b452-27e8-408c-9dd2-560bba1edec4" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AwardDateAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureTable_fa69d466-a334-4a58-83d8-c44b739572cd" xlink:to="loc_us-gaap_AwardDateAxis_0af9b452-27e8-408c-9dd2-560bba1edec4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardDateDomain_0af9b452-27e8-408c-9dd2-560bba1edec4_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AwardDateDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardDateAxis_0af9b452-27e8-408c-9dd2-560bba1edec4" xlink:to="loc_us-gaap_AwardDateDomain_0af9b452-27e8-408c-9dd2-560bba1edec4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardDateDomain_e49b8f7e-77e2-4d48-b5dc-27c7b5affb59" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AwardDateDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardDateAxis_0af9b452-27e8-408c-9dd2-560bba1edec4" xlink:to="loc_us-gaap_AwardDateDomain_e49b8f7e-77e2-4d48-b5dc-27c7b5affb59" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_AwardDateAllYearsExcluding2018Member_89a87de4-70da-400f-95a4-8e95f1bafefa" xlink:href="iclr-20211231.xsd#iclr_AwardDateAllYearsExcluding2018Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AwardDateDomain_e49b8f7e-77e2-4d48-b5dc-27c7b5affb59" xlink:to="loc_iclr_AwardDateAllYearsExcluding2018Member_89a87de4-70da-400f-95a4-8e95f1bafefa" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_AwardDate2018Member_93610e1c-c502-4632-ba6c-165c268a11af" xlink:href="iclr-20211231.xsd#iclr_AwardDate2018Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AwardDateDomain_e49b8f7e-77e2-4d48-b5dc-27c7b5affb59" xlink:to="loc_iclr_AwardDate2018Member_93610e1c-c502-4632-ba6c-165c268a11af" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LegalEntityAxis_5dab6fbe-9c14-4c4c-8deb-6cfbb7980a0e" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_LegalEntityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureTable_fa69d466-a334-4a58-83d8-c44b739572cd" xlink:to="loc_dei_LegalEntityAxis_5dab6fbe-9c14-4c4c-8deb-6cfbb7980a0e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_5dab6fbe-9c14-4c4c-8deb-6cfbb7980a0e_default" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_5dab6fbe-9c14-4c4c-8deb-6cfbb7980a0e" xlink:to="loc_dei_EntityDomain_5dab6fbe-9c14-4c4c-8deb-6cfbb7980a0e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_875c2ade-82e4-4967-98fa-8bf1d7760c27" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_5dab6fbe-9c14-4c4c-8deb-6cfbb7980a0e" xlink:to="loc_dei_EntityDomain_875c2ade-82e4-4967-98fa-8bf1d7760c27" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_PRAHealthSciencesIncMember_74c4e58e-fbd8-41b4-b272-a78400c89f2f" xlink:href="iclr-20211231.xsd#iclr_PRAHealthSciencesIncMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_875c2ade-82e4-4967-98fa-8bf1d7760c27" xlink:to="loc_iclr_PRAHealthSciencesIncMember_74c4e58e-fbd8-41b4-b272-a78400c89f2f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_cd81e8bb-3412-45d4-9cf2-4bfbfef446c6" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureTable_fa69d466-a334-4a58-83d8-c44b739572cd" xlink:to="loc_srt_RangeAxis_cd81e8bb-3412-45d4-9cf2-4bfbfef446c6" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_cd81e8bb-3412-45d4-9cf2-4bfbfef446c6_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_cd81e8bb-3412-45d4-9cf2-4bfbfef446c6" xlink:to="loc_srt_RangeMember_cd81e8bb-3412-45d4-9cf2-4bfbfef446c6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_24355fd8-5afc-4201-a82b-8819144f9912" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_cd81e8bb-3412-45d4-9cf2-4bfbfef446c6" xlink:to="loc_srt_RangeMember_24355fd8-5afc-4201-a82b-8819144f9912" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_d08b1bed-9fb3-46a4-99c5-2221d51ef7ba" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_24355fd8-5afc-4201-a82b-8819144f9912" xlink:to="loc_srt_MinimumMember_d08b1bed-9fb3-46a4-99c5-2221d51ef7ba" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_0e392388-c95e-4d7e-aa2a-abd01a174177" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_24355fd8-5afc-4201-a82b-8819144f9912" xlink:to="loc_srt_MaximumMember_0e392388-c95e-4d7e-aa2a-abd01a174177" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualAxis_9f523bf6-87f1-4b9e-98fe-3186af5647ea" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_TitleOfIndividualAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureTable_fa69d466-a334-4a58-83d8-c44b739572cd" xlink:to="loc_srt_TitleOfIndividualAxis_9f523bf6-87f1-4b9e-98fe-3186af5647ea" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_9f523bf6-87f1-4b9e-98fe-3186af5647ea_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_TitleOfIndividualWithRelationshipToEntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_TitleOfIndividualAxis_9f523bf6-87f1-4b9e-98fe-3186af5647ea" xlink:to="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_9f523bf6-87f1-4b9e-98fe-3186af5647ea_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_f11702e9-ed03-4104-b91e-cb309bcf7a06" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_TitleOfIndividualWithRelationshipToEntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_TitleOfIndividualAxis_9f523bf6-87f1-4b9e-98fe-3186af5647ea" xlink:to="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_f11702e9-ed03-4104-b91e-cb309bcf7a06" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_NonexecutiveDirectorMember_e1b9ea4a-f326-4050-ac17-866e1fee539a" xlink:href="iclr-20211231.xsd#iclr_NonexecutiveDirectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_f11702e9-ed03-4104-b91e-cb309bcf7a06" xlink:to="loc_iclr_NonexecutiveDirectorMember_e1b9ea4a-f326-4050-ac17-866e1fee539a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameAxis_2e4c2978-ccd1-490f-b6e7-0732c2b59c6e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PlanNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureTable_fa69d466-a334-4a58-83d8-c44b739572cd" xlink:to="loc_us-gaap_PlanNameAxis_2e4c2978-ccd1-490f-b6e7-0732c2b59c6e" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_2e4c2978-ccd1-490f-b6e7-0732c2b59c6e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PlanNameAxis_2e4c2978-ccd1-490f-b6e7-0732c2b59c6e" xlink:to="loc_us-gaap_PlanNameDomain_2e4c2978-ccd1-490f-b6e7-0732c2b59c6e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_dea6e79d-1d76-4f3e-b240-c30befdf90e3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PlanNameAxis_2e4c2978-ccd1-490f-b6e7-0732c2b59c6e" xlink:to="loc_us-gaap_PlanNameDomain_dea6e79d-1d76-4f3e-b240-c30befdf90e3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_The2020LegacyPRAPlanMember_2ddc235a-cbd5-4ffb-9269-832895f822e1" xlink:href="iclr-20211231.xsd#iclr_The2020LegacyPRAPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_dea6e79d-1d76-4f3e-b240-c30befdf90e3" xlink:to="loc_iclr_The2020LegacyPRAPlanMember_2ddc235a-cbd5-4ffb-9269-832895f822e1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_The2018LegacyPRAPlanMember_68e01796-da98-4951-8cfb-7cc2bcd1b42b" xlink:href="iclr-20211231.xsd#iclr_The2018LegacyPRAPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_dea6e79d-1d76-4f3e-b240-c30befdf90e3" xlink:to="loc_iclr_The2018LegacyPRAPlanMember_68e01796-da98-4951-8cfb-7cc2bcd1b42b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_A2013LegacyPRAPlanMember_6d978ce2-7ee2-4509-a70c-fc1981af6b4e" xlink:href="iclr-20211231.xsd#iclr_A2013LegacyPRAPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_dea6e79d-1d76-4f3e-b240-c30befdf90e3" xlink:to="loc_iclr_A2013LegacyPRAPlanMember_6d978ce2-7ee2-4509-a70c-fc1981af6b4e" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/EquityIncentiveSchemesandStockCompensationChargesSummaryofStockOptionActivityDetails" xlink:type="simple" xlink:href="iclr-20211231.xsd#EquityIncentiveSchemesandStockCompensationChargesSummaryofStockOptionActivityDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/EquityIncentiveSchemesandStockCompensationChargesSummaryofStockOptionActivityDetails" xlink:type="extended" id="i9af9a85414114bb988d54a1f4e179bf9_EquityIncentiveSchemesandStockCompensationChargesSummaryofStockOptionActivityDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_427aa97c-aef9-4987-9ce3-fd3e62725acd" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_89f7a83f-e92c-4910-9570-a3be76f8507b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_427aa97c-aef9-4987-9ce3-fd3e62725acd" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_89f7a83f-e92c-4910-9570-a3be76f8507b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_380453b1-091f-4576-8e7f-e6c635404c4a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_89f7a83f-e92c-4910-9570-a3be76f8507b" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_380453b1-091f-4576-8e7f-e6c635404c4a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAssumedThroughBusinessCombination_a3cb590f-b9d0-4064-992d-639b332bfdb2" xlink:href="iclr-20211231.xsd#iclr_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAssumedThroughBusinessCombination"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_89f7a83f-e92c-4910-9570-a3be76f8507b" xlink:to="loc_iclr_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAssumedThroughBusinessCombination_a3cb590f-b9d0-4064-992d-639b332bfdb2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross_cfc27302-e866-42bd-b291-156a3928ffe2" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_89f7a83f-e92c-4910-9570-a3be76f8507b" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross_cfc27302-e866-42bd-b291-156a3928ffe2" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_5e97661c-9065-4125-ba70-4f112d7c8ee9" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_89f7a83f-e92c-4910-9570-a3be76f8507b" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_5e97661c-9065-4125-ba70-4f112d7c8ee9" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod_ab6c829b-90fd-472e-8a6b-696f530350b7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_89f7a83f-e92c-4910-9570-a3be76f8507b" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod_ab6c829b-90fd-472e-8a6b-696f530350b7" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_de7fd376-ebb9-4830-966c-7adcb1cf5fef" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestExercisableNumber_94c7d389-8533-44b5-83f8-859a5d0cc831" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestExercisableNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_427aa97c-aef9-4987-9ce3-fd3e62725acd" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestExercisableNumber_94c7d389-8533-44b5-83f8-859a5d0cc831" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_3e831bf7-3d0b-4793-9405-e453a65d51de" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_427aa97c-aef9-4987-9ce3-fd3e62725acd" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_3e831bf7-3d0b-4793-9405-e453a65d51de" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice_79c51321-8e55-498d-8254-c6c67641e4bd" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_3e831bf7-3d0b-4793-9405-e453a65d51de" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice_79c51321-8e55-498d-8254-c6c67641e4bd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsAssumedInBusinessCombinationWeightedAverageExercisePrice_987d67fd-ac80-4c15-8e53-219861515453" xlink:href="iclr-20211231.xsd#iclr_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsAssumedInBusinessCombinationWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_3e831bf7-3d0b-4793-9405-e453a65d51de" xlink:to="loc_iclr_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsAssumedInBusinessCombinationWeightedAverageExercisePrice_987d67fd-ac80-4c15-8e53-219861515453" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice_7772d13c-c80a-46d1-b3cb-021952e60d0c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_3e831bf7-3d0b-4793-9405-e453a65d51de" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice_7772d13c-c80a-46d1-b3cb-021952e60d0c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice_312b7ca1-5a53-4f2c-b383-74fcee22fc1e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_3e831bf7-3d0b-4793-9405-e453a65d51de" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice_312b7ca1-5a53-4f2c-b383-74fcee22fc1e" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExpirationsInPeriodWeightedAverageExercisePrice_911aa29c-5497-4aee-b4a8-19d595232c21" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExpirationsInPeriodWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_3e831bf7-3d0b-4793-9405-e453a65d51de" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExpirationsInPeriodWeightedAverageExercisePrice_911aa29c-5497-4aee-b4a8-19d595232c21" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice_fb7fd09f-c4fe-41e7-9342-d29b7fe75a66" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestExercisableWeightedAverageExercisePrice_f40bc291-9733-4727-ac4b-c18606bdba0a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestExercisableWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_427aa97c-aef9-4987-9ce3-fd3e62725acd" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestExercisableWeightedAverageExercisePrice_f40bc291-9733-4727-ac4b-c18606bdba0a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_51b1abde-6b8e-40c2-914e-794d0b5b0383" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_427aa97c-aef9-4987-9ce3-fd3e62725acd" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_51b1abde-6b8e-40c2-914e-794d0b5b0383" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_9306667e-5408-417f-8977-c95400f8b1e3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_51b1abde-6b8e-40c2-914e-794d0b5b0383" xlink:to="loc_us-gaap_AwardTypeAxis_9306667e-5408-417f-8977-c95400f8b1e3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_9306667e-5408-417f-8977-c95400f8b1e3_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_9306667e-5408-417f-8977-c95400f8b1e3" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_9306667e-5408-417f-8977-c95400f8b1e3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_c8bad141-bf0e-48ac-b0a3-056b95a24d74" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_9306667e-5408-417f-8977-c95400f8b1e3" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_c8bad141-bf0e-48ac-b0a3-056b95a24d74" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_StockOptionAndAwardPlansMember_69387d92-8376-4e96-88b8-99a3f135a56d" xlink:href="iclr-20211231.xsd#iclr_StockOptionAndAwardPlansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_c8bad141-bf0e-48ac-b0a3-056b95a24d74" xlink:to="loc_iclr_StockOptionAndAwardPlansMember_69387d92-8376-4e96-88b8-99a3f135a56d" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/EquityIncentiveSchemesandStockCompensationChargesOutstandingandExercisableShareOptionsDetails" xlink:type="simple" xlink:href="iclr-20211231.xsd#EquityIncentiveSchemesandStockCompensationChargesOutstandingandExercisableShareOptionsDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/EquityIncentiveSchemesandStockCompensationChargesOutstandingandExercisableShareOptionsDetails" xlink:type="extended" id="id1a07a5d230a4749b8d3c9c497b08d42_EquityIncentiveSchemesandStockCompensationChargesOutstandingandExercisableShareOptionsDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_3ec3ed5e-abee-4212-a1e2-d80ffef32ee6" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLowerRangeLimit_f1afceab-caf8-4761-b597-49f802ea3d20" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLowerRangeLimit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_3ec3ed5e-abee-4212-a1e2-d80ffef32ee6" xlink:to="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLowerRangeLimit_f1afceab-caf8-4761-b597-49f802ea3d20" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeUpperRangeLimit_2e46f229-17d4-4591-9e49-6aa808a5b137" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeUpperRangeLimit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_3ec3ed5e-abee-4212-a1e2-d80ffef32ee6" xlink:to="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeUpperRangeLimit_2e46f229-17d4-4591-9e49-6aa808a5b137" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfOutstandingOptions_24ec654e-cec2-4583-9e83-2d5833b3a654" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfOutstandingOptions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_3ec3ed5e-abee-4212-a1e2-d80ffef32ee6" xlink:to="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfOutstandingOptions_24ec654e-cec2-4583-9e83-2d5833b3a654" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeOutstandingOptionsWeightedAverageRemainingContractualTerm2_542c260d-696c-4ee6-aca1-624134530d5f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeOutstandingOptionsWeightedAverageRemainingContractualTerm2"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_3ec3ed5e-abee-4212-a1e2-d80ffef32ee6" xlink:to="loc_us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeOutstandingOptionsWeightedAverageRemainingContractualTerm2_542c260d-696c-4ee6-aca1-624134530d5f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice_13793bcf-23d2-4370-bac7-4c6a25b54b35" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_3ec3ed5e-abee-4212-a1e2-d80ffef32ee6" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice_13793bcf-23d2-4370-bac7-4c6a25b54b35" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfExercisableOptions_69146082-2888-49fa-b865-70b04f353128" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfExercisableOptions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_3ec3ed5e-abee-4212-a1e2-d80ffef32ee6" xlink:to="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfExercisableOptions_69146082-2888-49fa-b865-70b04f353128" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestExercisableWeightedAverageExercisePrice_9d8b252d-064c-4c74-9080-9ada6d2f5024" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestExercisableWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_3ec3ed5e-abee-4212-a1e2-d80ffef32ee6" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestExercisableWeightedAverageExercisePrice_9d8b252d-064c-4c74-9080-9ada6d2f5024" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_9ec75c17-05e7-435a-a082-383795eba8cc" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_3ec3ed5e-abee-4212-a1e2-d80ffef32ee6" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_9ec75c17-05e7-435a-a082-383795eba8cc" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_c1576301-0a6c-4c02-a771-d4b58d87f646" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_9ec75c17-05e7-435a-a082-383795eba8cc" xlink:to="loc_us-gaap_AwardTypeAxis_c1576301-0a6c-4c02-a771-d4b58d87f646" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_c1576301-0a6c-4c02-a771-d4b58d87f646_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_c1576301-0a6c-4c02-a771-d4b58d87f646" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_c1576301-0a6c-4c02-a771-d4b58d87f646_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_e60c4500-095d-469b-b10e-50c0a89a95fa" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_c1576301-0a6c-4c02-a771-d4b58d87f646" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_e60c4500-095d-469b-b10e-50c0a89a95fa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_StockOptionAndAwardPlansMember_2796f5a4-ea9c-4a8e-b502-0c81c9e04f8d" xlink:href="iclr-20211231.xsd#iclr_StockOptionAndAwardPlansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_e60c4500-095d-469b-b10e-50c0a89a95fa" xlink:to="loc_iclr_StockOptionAndAwardPlansMember_2796f5a4-ea9c-4a8e-b502-0c81c9e04f8d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis_f3bca583-8a28-46d8-a1f9-9208a33293ef" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_9ec75c17-05e7-435a-a082-383795eba8cc" xlink:to="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis_f3bca583-8a28-46d8-a1f9-9208a33293ef" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain_f3bca583-8a28-46d8-a1f9-9208a33293ef_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis_f3bca583-8a28-46d8-a1f9-9208a33293ef" xlink:to="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain_f3bca583-8a28-46d8-a1f9-9208a33293ef_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain_77bd2413-0759-49f3-9065-a615929230c8" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis_f3bca583-8a28-46d8-a1f9-9208a33293ef" xlink:to="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain_77bd2413-0759-49f3-9065-a615929230c8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_Range1Member_ba751d33-0de0-4cac-bd83-ce0ecf3eaacd" xlink:href="iclr-20211231.xsd#iclr_Range1Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain_77bd2413-0759-49f3-9065-a615929230c8" xlink:to="loc_iclr_Range1Member_ba751d33-0de0-4cac-bd83-ce0ecf3eaacd" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_Range2Member_0a09a51f-0f6c-4862-923d-3b4ac0b4b37e" xlink:href="iclr-20211231.xsd#iclr_Range2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain_77bd2413-0759-49f3-9065-a615929230c8" xlink:to="loc_iclr_Range2Member_0a09a51f-0f6c-4862-923d-3b4ac0b4b37e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_Range3Member_c62e5413-f97e-420f-8535-805a836f0641" xlink:href="iclr-20211231.xsd#iclr_Range3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain_77bd2413-0759-49f3-9065-a615929230c8" xlink:to="loc_iclr_Range3Member_c62e5413-f97e-420f-8535-805a836f0641" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_Range4Member_3ccb1ad6-9728-433e-9ec0-b55c06940a78" xlink:href="iclr-20211231.xsd#iclr_Range4Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain_77bd2413-0759-49f3-9065-a615929230c8" xlink:to="loc_iclr_Range4Member_3ccb1ad6-9728-433e-9ec0-b55c06940a78" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/EquityIncentiveSchemesandStockCompensationChargesScheduleofWeightedAverageFairValuesandAssumptionsUsedDetails" xlink:type="simple" xlink:href="iclr-20211231.xsd#EquityIncentiveSchemesandStockCompensationChargesScheduleofWeightedAverageFairValuesandAssumptionsUsedDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/EquityIncentiveSchemesandStockCompensationChargesScheduleofWeightedAverageFairValuesandAssumptionsUsedDetails" xlink:type="extended" id="i1fdba5b4176c4f9dbb98948a33b3dd1b_EquityIncentiveSchemesandStockCompensationChargesScheduleofWeightedAverageFairValuesandAssumptionsUsedDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_462a79c7-8025-4619-a9d1-a55ab21a5088" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_b3b65784-1c44-4032-94d9-be4875bd1232" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_462a79c7-8025-4619-a9d1-a55ab21a5088" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_b3b65784-1c44-4032-94d9-be4875bd1232" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsAndMethodologyAbstract_56735f10-a337-4cf6-9ae9-642f18a24af5" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsAndMethodologyAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_462a79c7-8025-4619-a9d1-a55ab21a5088" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsAndMethodologyAbstract_56735f10-a337-4cf6-9ae9-642f18a24af5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_6515fdcb-cb77-486a-8c3d-b6e212c6617a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsAndMethodologyAbstract_56735f10-a337-4cf6-9ae9-642f18a24af5" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_6515fdcb-cb77-486a-8c3d-b6e212c6617a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_31561450-bc83-46bf-94fb-61f9b33304fd" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsAndMethodologyAbstract_56735f10-a337-4cf6-9ae9-642f18a24af5" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_31561450-bc83-46bf-94fb-61f9b33304fd" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_5624b8bb-78ae-4823-b44c-37ba8f05769f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsAndMethodologyAbstract_56735f10-a337-4cf6-9ae9-642f18a24af5" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_5624b8bb-78ae-4823-b44c-37ba8f05769f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_92a4ec40-00d6-42cd-b491-8bda31a9972e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsAndMethodologyAbstract_56735f10-a337-4cf6-9ae9-642f18a24af5" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_92a4ec40-00d6-42cd-b491-8bda31a9972e" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_839c693b-618d-439c-b9d8-d8cb14a1ed9d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_462a79c7-8025-4619-a9d1-a55ab21a5088" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_839c693b-618d-439c-b9d8-d8cb14a1ed9d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_bb63480f-be4d-4faf-b17d-079da9a72843" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_839c693b-618d-439c-b9d8-d8cb14a1ed9d" xlink:to="loc_us-gaap_AwardTypeAxis_bb63480f-be4d-4faf-b17d-079da9a72843" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_bb63480f-be4d-4faf-b17d-079da9a72843_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_bb63480f-be4d-4faf-b17d-079da9a72843" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_bb63480f-be4d-4faf-b17d-079da9a72843_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_a5fbcc56-d59b-4222-b87b-6b2eb15ab3b5" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_bb63480f-be4d-4faf-b17d-079da9a72843" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_a5fbcc56-d59b-4222-b87b-6b2eb15ab3b5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_e32c5b79-f11e-4ffc-97ae-6e595d82ffbf" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_a5fbcc56-d59b-4222-b87b-6b2eb15ab3b5" xlink:to="loc_us-gaap_EmployeeStockOptionMember_e32c5b79-f11e-4ffc-97ae-6e595d82ffbf" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/EquityIncentiveSchemesandStockCompensationChargesSummaryofRSUandPSUActivityDetails" xlink:type="simple" xlink:href="iclr-20211231.xsd#EquityIncentiveSchemesandStockCompensationChargesSummaryofRSUandPSUActivityDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/EquityIncentiveSchemesandStockCompensationChargesSummaryofRSUandPSUActivityDetails" xlink:type="extended" id="i7f91e7fb3c694759b040e27507f76a70_EquityIncentiveSchemesandStockCompensationChargesSummaryofRSUandPSUActivityDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_d75859ce-0cf3-4c94-b8dc-c1067b55cb0c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_7b21c0b8-7cbe-49ab-8191-102d90a0d53b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_d75859ce-0cf3-4c94-b8dc-c1067b55cb0c" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_7b21c0b8-7cbe-49ab-8191-102d90a0d53b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_8295a9a0-7af7-41ff-8acf-b53837c89581" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_7b21c0b8-7cbe-49ab-8191-102d90a0d53b" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_8295a9a0-7af7-41ff-8acf-b53837c89581" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAssumedThroughBusinessCombination_f2afc779-7b76-4cee-9541-27b5b721de9a" xlink:href="iclr-20211231.xsd#iclr_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAssumedThroughBusinessCombination"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_7b21c0b8-7cbe-49ab-8191-102d90a0d53b" xlink:to="loc_iclr_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAssumedThroughBusinessCombination_f2afc779-7b76-4cee-9541-27b5b721de9a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_cf471478-f315-4125-bc8c-117f4bde84ed" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_7b21c0b8-7cbe-49ab-8191-102d90a0d53b" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_cf471478-f315-4125-bc8c-117f4bde84ed" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_0f9fe06f-149f-44e8-8b68-35b03a1ebc85" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_7b21c0b8-7cbe-49ab-8191-102d90a0d53b" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_0f9fe06f-149f-44e8-8b68-35b03a1ebc85" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_2df7bf66-1349-42ac-bbb9-5f12a2dd6ef1" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_7b21c0b8-7cbe-49ab-8191-102d90a0d53b" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_2df7bf66-1349-42ac-bbb9-5f12a2dd6ef1" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_dd7bf316-d130-487a-b7fa-608cbe2d6aeb" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_a93c478c-c6b0-4cff-b329-91a40b2526a4" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_d75859ce-0cf3-4c94-b8dc-c1067b55cb0c" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_a93c478c-c6b0-4cff-b329-91a40b2526a4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_e82d228d-a8fb-4175-9a2f-236ecd1dca3c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_a93c478c-c6b0-4cff-b329-91a40b2526a4" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_e82d228d-a8fb-4175-9a2f-236ecd1dca3c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsAssumedInBusinessCombinationWeightedAverageExercisePrice_aecbe10e-08a7-4a98-9ff5-ac299314fe49" xlink:href="iclr-20211231.xsd#iclr_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsAssumedInBusinessCombinationWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_a93c478c-c6b0-4cff-b329-91a40b2526a4" xlink:to="loc_iclr_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsAssumedInBusinessCombinationWeightedAverageExercisePrice_aecbe10e-08a7-4a98-9ff5-ac299314fe49" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_86cdfcce-7d9a-4221-8c34-e8ed48ed2085" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_a93c478c-c6b0-4cff-b329-91a40b2526a4" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_86cdfcce-7d9a-4221-8c34-e8ed48ed2085" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_891f38df-7fc9-4330-904d-a4433092c5cb" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_a93c478c-c6b0-4cff-b329-91a40b2526a4" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_891f38df-7fc9-4330-904d-a4433092c5cb" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_573fa74e-47ec-43bf-b50d-3eb65c8f79f0" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_a93c478c-c6b0-4cff-b329-91a40b2526a4" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_573fa74e-47ec-43bf-b50d-3eb65c8f79f0" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_8bbac6b3-9333-4a37-8ce8-8c66cca5cf8f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_ed5da16c-0a75-4d9e-b636-b0915ce010cb" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_d75859ce-0cf3-4c94-b8dc-c1067b55cb0c" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_ed5da16c-0a75-4d9e-b636-b0915ce010cb" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_215f002f-7c01-411a-ad5f-141ee3954ce9" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_ed5da16c-0a75-4d9e-b636-b0915ce010cb" xlink:to="loc_us-gaap_AwardTypeAxis_215f002f-7c01-411a-ad5f-141ee3954ce9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_215f002f-7c01-411a-ad5f-141ee3954ce9_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_215f002f-7c01-411a-ad5f-141ee3954ce9" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_215f002f-7c01-411a-ad5f-141ee3954ce9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_3869a23e-0813-4819-88b5-b9cb333a9e56" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_215f002f-7c01-411a-ad5f-141ee3954ce9" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_3869a23e-0813-4819-88b5-b9cb333a9e56" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_PerformanceShareUnitMember_f64362b6-a651-4322-880d-2b6395e5a63b" xlink:href="iclr-20211231.xsd#iclr_PerformanceShareUnitMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_3869a23e-0813-4819-88b5-b9cb333a9e56" xlink:to="loc_iclr_PerformanceShareUnitMember_f64362b6-a651-4322-880d-2b6395e5a63b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_ffff41c0-3ad8-4e75-aff8-b7c09ba72bdb" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_3869a23e-0813-4819-88b5-b9cb333a9e56" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_ffff41c0-3ad8-4e75-aff8-b7c09ba72bdb" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/EquityIncentiveSchemesandStockCompensationChargesScheduleofNoncashStockCompensationExpenseDetails" xlink:type="simple" xlink:href="iclr-20211231.xsd#EquityIncentiveSchemesandStockCompensationChargesScheduleofNoncashStockCompensationExpenseDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/EquityIncentiveSchemesandStockCompensationChargesScheduleofNoncashStockCompensationExpenseDetails" xlink:type="extended" id="i230b7bda8cd24e91837a607af20060ea_EquityIncentiveSchemesandStockCompensationChargesScheduleofNoncashStockCompensationExpenseDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_a8b7ae2c-4023-4bcc-9a9d-be9d215e5ff1" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensation_81c66f09-4274-4af7-976a-7f547c57ab7e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_a8b7ae2c-4023-4bcc-9a9d-be9d215e5ff1" xlink:to="loc_us-gaap_ShareBasedCompensation_81c66f09-4274-4af7-976a-7f547c57ab7e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_4a09209c-fef7-4e9c-a23b-0d248b9b4c11" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_a8b7ae2c-4023-4bcc-9a9d-be9d215e5ff1" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_4a09209c-fef7-4e9c-a23b-0d248b9b4c11" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_0fa7ad2d-cba6-408b-b4c4-dfd59bbb06ce" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeStatementLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_4a09209c-fef7-4e9c-a23b-0d248b9b4c11" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_0fa7ad2d-cba6-408b-b4c4-dfd59bbb06ce" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_0fa7ad2d-cba6-408b-b4c4-dfd59bbb06ce_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_0fa7ad2d-cba6-408b-b4c4-dfd59bbb06ce" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_0fa7ad2d-cba6-408b-b4c4-dfd59bbb06ce_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_504ae7ba-46fe-4a81-ad16-1efd9e2fa38b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_0fa7ad2d-cba6-408b-b4c4-dfd59bbb06ce" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_504ae7ba-46fe-4a81-ad16-1efd9e2fa38b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostOfSalesMember_6e1bc4da-16db-4ef8-b992-82c786db2c48" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CostOfSalesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_504ae7ba-46fe-4a81-ad16-1efd9e2fa38b" xlink:to="loc_us-gaap_CostOfSalesMember_6e1bc4da-16db-4ef8-b992-82c786db2c48" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SellingGeneralAndAdministrativeExpensesMember_0b9c8751-cdc7-401d-b570-cd8a3ed0f20f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SellingGeneralAndAdministrativeExpensesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_504ae7ba-46fe-4a81-ad16-1efd9e2fa38b" xlink:to="loc_us-gaap_SellingGeneralAndAdministrativeExpensesMember_0b9c8751-cdc7-401d-b570-cd8a3ed0f20f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_TransactionAndIntegrationExpenseMember_f6ae58de-d95f-4f5f-868d-51d06de79be9" xlink:href="iclr-20211231.xsd#iclr_TransactionAndIntegrationExpenseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_504ae7ba-46fe-4a81-ad16-1efd9e2fa38b" xlink:to="loc_iclr_TransactionAndIntegrationExpenseMember_f6ae58de-d95f-4f5f-868d-51d06de79be9" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/ShareCapitalDetails" xlink:type="simple" xlink:href="iclr-20211231.xsd#ShareCapitalDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/ShareCapitalDetails" xlink:type="extended" id="i8ef14ea96cf94901a1739500994d43d7_ShareCapitalDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityClassOfTreasuryStockLineItems_42b8b1ca-1866-4212-9eb3-6ade1da4b9fe" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EquityClassOfTreasuryStockLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_NumberOfVotes_3cabfcb8-9761-478d-ad30-82615a496e53" xlink:href="iclr-20211231.xsd#iclr_NumberOfVotes"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityClassOfTreasuryStockLineItems_42b8b1ca-1866-4212-9eb3-6ade1da4b9fe" xlink:to="loc_iclr_NumberOfVotes_3cabfcb8-9761-478d-ad30-82615a496e53" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesAcquisitions_8498927b-38d0-4a3b-a840-73d7c632dcfe" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesAcquisitions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityClassOfTreasuryStockLineItems_42b8b1ca-1866-4212-9eb3-6ade1da4b9fe" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesAcquisitions_8498927b-38d0-4a3b-a840-73d7c632dcfe" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_1620cfb7-ce67-4aaf-a1aa-eee21bbfeffa" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityClassOfTreasuryStockLineItems_42b8b1ca-1866-4212-9eb3-6ade1da4b9fe" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_1620cfb7-ce67-4aaf-a1aa-eee21bbfeffa" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice_d23dc9bf-e638-4120-a650-5377582b848a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityClassOfTreasuryStockLineItems_42b8b1ca-1866-4212-9eb3-6ade1da4b9fe" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice_d23dc9bf-e638-4120-a650-5377582b848a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromStockOptionsExercised_acb2f5a7-1173-411b-b004-65a2fd22be92" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ProceedsFromStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityClassOfTreasuryStockLineItems_42b8b1ca-1866-4212-9eb3-6ade1da4b9fe" xlink:to="loc_us-gaap_ProceedsFromStockOptionsExercised_acb2f5a7-1173-411b-b004-65a2fd22be92" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardNetOfForfeitures_3b331642-9119-4750-8ca4-a4127c87de36" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardNetOfForfeitures"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityClassOfTreasuryStockLineItems_42b8b1ca-1866-4212-9eb3-6ade1da4b9fe" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardNetOfForfeitures_3b331642-9119-4750-8ca4-a4127c87de36" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensationGross_81471060-889b-409e-b291-6d2fe12c7bd0" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensationGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityClassOfTreasuryStockLineItems_42b8b1ca-1866-4212-9eb3-6ade1da4b9fe" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensationGross_81471060-889b-409e-b291-6d2fe12c7bd0" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_StockRepurchaseProgramAuthorizedPercentage_0044edaf-aea4-4edf-8d4e-0ae48a49e708" xlink:href="iclr-20211231.xsd#iclr_StockRepurchaseProgramAuthorizedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityClassOfTreasuryStockLineItems_42b8b1ca-1866-4212-9eb3-6ade1da4b9fe" xlink:to="loc_iclr_StockRepurchaseProgramAuthorizedPercentage_0044edaf-aea4-4edf-8d4e-0ae48a49e708" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchaseProgramAuthorizedAmount1_48c6a32f-ce04-4f2f-b4bf-afd75fab99d3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockRepurchaseProgramAuthorizedAmount1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityClassOfTreasuryStockLineItems_42b8b1ca-1866-4212-9eb3-6ade1da4b9fe" xlink:to="loc_us-gaap_StockRepurchaseProgramAuthorizedAmount1_48c6a32f-ce04-4f2f-b4bf-afd75fab99d3" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodShares_04c2d87c-feb0-4f74-aeaf-e8567154d2d7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockRepurchasedAndRetiredDuringPeriodShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityClassOfTreasuryStockLineItems_42b8b1ca-1866-4212-9eb3-6ade1da4b9fe" xlink:to="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodShares_04c2d87c-feb0-4f74-aeaf-e8567154d2d7" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodValue_26d214bd-1b3c-45ac-afd5-319768312271" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockRepurchasedAndRetiredDuringPeriodValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityClassOfTreasuryStockLineItems_42b8b1ca-1866-4212-9eb3-6ade1da4b9fe" xlink:to="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodValue_26d214bd-1b3c-45ac-afd5-319768312271" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchaseProgramNumberOfSharesAuthorizedToBeRepurchased_6c2402a1-3e3a-40bb-aeb6-211afd5d48d1" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockRepurchaseProgramNumberOfSharesAuthorizedToBeRepurchased"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityClassOfTreasuryStockLineItems_42b8b1ca-1866-4212-9eb3-6ade1da4b9fe" xlink:to="loc_us-gaap_StockRepurchaseProgramNumberOfSharesAuthorizedToBeRepurchased_6c2402a1-3e3a-40bb-aeb6-211afd5d48d1" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfTreasuryStockTable_c0a9e1ba-2f84-4f29-b5a4-dc4caad241eb" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ClassOfTreasuryStockTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EquityClassOfTreasuryStockLineItems_42b8b1ca-1866-4212-9eb3-6ade1da4b9fe" xlink:to="loc_us-gaap_ClassOfTreasuryStockTable_c0a9e1ba-2f84-4f29-b5a4-dc4caad241eb" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_8bab64cb-3002-42e1-9b62-6a0180196905" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ClassOfTreasuryStockTable_c0a9e1ba-2f84-4f29-b5a4-dc4caad241eb" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_8bab64cb-3002-42e1-9b62-6a0180196905" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_8bab64cb-3002-42e1-9b62-6a0180196905_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_8bab64cb-3002-42e1-9b62-6a0180196905" xlink:to="loc_us-gaap_EquityComponentDomain_8bab64cb-3002-42e1-9b62-6a0180196905_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_ce0fcbd0-f4f1-4752-a0e7-c3852c25f2c7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_8bab64cb-3002-42e1-9b62-6a0180196905" xlink:to="loc_us-gaap_EquityComponentDomain_ce0fcbd0-f4f1-4752-a0e7-c3852c25f2c7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_930e2103-3d60-42a0-a6e3-34689e8c975d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_ce0fcbd0-f4f1-4752-a0e7-c3852c25f2c7" xlink:to="loc_us-gaap_CommonStockMember_930e2103-3d60-42a0-a6e3-34689e8c975d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_29dbc014-a86d-450e-a6f9-a8d713a83455" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ClassOfTreasuryStockTable_c0a9e1ba-2f84-4f29-b5a4-dc4caad241eb" xlink:to="loc_us-gaap_AwardTypeAxis_29dbc014-a86d-450e-a6f9-a8d713a83455" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_29dbc014-a86d-450e-a6f9-a8d713a83455_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_29dbc014-a86d-450e-a6f9-a8d713a83455" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_29dbc014-a86d-450e-a6f9-a8d713a83455_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_77218520-c437-4764-a374-c519b2d91639" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_29dbc014-a86d-450e-a6f9-a8d713a83455" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_77218520-c437-4764-a374-c519b2d91639" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_4cd6e968-a429-47ef-9b44-414aece103ab" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_77218520-c437-4764-a374-c519b2d91639" xlink:to="loc_us-gaap_EmployeeStockOptionMember_4cd6e968-a429-47ef-9b44-414aece103ab" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_40e8a482-1154-42be-a78f-3ffb56fdc7f1" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_77218520-c437-4764-a374-c519b2d91639" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_40e8a482-1154-42be-a78f-3ffb56fdc7f1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_PerformanceShareUnitMember_e4521dba-fce9-42fd-947f-1f27f9c3f1fd" xlink:href="iclr-20211231.xsd#iclr_PerformanceShareUnitMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_77218520-c437-4764-a374-c519b2d91639" xlink:to="loc_iclr_PerformanceShareUnitMember_e4521dba-fce9-42fd-947f-1f27f9c3f1fd" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_a85bbd7d-8983-4f13-8fda-efc9b3a2fcf1" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ClassOfTreasuryStockTable_c0a9e1ba-2f84-4f29-b5a4-dc4caad241eb" xlink:to="loc_srt_RangeAxis_a85bbd7d-8983-4f13-8fda-efc9b3a2fcf1" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_a85bbd7d-8983-4f13-8fda-efc9b3a2fcf1_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_a85bbd7d-8983-4f13-8fda-efc9b3a2fcf1" xlink:to="loc_srt_RangeMember_a85bbd7d-8983-4f13-8fda-efc9b3a2fcf1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_3c59cdd5-5a90-4992-ac19-4865f3ea55a3" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_a85bbd7d-8983-4f13-8fda-efc9b3a2fcf1" xlink:to="loc_srt_RangeMember_3c59cdd5-5a90-4992-ac19-4865f3ea55a3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_9e616d47-cb02-4aeb-9b92-8a4159d05217" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_3c59cdd5-5a90-4992-ac19-4865f3ea55a3" xlink:to="loc_srt_MaximumMember_9e616d47-cb02-4aeb-9b92-8a4159d05217" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareRepurchaseProgramAxis_f6952d21-01a8-43ee-9955-82c85375fed7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareRepurchaseProgramAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ClassOfTreasuryStockTable_c0a9e1ba-2f84-4f29-b5a4-dc4caad241eb" xlink:to="loc_us-gaap_ShareRepurchaseProgramAxis_f6952d21-01a8-43ee-9955-82c85375fed7" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareRepurchaseProgramDomain_f6952d21-01a8-43ee-9955-82c85375fed7_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareRepurchaseProgramDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ShareRepurchaseProgramAxis_f6952d21-01a8-43ee-9955-82c85375fed7" xlink:to="loc_us-gaap_ShareRepurchaseProgramDomain_f6952d21-01a8-43ee-9955-82c85375fed7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareRepurchaseProgramDomain_5c7181dd-ffd5-41a5-9f42-36740655f468" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareRepurchaseProgramDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ShareRepurchaseProgramAxis_f6952d21-01a8-43ee-9955-82c85375fed7" xlink:to="loc_us-gaap_ShareRepurchaseProgramDomain_5c7181dd-ffd5-41a5-9f42-36740655f468" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BuybackProgramMember_4f0cb25e-3f1f-43b6-9264-717b6c08e04f" xlink:href="iclr-20211231.xsd#iclr_BuybackProgramMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareRepurchaseProgramDomain_5c7181dd-ffd5-41a5-9f42-36740655f468" xlink:to="loc_iclr_BuybackProgramMember_4f0cb25e-3f1f-43b6-9264-717b6c08e04f" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/IncomeTaxesComponentsofIncomeBeforeProvisionforIncomeTaxesDetails" xlink:type="simple" xlink:href="iclr-20211231.xsd#IncomeTaxesComponentsofIncomeBeforeProvisionforIncomeTaxesDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/IncomeTaxesComponentsofIncomeBeforeProvisionforIncomeTaxesDetails" xlink:type="extended" id="i2451a339776e4b309a8b2e288fe184ce_IncomeTaxesComponentsofIncomeBeforeProvisionforIncomeTaxesDetails">
    <link:loc xlink:type="locator" xlink:label="loc_iclr_ScheduleOfComponentsOfIncomeBeforeIncomeTaxExpenseBenefitLineItems_71c37797-0c33-44f2-a1f1-8f3422488e1c" xlink:href="iclr-20211231.xsd#iclr_ScheduleOfComponentsOfIncomeBeforeIncomeTaxExpenseBenefitLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments_a62702fa-eda0-4ef0-9b04-4c90d50c5fa1" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iclr_ScheduleOfComponentsOfIncomeBeforeIncomeTaxExpenseBenefitLineItems_71c37797-0c33-44f2-a1f1-8f3422488e1c" xlink:to="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments_a62702fa-eda0-4ef0-9b04-4c90d50c5fa1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_ScheduleOfComponentsOfIncomeBeforeIncomeTaxExpenseBenefitTable_6edb4f08-7ec5-45ce-afb9-8c3d88a5fbef" xlink:href="iclr-20211231.xsd#iclr_ScheduleOfComponentsOfIncomeBeforeIncomeTaxExpenseBenefitTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_iclr_ScheduleOfComponentsOfIncomeBeforeIncomeTaxExpenseBenefitLineItems_71c37797-0c33-44f2-a1f1-8f3422488e1c" xlink:to="loc_iclr_ScheduleOfComponentsOfIncomeBeforeIncomeTaxExpenseBenefitTable_6edb4f08-7ec5-45ce-afb9-8c3d88a5fbef" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementGeographicalAxis_1514d104-f942-4861-8ca3-8fd6709bc735" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_StatementGeographicalAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_iclr_ScheduleOfComponentsOfIncomeBeforeIncomeTaxExpenseBenefitTable_6edb4f08-7ec5-45ce-afb9-8c3d88a5fbef" xlink:to="loc_srt_StatementGeographicalAxis_1514d104-f942-4861-8ca3-8fd6709bc735" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_1514d104-f942-4861-8ca3-8fd6709bc735_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_1514d104-f942-4861-8ca3-8fd6709bc735" xlink:to="loc_srt_SegmentGeographicalDomain_1514d104-f942-4861-8ca3-8fd6709bc735_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_2b4a72f2-8a74-4655-af10-ceb3aefd80c8" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_1514d104-f942-4861-8ca3-8fd6709bc735" xlink:to="loc_srt_SegmentGeographicalDomain_2b4a72f2-8a74-4655-af10-ceb3aefd80c8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_IE_ac131d7a-68ab-4a35-924e-f7340d74ce1f" xlink:href="https://xbrl.sec.gov/country/2021/country-2021.xsd#country_IE"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_2b4a72f2-8a74-4655-af10-ceb3aefd80c8" xlink:to="loc_country_IE_ac131d7a-68ab-4a35-924e-f7340d74ce1f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_US_6b8da3ac-7e2b-4eea-9a2d-70a2e0074585" xlink:href="https://xbrl.sec.gov/country/2021/country-2021.xsd#country_US"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_2b4a72f2-8a74-4655-af10-ceb3aefd80c8" xlink:to="loc_country_US_6b8da3ac-7e2b-4eea-9a2d-70a2e0074585" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_RestOfEuropeAndOtherCountriesMember_07e5d0fa-fbaf-4566-96bd-02d426ed44ee" xlink:href="iclr-20211231.xsd#iclr_RestOfEuropeAndOtherCountriesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_2b4a72f2-8a74-4655-af10-ceb3aefd80c8" xlink:to="loc_iclr_RestOfEuropeAndOtherCountriesMember_07e5d0fa-fbaf-4566-96bd-02d426ed44ee" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/IncomeTaxesComponentsofProvisionforIncomeTaxesDetails" xlink:type="simple" xlink:href="iclr-20211231.xsd#IncomeTaxesComponentsofProvisionforIncomeTaxesDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/IncomeTaxesComponentsofProvisionforIncomeTaxesDetails" xlink:type="extended" id="i5a0c15e3903646c298cca8d07fe002aa_IncomeTaxesComponentsofProvisionforIncomeTaxesDetails">
    <link:loc xlink:type="locator" xlink:label="loc_iclr_ReconciliationOfProvisionOfIncomeTaxesLineItems_45b8cc9e-3e7a-48e4-8f0b-337a897da42d" xlink:href="iclr-20211231.xsd#iclr_ReconciliationOfProvisionOfIncomeTaxesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxExpenseBenefitContinuingOperationsAbstract_c67b40bb-2b78-403c-8d70-1fda6a87949b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeTaxExpenseBenefitContinuingOperationsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iclr_ReconciliationOfProvisionOfIncomeTaxesLineItems_45b8cc9e-3e7a-48e4-8f0b-337a897da42d" xlink:to="loc_us-gaap_IncomeTaxExpenseBenefitContinuingOperationsAbstract_c67b40bb-2b78-403c-8d70-1fda6a87949b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CurrentIncomeTaxExpenseBenefitContinuingOperationsAbstract_97d65f8a-b9cc-471f-bd12-9a95c8a8c25e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CurrentIncomeTaxExpenseBenefitContinuingOperationsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxExpenseBenefitContinuingOperationsAbstract_c67b40bb-2b78-403c-8d70-1fda6a87949b" xlink:to="loc_us-gaap_CurrentIncomeTaxExpenseBenefitContinuingOperationsAbstract_97d65f8a-b9cc-471f-bd12-9a95c8a8c25e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CurrentIncomeTaxExpenseBenefit_51abc0be-1f62-4ab0-a05d-b90b16877db4" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CurrentIncomeTaxExpenseBenefit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CurrentIncomeTaxExpenseBenefitContinuingOperationsAbstract_97d65f8a-b9cc-471f-bd12-9a95c8a8c25e" xlink:to="loc_us-gaap_CurrentIncomeTaxExpenseBenefit_51abc0be-1f62-4ab0-a05d-b90b16877db4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredIncomeTaxExpenseBenefitContinuingOperationsAbstract_4b351f25-3f2c-4b0c-a857-6286cf56fd3e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DeferredIncomeTaxExpenseBenefitContinuingOperationsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxExpenseBenefitContinuingOperationsAbstract_c67b40bb-2b78-403c-8d70-1fda6a87949b" xlink:to="loc_us-gaap_DeferredIncomeTaxExpenseBenefitContinuingOperationsAbstract_4b351f25-3f2c-4b0c-a857-6286cf56fd3e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredIncomeTaxExpenseBenefit_e1bc701f-272b-4280-815a-ef313c04cd25" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DeferredIncomeTaxExpenseBenefit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DeferredIncomeTaxExpenseBenefitContinuingOperationsAbstract_4b351f25-3f2c-4b0c-a857-6286cf56fd3e" xlink:to="loc_us-gaap_DeferredIncomeTaxExpenseBenefit_e1bc701f-272b-4280-815a-ef313c04cd25" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxExpenseBenefit_f09ff226-e52a-48e4-86a4-703118699005" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeTaxExpenseBenefit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxExpenseBenefitContinuingOperationsAbstract_c67b40bb-2b78-403c-8d70-1fda6a87949b" xlink:to="loc_us-gaap_IncomeTaxExpenseBenefit_f09ff226-e52a-48e4-86a4-703118699005" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentToAdditionalPaidInCapitalIncomeTaxEffectFromShareBasedCompensationNetAbstract_2986bc59-4b9a-420a-a2da-d1499276ee32" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AdjustmentToAdditionalPaidInCapitalIncomeTaxEffectFromShareBasedCompensationNetAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iclr_ReconciliationOfProvisionOfIncomeTaxesLineItems_45b8cc9e-3e7a-48e4-8f0b-337a897da42d" xlink:to="loc_us-gaap_AdjustmentToAdditionalPaidInCapitalIncomeTaxEffectFromShareBasedCompensationNetAbstract_2986bc59-4b9a-420a-a2da-d1499276ee32" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeForeignCurrencyTranslationGainLossArisingDuringPeriodTax_bbc3f1ec-8844-449b-b7d2-64aacc122cda" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OtherComprehensiveIncomeForeignCurrencyTranslationGainLossArisingDuringPeriodTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentToAdditionalPaidInCapitalIncomeTaxEffectFromShareBasedCompensationNetAbstract_2986bc59-4b9a-420a-a2da-d1499276ee32" xlink:to="loc_us-gaap_OtherComprehensiveIncomeForeignCurrencyTranslationGainLossArisingDuringPeriodTax_bbc3f1ec-8844-449b-b7d2-64aacc122cda" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxEffectsAllocatedDirectlyToEquity_34288689-d6fe-48ea-af9d-1efa9892f46e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeTaxEffectsAllocatedDirectlyToEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iclr_ReconciliationOfProvisionOfIncomeTaxesLineItems_45b8cc9e-3e7a-48e4-8f0b-337a897da42d" xlink:to="loc_us-gaap_IncomeTaxEffectsAllocatedDirectlyToEquity_34288689-d6fe-48ea-af9d-1efa9892f46e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_ReconciliationOfProvisionOfIncomeTaxesTable_f7e74641-f824-4286-835e-0526defd02d7" xlink:href="iclr-20211231.xsd#iclr_ReconciliationOfProvisionOfIncomeTaxesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_iclr_ReconciliationOfProvisionOfIncomeTaxesLineItems_45b8cc9e-3e7a-48e4-8f0b-337a897da42d" xlink:to="loc_iclr_ReconciliationOfProvisionOfIncomeTaxesTable_f7e74641-f824-4286-835e-0526defd02d7" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementGeographicalAxis_973594bb-7c50-4a30-a13c-3e94bf0fdb9a" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_StatementGeographicalAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_iclr_ReconciliationOfProvisionOfIncomeTaxesTable_f7e74641-f824-4286-835e-0526defd02d7" xlink:to="loc_srt_StatementGeographicalAxis_973594bb-7c50-4a30-a13c-3e94bf0fdb9a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_973594bb-7c50-4a30-a13c-3e94bf0fdb9a_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_973594bb-7c50-4a30-a13c-3e94bf0fdb9a" xlink:to="loc_srt_SegmentGeographicalDomain_973594bb-7c50-4a30-a13c-3e94bf0fdb9a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_4af77b30-3ab0-487e-8294-b3af80ce8ce6" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_973594bb-7c50-4a30-a13c-3e94bf0fdb9a" xlink:to="loc_srt_SegmentGeographicalDomain_4af77b30-3ab0-487e-8294-b3af80ce8ce6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_IE_c17b47c0-28ed-460d-b5e7-1c58369a44bf" xlink:href="https://xbrl.sec.gov/country/2021/country-2021.xsd#country_IE"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_4af77b30-3ab0-487e-8294-b3af80ce8ce6" xlink:to="loc_country_IE_c17b47c0-28ed-460d-b5e7-1c58369a44bf" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_US_a529e870-457a-47b9-9740-176e923b7641" xlink:href="https://xbrl.sec.gov/country/2021/country-2021.xsd#country_US"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_4af77b30-3ab0-487e-8294-b3af80ce8ce6" xlink:to="loc_country_US_a529e870-457a-47b9-9740-176e923b7641" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_RestOfEuropeAndOtherCountriesMember_d4f1d40e-af8d-44d9-ab90-ca74a18dafdb" xlink:href="iclr-20211231.xsd#iclr_RestOfEuropeAndOtherCountriesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_4af77b30-3ab0-487e-8294-b3af80ce8ce6" xlink:to="loc_iclr_RestOfEuropeAndOtherCountriesMember_d4f1d40e-af8d-44d9-ab90-ca74a18dafdb" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/IncomeTaxesNarrativeDetails" xlink:type="simple" xlink:href="iclr-20211231.xsd#IncomeTaxesNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/IncomeTaxesNarrativeDetails" xlink:type="extended" id="ia2fee3898207481d89dc1d57de82329b_IncomeTaxesNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleLineItems_a33fb041-c21a-4cc3-a127-b5756337fd6d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_DeferredTaxAssetsAdditionalTaxCreditCarryforwardsNotSubjectToExpiration_cf43e1bf-dae3-47e4-92b1-f55765dea83e" xlink:href="iclr-20211231.xsd#iclr_DeferredTaxAssetsAdditionalTaxCreditCarryforwardsNotSubjectToExpiration"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleLineItems_a33fb041-c21a-4cc3-a127-b5756337fd6d" xlink:to="loc_iclr_DeferredTaxAssetsAdditionalTaxCreditCarryforwardsNotSubjectToExpiration_cf43e1bf-dae3-47e4-92b1-f55765dea83e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredTaxAssetsOperatingLossCarryforwards_9a3701f0-fa48-40cd-a4c3-3945e65c64da" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DeferredTaxAssetsOperatingLossCarryforwards"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleLineItems_a33fb041-c21a-4cc3-a127-b5756337fd6d" xlink:to="loc_us-gaap_DeferredTaxAssetsOperatingLossCarryforwards_9a3701f0-fa48-40cd-a4c3-3945e65c64da" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_OperatingLossCarryforwardsUSFederal_2e293457-7162-4df3-88a0-98daf6500680" xlink:href="iclr-20211231.xsd#iclr_OperatingLossCarryforwardsUSFederal"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleLineItems_a33fb041-c21a-4cc3-a127-b5756337fd6d" xlink:to="loc_iclr_OperatingLossCarryforwardsUSFederal_2e293457-7162-4df3-88a0-98daf6500680" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLossCarryforwards_11f85a33-5d5e-45f9-b9c6-2671031bce10" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OperatingLossCarryforwards"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleLineItems_a33fb041-c21a-4cc3-a127-b5756337fd6d" xlink:to="loc_us-gaap_OperatingLossCarryforwards_11f85a33-5d5e-45f9-b9c6-2671031bce10" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TaxCreditCarryforwardAmount_3db68401-f3e7-4f9d-b3a0-52bc52a43234" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TaxCreditCarryforwardAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleLineItems_a33fb041-c21a-4cc3-a127-b5756337fd6d" xlink:to="loc_us-gaap_TaxCreditCarryforwardAmount_3db68401-f3e7-4f9d-b3a0-52bc52a43234" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsNotSubjectToExpiration_12d086d8-bc89-4611-a19b-7cdb3791625b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsNotSubjectToExpiration"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleLineItems_a33fb041-c21a-4cc3-a127-b5756337fd6d" xlink:to="loc_us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsNotSubjectToExpiration_12d086d8-bc89-4611-a19b-7cdb3791625b" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsSubjectToExpiration_5b614132-a9d7-4961-9dd5-ebb0f27305d2" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsSubjectToExpiration"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleLineItems_a33fb041-c21a-4cc3-a127-b5756337fd6d" xlink:to="loc_us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsSubjectToExpiration_5b614132-a9d7-4961-9dd5-ebb0f27305d2" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsAlternativeMinimumTax_dc0e994e-aa07-41d4-92eb-2a10aa31ce13" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsAlternativeMinimumTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleLineItems_a33fb041-c21a-4cc3-a127-b5756337fd6d" xlink:to="loc_us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsAlternativeMinimumTax_dc0e994e-aa07-41d4-92eb-2a10aa31ce13" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredTaxAssetsValuationAllowance_648d3d4b-a964-45f0-9a66-9a4b890db1e6" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DeferredTaxAssetsValuationAllowance"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleLineItems_a33fb041-c21a-4cc3-a127-b5756337fd6d" xlink:to="loc_us-gaap_DeferredTaxAssetsValuationAllowance_648d3d4b-a964-45f0-9a66-9a4b890db1e6" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount_1d335ceb-2df3-4f96-9053-821221cc14d7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleLineItems_a33fb041-c21a-4cc3-a127-b5756337fd6d" xlink:to="loc_us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount_1d335ceb-2df3-4f96-9053-821221cc14d7" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredTaxLiabilitiesUndistributedForeignEarnings_4baaaa76-ee30-412f-ae35-3b09f4c2d6f7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DeferredTaxLiabilitiesUndistributedForeignEarnings"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleLineItems_a33fb041-c21a-4cc3-a127-b5756337fd6d" xlink:to="loc_us-gaap_DeferredTaxLiabilitiesUndistributedForeignEarnings_4baaaa76-ee30-412f-ae35-3b09f4c2d6f7" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleAmountOfUnrecordedBenefit_774ed01d-781c-419b-9bcf-95f3b5a67601" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleAmountOfUnrecordedBenefit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleLineItems_a33fb041-c21a-4cc3-a127-b5756337fd6d" xlink:to="loc_us-gaap_SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleAmountOfUnrecordedBenefit_774ed01d-781c-419b-9bcf-95f3b5a67601" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate_580cc4b3-cd10-44da-b735-06c3b2963a7d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleLineItems_a33fb041-c21a-4cc3-a127-b5756337fd6d" xlink:to="loc_us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate_580cc4b3-cd10-44da-b735-06c3b2963a7d" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestExpense_d4cc66ec-bab9-4c41-af39-d4176ee66139" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleLineItems_a33fb041-c21a-4cc3-a127-b5756337fd6d" xlink:to="loc_us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestExpense_d4cc66ec-bab9-4c41-af39-d4176ee66139" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued_cffe1374-15ba-4016-aaca-0e92cd89a692" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleLineItems_a33fb041-c21a-4cc3-a127-b5756337fd6d" xlink:to="loc_us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued_cffe1374-15ba-4016-aaca-0e92cd89a692" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleTable_f0c8888f-c884-43e0-b49d-fd47d300ecf7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleLineItems_a33fb041-c21a-4cc3-a127-b5756337fd6d" xlink:to="loc_us-gaap_SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleTable_f0c8888f-c884-43e0-b49d-fd47d300ecf7" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementGeographicalAxis_eafeadb6-d901-44ec-99b3-36113709d901" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_StatementGeographicalAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleTable_f0c8888f-c884-43e0-b49d-fd47d300ecf7" xlink:to="loc_srt_StatementGeographicalAxis_eafeadb6-d901-44ec-99b3-36113709d901" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_eafeadb6-d901-44ec-99b3-36113709d901_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_eafeadb6-d901-44ec-99b3-36113709d901" xlink:to="loc_srt_SegmentGeographicalDomain_eafeadb6-d901-44ec-99b3-36113709d901_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_4b6feb18-bc5d-4fc1-8551-063afe383f11" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_eafeadb6-d901-44ec-99b3-36113709d901" xlink:to="loc_srt_SegmentGeographicalDomain_4b6feb18-bc5d-4fc1-8551-063afe383f11" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_IE_2f1eb772-34ba-4e3e-81ed-3fce5c9fca54" xlink:href="https://xbrl.sec.gov/country/2021/country-2021.xsd#country_IE"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_4b6feb18-bc5d-4fc1-8551-063afe383f11" xlink:to="loc_country_IE_2f1eb772-34ba-4e3e-81ed-3fce5c9fca54" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_US_51466191-7733-44ae-9438-50fdfc926c69" xlink:href="https://xbrl.sec.gov/country/2021/country-2021.xsd#country_US"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_4b6feb18-bc5d-4fc1-8551-063afe383f11" xlink:to="loc_country_US_51466191-7733-44ae-9438-50fdfc926c69" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_OtherCountriesMember_8206f299-ac43-4a1b-b8db-431796680ed1" xlink:href="iclr-20211231.xsd#iclr_OtherCountriesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_4b6feb18-bc5d-4fc1-8551-063afe383f11" xlink:to="loc_iclr_OtherCountriesMember_8206f299-ac43-4a1b-b8db-431796680ed1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityAxis_1cd53a76-b5c8-4bfc-ba88-b156b81ebe35" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeTaxAuthorityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleTable_f0c8888f-c884-43e0-b49d-fd47d300ecf7" xlink:to="loc_us-gaap_IncomeTaxAuthorityAxis_1cd53a76-b5c8-4bfc-ba88-b156b81ebe35" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityDomain_1cd53a76-b5c8-4bfc-ba88-b156b81ebe35_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeTaxAuthorityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeTaxAuthorityAxis_1cd53a76-b5c8-4bfc-ba88-b156b81ebe35" xlink:to="loc_us-gaap_IncomeTaxAuthorityDomain_1cd53a76-b5c8-4bfc-ba88-b156b81ebe35_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityDomain_42953620-cd8c-4a5f-a355-0c2e3d7778ad" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeTaxAuthorityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeTaxAuthorityAxis_1cd53a76-b5c8-4bfc-ba88-b156b81ebe35" xlink:to="loc_us-gaap_IncomeTaxAuthorityDomain_42953620-cd8c-4a5f-a355-0c2e3d7778ad" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForeignCountryMember_0c176507-2a2f-4217-84a3-ae0239a8e9bc" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ForeignCountryMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityDomain_42953620-cd8c-4a5f-a355-0c2e3d7778ad" xlink:to="loc_us-gaap_ForeignCountryMember_0c176507-2a2f-4217-84a3-ae0239a8e9bc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StateAndLocalJurisdictionMember_3d57e98b-5706-4e05-83f9-0dd1e0211ed6" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StateAndLocalJurisdictionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityDomain_42953620-cd8c-4a5f-a355-0c2e3d7778ad" xlink:to="loc_us-gaap_StateAndLocalJurisdictionMember_3d57e98b-5706-4e05-83f9-0dd1e0211ed6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLocationAxis_ebefe0fe-e9ee-4432-adad-5503e0dd2fd4" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OtherComprehensiveIncomeLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleTable_f0c8888f-c884-43e0-b49d-fd47d300ecf7" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLocationAxis_ebefe0fe-e9ee-4432-adad-5503e0dd2fd4" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLocationDomain_ebefe0fe-e9ee-4432-adad-5503e0dd2fd4_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OtherComprehensiveIncomeLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_OtherComprehensiveIncomeLocationAxis_ebefe0fe-e9ee-4432-adad-5503e0dd2fd4" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLocationDomain_ebefe0fe-e9ee-4432-adad-5503e0dd2fd4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLocationDomain_8fea87b1-97b8-4950-8074-da24bbb2a7a4" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OtherComprehensiveIncomeLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_OtherComprehensiveIncomeLocationAxis_ebefe0fe-e9ee-4432-adad-5503e0dd2fd4" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLocationDomain_8fea87b1-97b8-4950-8074-da24bbb2a7a4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeMember_173efbd3-0e22-4855-bbd7-56254e9b38a9" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OtherComprehensiveIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherComprehensiveIncomeLocationDomain_8fea87b1-97b8-4950-8074-da24bbb2a7a4" xlink:to="loc_us-gaap_OtherComprehensiveIncomeMember_173efbd3-0e22-4855-bbd7-56254e9b38a9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TaxPeriodAxis_3e8d3d1b-aa94-4631-b27a-293cb61a50a2" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TaxPeriodAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleTable_f0c8888f-c884-43e0-b49d-fd47d300ecf7" xlink:to="loc_us-gaap_TaxPeriodAxis_3e8d3d1b-aa94-4631-b27a-293cb61a50a2" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TaxPeriodDomain_3e8d3d1b-aa94-4631-b27a-293cb61a50a2_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TaxPeriodDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_TaxPeriodAxis_3e8d3d1b-aa94-4631-b27a-293cb61a50a2" xlink:to="loc_us-gaap_TaxPeriodDomain_3e8d3d1b-aa94-4631-b27a-293cb61a50a2_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TaxPeriodDomain_46dcd2f0-4bcb-4598-b88c-7567a7c82be5" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TaxPeriodDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_TaxPeriodAxis_3e8d3d1b-aa94-4631-b27a-293cb61a50a2" xlink:to="loc_us-gaap_TaxPeriodDomain_46dcd2f0-4bcb-4598-b88c-7567a7c82be5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_TaxYear2022to2035Member_4bffa3bd-2fef-492f-a130-cf05ff08ac1a" xlink:href="iclr-20211231.xsd#iclr_TaxYear2022to2035Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TaxPeriodDomain_46dcd2f0-4bcb-4598-b88c-7567a7c82be5" xlink:to="loc_iclr_TaxYear2022to2035Member_4bffa3bd-2fef-492f-a130-cf05ff08ac1a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_TaxYear20362040Member_b67d96a4-fec8-49db-96a7-6e452cc781bb" xlink:href="iclr-20211231.xsd#iclr_TaxYear20362040Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TaxPeriodDomain_46dcd2f0-4bcb-4598-b88c-7567a7c82be5" xlink:to="loc_iclr_TaxYear20362040Member_b67d96a4-fec8-49db-96a7-6e452cc781bb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_TaxYear2022To2028Member_872adbb7-0b3f-410c-86d9-febf196a5cfa" xlink:href="iclr-20211231.xsd#iclr_TaxYear2022To2028Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TaxPeriodDomain_46dcd2f0-4bcb-4598-b88c-7567a7c82be5" xlink:to="loc_iclr_TaxYear2022To2028Member_872adbb7-0b3f-410c-86d9-febf196a5cfa" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_TaxYear2029To2038Member_34f32737-b827-4320-bdb5-ffebcb027299" xlink:href="iclr-20211231.xsd#iclr_TaxYear2029To2038Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TaxPeriodDomain_46dcd2f0-4bcb-4598-b88c-7567a7c82be5" xlink:to="loc_iclr_TaxYear2029To2038Member_34f32737-b827-4320-bdb5-ffebcb027299" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_a26ea303-da4e-4658-982e-0af1b7314523" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeStatementLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleTable_f0c8888f-c884-43e0-b49d-fd47d300ecf7" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_a26ea303-da4e-4658-982e-0af1b7314523" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_a26ea303-da4e-4658-982e-0af1b7314523_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_a26ea303-da4e-4658-982e-0af1b7314523" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_a26ea303-da4e-4658-982e-0af1b7314523_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_c464e1ab-a786-47fe-85ca-e9a96093ca03" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_a26ea303-da4e-4658-982e-0af1b7314523" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_c464e1ab-a786-47fe-85ca-e9a96093ca03" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_IncomeTaxBenefitMember_2e1b380a-90f0-4ed9-8979-c58afc16eb00" xlink:href="iclr-20211231.xsd#iclr_IncomeTaxBenefitMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_c464e1ab-a786-47fe-85ca-e9a96093ca03" xlink:to="loc_iclr_IncomeTaxBenefitMember_2e1b380a-90f0-4ed9-8979-c58afc16eb00" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityNameAxis_de87fa5f-e452-493c-879b-88984fcdbe89" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeTaxAuthorityNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleTable_f0c8888f-c884-43e0-b49d-fd47d300ecf7" xlink:to="loc_us-gaap_IncomeTaxAuthorityNameAxis_de87fa5f-e452-493c-879b-88984fcdbe89" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityNameDomain_de87fa5f-e452-493c-879b-88984fcdbe89_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeTaxAuthorityNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeTaxAuthorityNameAxis_de87fa5f-e452-493c-879b-88984fcdbe89" xlink:to="loc_us-gaap_IncomeTaxAuthorityNameDomain_de87fa5f-e452-493c-879b-88984fcdbe89_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityNameDomain_0925963c-a4d3-4cf5-9363-cf5193918403" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeTaxAuthorityNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeTaxAuthorityNameAxis_de87fa5f-e452-493c-879b-88984fcdbe89" xlink:to="loc_us-gaap_IncomeTaxAuthorityNameDomain_0925963c-a4d3-4cf5-9363-cf5193918403" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InternalRevenueServiceIRSMember_81fde330-c6ba-4ecf-8847-c65e240875b6" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_InternalRevenueServiceIRSMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityNameDomain_0925963c-a4d3-4cf5-9363-cf5193918403" xlink:to="loc_us-gaap_InternalRevenueServiceIRSMember_81fde330-c6ba-4ecf-8847-c65e240875b6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_a6522125-fddb-4730-b6ad-5259b2d4ec23" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleTable_f0c8888f-c884-43e0-b49d-fd47d300ecf7" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_a6522125-fddb-4730-b6ad-5259b2d4ec23" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_a6522125-fddb-4730-b6ad-5259b2d4ec23_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_a6522125-fddb-4730-b6ad-5259b2d4ec23" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_a6522125-fddb-4730-b6ad-5259b2d4ec23_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_ec7b7d78-e5fb-49de-be61-8c1a44cb64f6" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_a6522125-fddb-4730-b6ad-5259b2d4ec23" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_ec7b7d78-e5fb-49de-be61-8c1a44cb64f6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_PRAHealthSciencesIncMember_09b65086-b8f5-4353-8881-db3cd4ff3c73" xlink:href="iclr-20211231.xsd#iclr_PRAHealthSciencesIncMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_ec7b7d78-e5fb-49de-be61-8c1a44cb64f6" xlink:to="loc_iclr_PRAHealthSciencesIncMember_09b65086-b8f5-4353-8881-db3cd4ff3c73" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/IncomeTaxesExpectedExpiryDatesofNOLsDetails" xlink:type="simple" xlink:href="iclr-20211231.xsd#IncomeTaxesExpectedExpiryDatesofNOLsDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/IncomeTaxesExpectedExpiryDatesofNOLsDetails" xlink:type="extended" id="ic233eecf6cb54794995799037a1ef45f_IncomeTaxesExpectedExpiryDatesofNOLsDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLossCarryforwardsLineItems_70105e61-25fd-4d5b-953f-c5f3a1375e3c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OperatingLossCarryforwardsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsSubjectToExpiration_11e1fda8-a8e7-4992-a278-c08ddd7ee8da" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsSubjectToExpiration"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_70105e61-25fd-4d5b-953f-c5f3a1375e3c" xlink:to="loc_us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsSubjectToExpiration_11e1fda8-a8e7-4992-a278-c08ddd7ee8da" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsNotSubjectToExpiration_e3cce967-e3a6-45fc-a80e-4f02031b7c4d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsNotSubjectToExpiration"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_70105e61-25fd-4d5b-953f-c5f3a1375e3c" xlink:to="loc_us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsNotSubjectToExpiration_e3cce967-e3a6-45fc-a80e-4f02031b7c4d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredTaxAssetsOperatingLossCarryforwards_03f0066a-d262-4d7e-9371-2191e477f52c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DeferredTaxAssetsOperatingLossCarryforwards"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_70105e61-25fd-4d5b-953f-c5f3a1375e3c" xlink:to="loc_us-gaap_DeferredTaxAssetsOperatingLossCarryforwards_03f0066a-d262-4d7e-9371-2191e477f52c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLossCarryforwardsTable_12ac1d92-3d60-41b1-8970-0b5e4f684df6" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OperatingLossCarryforwardsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_70105e61-25fd-4d5b-953f-c5f3a1375e3c" xlink:to="loc_us-gaap_OperatingLossCarryforwardsTable_12ac1d92-3d60-41b1-8970-0b5e4f684df6" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementGeographicalAxis_94662560-1267-4413-8a80-abfe615e9790" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_StatementGeographicalAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_OperatingLossCarryforwardsTable_12ac1d92-3d60-41b1-8970-0b5e4f684df6" xlink:to="loc_srt_StatementGeographicalAxis_94662560-1267-4413-8a80-abfe615e9790" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_94662560-1267-4413-8a80-abfe615e9790_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_94662560-1267-4413-8a80-abfe615e9790" xlink:to="loc_srt_SegmentGeographicalDomain_94662560-1267-4413-8a80-abfe615e9790_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_d8a273a1-cb0c-4a52-ae27-2eccf8f5e81d" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_94662560-1267-4413-8a80-abfe615e9790" xlink:to="loc_srt_SegmentGeographicalDomain_d8a273a1-cb0c-4a52-ae27-2eccf8f5e81d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_US_1c820383-1740-4a6d-9521-7c1441fed347" xlink:href="https://xbrl.sec.gov/country/2021/country-2021.xsd#country_US"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_d8a273a1-cb0c-4a52-ae27-2eccf8f5e81d" xlink:to="loc_country_US_1c820383-1740-4a6d-9521-7c1441fed347" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityNameAxis_1c20dc02-9e3c-4fdb-bf6f-4278ae533df9" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeTaxAuthorityNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_OperatingLossCarryforwardsTable_12ac1d92-3d60-41b1-8970-0b5e4f684df6" xlink:to="loc_us-gaap_IncomeTaxAuthorityNameAxis_1c20dc02-9e3c-4fdb-bf6f-4278ae533df9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityNameDomain_1c20dc02-9e3c-4fdb-bf6f-4278ae533df9_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeTaxAuthorityNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeTaxAuthorityNameAxis_1c20dc02-9e3c-4fdb-bf6f-4278ae533df9" xlink:to="loc_us-gaap_IncomeTaxAuthorityNameDomain_1c20dc02-9e3c-4fdb-bf6f-4278ae533df9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityNameDomain_2b720641-2fab-4b14-ba10-f7b7899c3999" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeTaxAuthorityNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeTaxAuthorityNameAxis_1c20dc02-9e3c-4fdb-bf6f-4278ae533df9" xlink:to="loc_us-gaap_IncomeTaxAuthorityNameDomain_2b720641-2fab-4b14-ba10-f7b7899c3999" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InternalRevenueServiceIRSMember_8c906271-9cb7-437e-b94a-9ff570b8b44e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_InternalRevenueServiceIRSMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityNameDomain_2b720641-2fab-4b14-ba10-f7b7899c3999" xlink:to="loc_us-gaap_InternalRevenueServiceIRSMember_8c906271-9cb7-437e-b94a-9ff570b8b44e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityAxis_58bde70b-0165-4bba-875a-27f12213e9ba" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeTaxAuthorityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_OperatingLossCarryforwardsTable_12ac1d92-3d60-41b1-8970-0b5e4f684df6" xlink:to="loc_us-gaap_IncomeTaxAuthorityAxis_58bde70b-0165-4bba-875a-27f12213e9ba" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityDomain_58bde70b-0165-4bba-875a-27f12213e9ba_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeTaxAuthorityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeTaxAuthorityAxis_58bde70b-0165-4bba-875a-27f12213e9ba" xlink:to="loc_us-gaap_IncomeTaxAuthorityDomain_58bde70b-0165-4bba-875a-27f12213e9ba_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityDomain_7191fbd8-a3f6-4ade-9342-14d4bf1d8ff7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeTaxAuthorityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeTaxAuthorityAxis_58bde70b-0165-4bba-875a-27f12213e9ba" xlink:to="loc_us-gaap_IncomeTaxAuthorityDomain_7191fbd8-a3f6-4ade-9342-14d4bf1d8ff7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StateAndLocalJurisdictionMember_49d99bf7-7c93-4926-9f1c-187c5f66dceb" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StateAndLocalJurisdictionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityDomain_7191fbd8-a3f6-4ade-9342-14d4bf1d8ff7" xlink:to="loc_us-gaap_StateAndLocalJurisdictionMember_49d99bf7-7c93-4926-9f1c-187c5f66dceb" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TaxPeriodAxis_30049257-4a1c-41ca-8af7-3c38b9759fc9" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TaxPeriodAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_OperatingLossCarryforwardsTable_12ac1d92-3d60-41b1-8970-0b5e4f684df6" xlink:to="loc_us-gaap_TaxPeriodAxis_30049257-4a1c-41ca-8af7-3c38b9759fc9" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TaxPeriodDomain_30049257-4a1c-41ca-8af7-3c38b9759fc9_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TaxPeriodDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_TaxPeriodAxis_30049257-4a1c-41ca-8af7-3c38b9759fc9" xlink:to="loc_us-gaap_TaxPeriodDomain_30049257-4a1c-41ca-8af7-3c38b9759fc9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TaxPeriodDomain_34f932d2-fca0-47a6-b7e4-3681256f4ec9" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TaxPeriodDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_TaxPeriodAxis_30049257-4a1c-41ca-8af7-3c38b9759fc9" xlink:to="loc_us-gaap_TaxPeriodDomain_34f932d2-fca0-47a6-b7e4-3681256f4ec9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_TaxYear2022to2035Member_df071b16-4cd0-4077-a1a8-057ff60945ef" xlink:href="iclr-20211231.xsd#iclr_TaxYear2022to2035Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TaxPeriodDomain_34f932d2-fca0-47a6-b7e4-3681256f4ec9" xlink:to="loc_iclr_TaxYear2022to2035Member_df071b16-4cd0-4077-a1a8-057ff60945ef" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_TaxYear20362040Member_930cbe31-4c54-4202-8519-85f651aaa45d" xlink:href="iclr-20211231.xsd#iclr_TaxYear20362040Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TaxPeriodDomain_34f932d2-fca0-47a6-b7e4-3681256f4ec9" xlink:to="loc_iclr_TaxYear20362040Member_930cbe31-4c54-4202-8519-85f651aaa45d" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/RestructuringchargesNarrativeDetails" xlink:type="simple" xlink:href="iclr-20211231.xsd#RestructuringchargesNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/RestructuringchargesNarrativeDetails" xlink:type="extended" id="i17b85aa4cfbd456caa73ba46501eda8c_RestructuringchargesNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCostAndReserveLineItems_e0ebe25e-c851-4d68-b656-a580a748e416" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RestructuringCostAndReserveLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCosts_8e71a712-330f-47a2-9889-b5015ce34f03" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RestructuringCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_e0ebe25e-c851-4d68-b656-a580a748e416" xlink:to="loc_us-gaap_RestructuringCosts_8e71a712-330f-47a2-9889-b5015ce34f03" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseImpairmentLoss_633ecfd1-c75a-4175-96b1-0321d79f510e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OperatingLeaseImpairmentLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_e0ebe25e-c851-4d68-b656-a580a748e416" xlink:to="loc_us-gaap_OperatingLeaseImpairmentLoss_633ecfd1-c75a-4175-96b1-0321d79f510e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ImpairmentOfLongLivedAssetsHeldForUse_a5bf48b9-0b06-4744-b1a6-f462a2aef286" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ImpairmentOfLongLivedAssetsHeldForUse"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_e0ebe25e-c851-4d68-b656-a580a748e416" xlink:to="loc_us-gaap_ImpairmentOfLongLivedAssetsHeldForUse_a5bf48b9-0b06-4744-b1a6-f462a2aef286" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringReserve_7844ec22-786c-4065-87ca-b54bc71fb038" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RestructuringReserve"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_e0ebe25e-c851-4d68-b656-a580a748e416" xlink:to="loc_us-gaap_RestructuringReserve_7844ec22-786c-4065-87ca-b54bc71fb038" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_TransactionAndOneTimeFinancingCosts_96f111be-5f0e-4a35-9c4f-52fdd2dc1f94" xlink:href="iclr-20211231.xsd#iclr_TransactionAndOneTimeFinancingCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_e0ebe25e-c851-4d68-b656-a580a748e416" xlink:to="loc_iclr_TransactionAndOneTimeFinancingCosts_96f111be-5f0e-4a35-9c4f-52fdd2dc1f94" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_77dbec93-ba90-44f7-a365-9131622002da" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfRestructuringAndRelatedCostsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_e0ebe25e-c851-4d68-b656-a580a748e416" xlink:to="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_77dbec93-ba90-44f7-a365-9131622002da" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCostAndReserveAxis_c52c1bc4-5c2d-4130-8b75-5b857cf4371a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RestructuringCostAndReserveAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_77dbec93-ba90-44f7-a365-9131622002da" xlink:to="loc_us-gaap_RestructuringCostAndReserveAxis_c52c1bc4-5c2d-4130-8b75-5b857cf4371a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfRestructuringDomain_c52c1bc4-5c2d-4130-8b75-5b857cf4371a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TypeOfRestructuringDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RestructuringCostAndReserveAxis_c52c1bc4-5c2d-4130-8b75-5b857cf4371a" xlink:to="loc_us-gaap_TypeOfRestructuringDomain_c52c1bc4-5c2d-4130-8b75-5b857cf4371a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfRestructuringDomain_00865fb7-8289-471d-9b48-37f45fd7eae7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TypeOfRestructuringDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RestructuringCostAndReserveAxis_c52c1bc4-5c2d-4130-8b75-5b857cf4371a" xlink:to="loc_us-gaap_TypeOfRestructuringDomain_00865fb7-8289-471d-9b48-37f45fd7eae7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeSeveranceMember_ea2f7c3c-461a-430c-9597-08d996dc3480" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EmployeeSeveranceMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_00865fb7-8289-471d-9b48-37f45fd7eae7" xlink:to="loc_us-gaap_EmployeeSeveranceMember_ea2f7c3c-461a-430c-9597-08d996dc3480" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_LeaseLiabilityMember_7b3ba5f6-1029-405a-b25e-322c3fe4a304" xlink:href="iclr-20211231.xsd#iclr_LeaseLiabilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_00865fb7-8289-471d-9b48-37f45fd7eae7" xlink:to="loc_iclr_LeaseLiabilityMember_7b3ba5f6-1029-405a-b25e-322c3fe4a304" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringPlanAxis_bff38a16-43b7-4a8f-a5e3-49e86c6f7137" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RestructuringPlanAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_77dbec93-ba90-44f7-a365-9131622002da" xlink:to="loc_us-gaap_RestructuringPlanAxis_bff38a16-43b7-4a8f-a5e3-49e86c6f7137" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringPlanDomain_bff38a16-43b7-4a8f-a5e3-49e86c6f7137_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RestructuringPlanDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RestructuringPlanAxis_bff38a16-43b7-4a8f-a5e3-49e86c6f7137" xlink:to="loc_us-gaap_RestructuringPlanDomain_bff38a16-43b7-4a8f-a5e3-49e86c6f7137_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringPlanDomain_f6826d8f-0054-4665-b764-46297c44dd5d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RestructuringPlanDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RestructuringPlanAxis_bff38a16-43b7-4a8f-a5e3-49e86c6f7137" xlink:to="loc_us-gaap_RestructuringPlanDomain_f6826d8f-0054-4665-b764-46297c44dd5d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_A2021RestructuringPlanMember_d1ceb95b-3b8e-479f-b96f-01cd57dfcf5e" xlink:href="iclr-20211231.xsd#iclr_A2021RestructuringPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringPlanDomain_f6826d8f-0054-4665-b764-46297c44dd5d" xlink:to="loc_iclr_A2021RestructuringPlanMember_d1ceb95b-3b8e-479f-b96f-01cd57dfcf5e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationAxis_1d70e178-e145-49a2-b244-cf74c09cf7d4" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BalanceSheetLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_77dbec93-ba90-44f7-a365-9131622002da" xlink:to="loc_us-gaap_BalanceSheetLocationAxis_1d70e178-e145-49a2-b244-cf74c09cf7d4" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_1d70e178-e145-49a2-b244-cf74c09cf7d4_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BalanceSheetLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_1d70e178-e145-49a2-b244-cf74c09cf7d4" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_1d70e178-e145-49a2-b244-cf74c09cf7d4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_a38dcdf8-81d9-40cd-a690-b662c02aeced" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BalanceSheetLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_1d70e178-e145-49a2-b244-cf74c09cf7d4" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_a38dcdf8-81d9-40cd-a690-b662c02aeced" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherCurrentLiabilitiesMember_d2f0567d-943e-43fc-863b-a72a67f72511" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OtherCurrentLiabilitiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_a38dcdf8-81d9-40cd-a690-b662c02aeced" xlink:to="loc_us-gaap_OtherCurrentLiabilitiesMember_d2f0567d-943e-43fc-863b-a72a67f72511" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherNoncurrentLiabilitiesMember_a3b675ff-bcb8-439d-bf6b-ef52257aac4b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OtherNoncurrentLiabilitiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_a38dcdf8-81d9-40cd-a690-b662c02aeced" xlink:to="loc_us-gaap_OtherNoncurrentLiabilitiesMember_a3b675ff-bcb8-439d-bf6b-ef52257aac4b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_04234f33-81ae-4403-9056-bfac4c64238c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_77dbec93-ba90-44f7-a365-9131622002da" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_04234f33-81ae-4403-9056-bfac4c64238c" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_04234f33-81ae-4403-9056-bfac4c64238c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_04234f33-81ae-4403-9056-bfac4c64238c" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_04234f33-81ae-4403-9056-bfac4c64238c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_0eabdd01-5d40-42f8-a5e4-99d2a4890e1e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_04234f33-81ae-4403-9056-bfac4c64238c" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_0eabdd01-5d40-42f8-a5e4-99d2a4890e1e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_PRAHealthSciencesIncMember_32c0750a-b311-4f80-87ff-fc288e5a3f3d" xlink:href="iclr-20211231.xsd#iclr_PRAHealthSciencesIncMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_0eabdd01-5d40-42f8-a5e4-99d2a4890e1e" xlink:to="loc_iclr_PRAHealthSciencesIncMember_32c0750a-b311-4f80-87ff-fc288e5a3f3d" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/DisaggregationofRevenueDetails" xlink:type="simple" xlink:href="iclr-20211231.xsd#DisaggregationofRevenueDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/DisaggregationofRevenueDetails" xlink:type="extended" id="ie1397788d6904d13a859614f5559e73f_DisaggregationofRevenueDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueLineItems_17f5f86d-f4d9-43f8-827f-26f7ce093866" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DisaggregationOfRevenueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_1b51d9c7-a74e-4d40-9cb7-56f65df5eb54" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_17f5f86d-f4d9-43f8-827f-26f7ce093866" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_1b51d9c7-a74e-4d40-9cb7-56f65df5eb54" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueTable_34ef2fd4-c4e5-4acb-9610-fdbf87dd42d3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DisaggregationOfRevenueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_17f5f86d-f4d9-43f8-827f-26f7ce093866" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_34ef2fd4-c4e5-4acb-9610-fdbf87dd42d3" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MajorCustomersAxis_17568a89-e1d3-4219-921c-5505d98989e0" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_MajorCustomersAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_34ef2fd4-c4e5-4acb-9610-fdbf87dd42d3" xlink:to="loc_srt_MajorCustomersAxis_17568a89-e1d3-4219-921c-5505d98989e0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_NameOfMajorCustomerDomain_17568a89-e1d3-4219-921c-5505d98989e0_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_NameOfMajorCustomerDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_MajorCustomersAxis_17568a89-e1d3-4219-921c-5505d98989e0" xlink:to="loc_srt_NameOfMajorCustomerDomain_17568a89-e1d3-4219-921c-5505d98989e0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_NameOfMajorCustomerDomain_4460e831-23b0-4d14-bb8a-a0b89e1c3a4d" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_NameOfMajorCustomerDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_MajorCustomersAxis_17568a89-e1d3-4219-921c-5505d98989e0" xlink:to="loc_srt_NameOfMajorCustomerDomain_4460e831-23b0-4d14-bb8a-a0b89e1c3a4d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_TopClientMember_c29215c4-3b0a-41f2-986e-029b6eda28be" xlink:href="iclr-20211231.xsd#iclr_TopClientMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_4460e831-23b0-4d14-bb8a-a0b89e1c3a4d" xlink:to="loc_iclr_TopClientMember_c29215c4-3b0a-41f2-986e-029b6eda28be" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_Clients25Member_46ad258f-24b5-4030-82ca-545fa0f88a01" xlink:href="iclr-20211231.xsd#iclr_Clients25Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_4460e831-23b0-4d14-bb8a-a0b89e1c3a4d" xlink:to="loc_iclr_Clients25Member_46ad258f-24b5-4030-82ca-545fa0f88a01" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_Clients610Member_f6818516-8a59-4053-8693-a72940c1cf32" xlink:href="iclr-20211231.xsd#iclr_Clients610Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_4460e831-23b0-4d14-bb8a-a0b89e1c3a4d" xlink:to="loc_iclr_Clients610Member_f6818516-8a59-4053-8693-a72940c1cf32" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_Clients1125Member_463dddde-5803-443b-b4df-ffa5226c8c38" xlink:href="iclr-20211231.xsd#iclr_Clients1125Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_4460e831-23b0-4d14-bb8a-a0b89e1c3a4d" xlink:to="loc_iclr_Clients1125Member_463dddde-5803-443b-b4df-ffa5226c8c38" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_OtherCustomersMember_b5476e02-c0c3-493e-a553-0768bb2ef8ea" xlink:href="iclr-20211231.xsd#iclr_OtherCustomersMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_4460e831-23b0-4d14-bb8a-a0b89e1c3a4d" xlink:to="loc_iclr_OtherCustomersMember_b5476e02-c0c3-493e-a553-0768bb2ef8ea" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/AccountsreceivableunbilledrevenuecontractassetsandunearnedrevenueorpaymentsonaccountcontractliabilitiesNarrativeDetails" xlink:type="simple" xlink:href="iclr-20211231.xsd#AccountsreceivableunbilledrevenuecontractassetsandunearnedrevenueorpaymentsonaccountcontractliabilitiesNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/AccountsreceivableunbilledrevenuecontractassetsandunearnedrevenueorpaymentsonaccountcontractliabilitiesNarrativeDetails" xlink:type="extended" id="iba5f45162e4e44f397bf09d0fc0208e2_AccountsreceivableunbilledrevenuecontractassetsandunearnedrevenueorpaymentsonaccountcontractliabilitiesNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_e95e4432-996e-41b1-9da0-f53c09ef2e66" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_OtherLiabilitiesAmountPayableToThirdPartiesForReimbursableExpenses_29f96407-c699-4a5a-a382-82dd5798f055" xlink:href="iclr-20211231.xsd#iclr_OtherLiabilitiesAmountPayableToThirdPartiesForReimbursableExpenses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_e95e4432-996e-41b1-9da0-f53c09ef2e66" xlink:to="loc_iclr_OtherLiabilitiesAmountPayableToThirdPartiesForReimbursableExpenses_29f96407-c699-4a5a-a382-82dd5798f055" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_IncreaseDecreaseInContractwithCustomerAssetGrossCurrent_2e7a6ceb-bd5e-44b4-a8f6-c746aa395bc4" xlink:href="iclr-20211231.xsd#iclr_IncreaseDecreaseInContractwithCustomerAssetGrossCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_e95e4432-996e-41b1-9da0-f53c09ef2e66" xlink:to="loc_iclr_IncreaseDecreaseInContractwithCustomerAssetGrossCurrent_2e7a6ceb-bd5e-44b4-a8f6-c746aa395bc4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_IncreaseDecreaseInContractsWithCustomersLiabilityCurrent_245c37da-9d55-47ba-b400-62b3685f49a6" xlink:href="iclr-20211231.xsd#iclr_IncreaseDecreaseInContractsWithCustomersLiabilityCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_e95e4432-996e-41b1-9da0-f53c09ef2e66" xlink:to="loc_iclr_IncreaseDecreaseInContractsWithCustomersLiabilityCurrent_245c37da-9d55-47ba-b400-62b3685f49a6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_ChangeinAdvancePaymentsNettedAgainstUnbilledContractsReceivable_4502bb07-4f0d-4964-9976-45ad3947613c" xlink:href="iclr-20211231.xsd#iclr_ChangeinAdvancePaymentsNettedAgainstUnbilledContractsReceivable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_e95e4432-996e-41b1-9da0-f53c09ef2e66" xlink:to="loc_iclr_ChangeinAdvancePaymentsNettedAgainstUnbilledContractsReceivable_4502bb07-4f0d-4964-9976-45ad3947613c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerAssetCreditLossExpense_6c5d74bc-fa33-4227-86c0-f86ba792d66f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ContractWithCustomerAssetCreditLossExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_e95e4432-996e-41b1-9da0-f53c09ef2e66" xlink:to="loc_us-gaap_ContractWithCustomerAssetCreditLossExpense_6c5d74bc-fa33-4227-86c0-f86ba792d66f" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligation_53c46c14-f348-4cfd-8a1f-e11b34b0ea10" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RevenueRemainingPerformanceObligation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_e95e4432-996e-41b1-9da0-f53c09ef2e66" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligation_53c46c14-f348-4cfd-8a1f-e11b34b0ea10" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationPercentage_cc180bd1-40c8-4148-a95b-11620fcf95c2" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RevenueRemainingPerformanceObligationPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_e95e4432-996e-41b1-9da0-f53c09ef2e66" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationPercentage_cc180bd1-40c8-4148-a95b-11620fcf95c2" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_6f7c8a6d-4c8a-4221-a17b-1c13f0474b3d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_e95e4432-996e-41b1-9da0-f53c09ef2e66" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_6f7c8a6d-4c8a-4221-a17b-1c13f0474b3d" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_d8a4560e-9e66-4e8b-8b49-0d35620c303b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_e95e4432-996e-41b1-9da0-f53c09ef2e66" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_d8a4560e-9e66-4e8b-8b49-0d35620c303b" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionStartDateAxis_998cd3c2-530b-4361-acbc-869f676f9f61" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionStartDateAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_d8a4560e-9e66-4e8b-8b49-0d35620c303b" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionStartDateAxis_998cd3c2-530b-4361-acbc-869f676f9f61" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/AccountsreceivableunbilledrevenuecontractassetsandunearnedrevenueorpaymentsonaccountcontractliabilitiesNarrativeDetails_1" xlink:type="simple" xlink:href="iclr-20211231.xsd#AccountsreceivableunbilledrevenuecontractassetsandunearnedrevenueorpaymentsonaccountcontractliabilitiesNarrativeDetails_1"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/AccountsreceivableunbilledrevenuecontractassetsandunearnedrevenueorpaymentsonaccountcontractliabilitiesNarrativeDetails_1" xlink:type="extended" id="i4844508a2ec24fc89c7a44dfce069dc6_AccountsreceivableunbilledrevenuecontractassetsandunearnedrevenueorpaymentsonaccountcontractliabilitiesNarrativeDetails_1">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_e95e4432-996e-41b1-9da0-f53c09ef2e66" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_OtherLiabilitiesAmountPayableToThirdPartiesForReimbursableExpenses_29f96407-c699-4a5a-a382-82dd5798f055" xlink:href="iclr-20211231.xsd#iclr_OtherLiabilitiesAmountPayableToThirdPartiesForReimbursableExpenses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_e95e4432-996e-41b1-9da0-f53c09ef2e66" xlink:to="loc_iclr_OtherLiabilitiesAmountPayableToThirdPartiesForReimbursableExpenses_29f96407-c699-4a5a-a382-82dd5798f055" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_IncreaseDecreaseInContractwithCustomerAssetGrossCurrent_2e7a6ceb-bd5e-44b4-a8f6-c746aa395bc4" xlink:href="iclr-20211231.xsd#iclr_IncreaseDecreaseInContractwithCustomerAssetGrossCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_e95e4432-996e-41b1-9da0-f53c09ef2e66" xlink:to="loc_iclr_IncreaseDecreaseInContractwithCustomerAssetGrossCurrent_2e7a6ceb-bd5e-44b4-a8f6-c746aa395bc4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_IncreaseDecreaseInContractsWithCustomersLiabilityCurrent_245c37da-9d55-47ba-b400-62b3685f49a6" xlink:href="iclr-20211231.xsd#iclr_IncreaseDecreaseInContractsWithCustomersLiabilityCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_e95e4432-996e-41b1-9da0-f53c09ef2e66" xlink:to="loc_iclr_IncreaseDecreaseInContractsWithCustomersLiabilityCurrent_245c37da-9d55-47ba-b400-62b3685f49a6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_ChangeinAdvancePaymentsNettedAgainstUnbilledContractsReceivable_4502bb07-4f0d-4964-9976-45ad3947613c" xlink:href="iclr-20211231.xsd#iclr_ChangeinAdvancePaymentsNettedAgainstUnbilledContractsReceivable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_e95e4432-996e-41b1-9da0-f53c09ef2e66" xlink:to="loc_iclr_ChangeinAdvancePaymentsNettedAgainstUnbilledContractsReceivable_4502bb07-4f0d-4964-9976-45ad3947613c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerAssetCreditLossExpense_6c5d74bc-fa33-4227-86c0-f86ba792d66f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ContractWithCustomerAssetCreditLossExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_e95e4432-996e-41b1-9da0-f53c09ef2e66" xlink:to="loc_us-gaap_ContractWithCustomerAssetCreditLossExpense_6c5d74bc-fa33-4227-86c0-f86ba792d66f" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligation_53c46c14-f348-4cfd-8a1f-e11b34b0ea10" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RevenueRemainingPerformanceObligation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_e95e4432-996e-41b1-9da0-f53c09ef2e66" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligation_53c46c14-f348-4cfd-8a1f-e11b34b0ea10" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationPercentage_cc180bd1-40c8-4148-a95b-11620fcf95c2" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RevenueRemainingPerformanceObligationPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_e95e4432-996e-41b1-9da0-f53c09ef2e66" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationPercentage_cc180bd1-40c8-4148-a95b-11620fcf95c2" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_6f7c8a6d-4c8a-4221-a17b-1c13f0474b3d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_e95e4432-996e-41b1-9da0-f53c09ef2e66" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_6f7c8a6d-4c8a-4221-a17b-1c13f0474b3d" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_d8a4560e-9e66-4e8b-8b49-0d35620c303b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_e95e4432-996e-41b1-9da0-f53c09ef2e66" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_d8a4560e-9e66-4e8b-8b49-0d35620c303b" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/BusinessSegmentandGeographicalInformationNarrativeDetails" xlink:type="simple" xlink:href="iclr-20211231.xsd#BusinessSegmentandGeographicalInformationNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/BusinessSegmentandGeographicalInformationNarrativeDetails" xlink:type="extended" id="i934fea11f6094037ad58de1af37c6ea8_BusinessSegmentandGeographicalInformationNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_2a06c29c-f01d-4626-a9a6-25626ed5cf4e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NumberOfOperatingSegments_8d173fa7-e00e-4ff4-a2b4-1e3ab696259c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_NumberOfOperatingSegments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_2a06c29c-f01d-4626-a9a6-25626ed5cf4e" xlink:to="loc_us-gaap_NumberOfOperatingSegments_8d173fa7-e00e-4ff4-a2b4-1e3ab696259c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_IncomeLossFromOperationsExcludingRestructuringTransactionAndIntegrationCosts_865ea523-bc6e-4887-8f1f-1253e5c52a3d" xlink:href="iclr-20211231.xsd#iclr_IncomeLossFromOperationsExcludingRestructuringTransactionAndIntegrationCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_2a06c29c-f01d-4626-a9a6-25626ed5cf4e" xlink:to="loc_iclr_IncomeLossFromOperationsExcludingRestructuringTransactionAndIntegrationCosts_865ea523-bc6e-4887-8f1f-1253e5c52a3d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCosts_e956dcc5-a08a-4a77-9249-352077cb5275" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RestructuringCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_2a06c29c-f01d-4626-a9a6-25626ed5cf4e" xlink:to="loc_us-gaap_RestructuringCosts_e956dcc5-a08a-4a77-9249-352077cb5275" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationIntegrationRelatedCosts_5ea6dc89-694b-431d-9305-d198daaa8493" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationIntegrationRelatedCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_2a06c29c-f01d-4626-a9a6-25626ed5cf4e" xlink:to="loc_us-gaap_BusinessCombinationIntegrationRelatedCosts_5ea6dc89-694b-431d-9305-d198daaa8493" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingIncomeLoss_1cfd3269-a548-4422-8369-3d1fb2b62fb1" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OperatingIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_2a06c29c-f01d-4626-a9a6-25626ed5cf4e" xlink:to="loc_us-gaap_OperatingIncomeLoss_1cfd3269-a548-4422-8369-3d1fb2b62fb1" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_72645d27-af7f-49b9-9a00-f1266917801c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_2a06c29c-f01d-4626-a9a6-25626ed5cf4e" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_72645d27-af7f-49b9-9a00-f1266917801c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringPlanAxis_8e0a6d83-a11d-4e6f-a98f-f6e7c5ee776f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RestructuringPlanAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_72645d27-af7f-49b9-9a00-f1266917801c" xlink:to="loc_us-gaap_RestructuringPlanAxis_8e0a6d83-a11d-4e6f-a98f-f6e7c5ee776f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringPlanDomain_8e0a6d83-a11d-4e6f-a98f-f6e7c5ee776f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RestructuringPlanDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RestructuringPlanAxis_8e0a6d83-a11d-4e6f-a98f-f6e7c5ee776f" xlink:to="loc_us-gaap_RestructuringPlanDomain_8e0a6d83-a11d-4e6f-a98f-f6e7c5ee776f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringPlanDomain_0caa152c-face-4ef5-a0d8-c9f2523a42a7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RestructuringPlanDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RestructuringPlanAxis_8e0a6d83-a11d-4e6f-a98f-f6e7c5ee776f" xlink:to="loc_us-gaap_RestructuringPlanDomain_0caa152c-face-4ef5-a0d8-c9f2523a42a7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_A2021RestructuringPlanMember_316a4c77-f626-4994-85f4-d28c1492a477" xlink:href="iclr-20211231.xsd#iclr_A2021RestructuringPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringPlanDomain_0caa152c-face-4ef5-a0d8-c9f2523a42a7" xlink:to="loc_iclr_A2021RestructuringPlanMember_316a4c77-f626-4994-85f4-d28c1492a477" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_34dbb599-2000-4a9e-be6c-033979e75fff" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_72645d27-af7f-49b9-9a00-f1266917801c" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_34dbb599-2000-4a9e-be6c-033979e75fff" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_34dbb599-2000-4a9e-be6c-033979e75fff_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_34dbb599-2000-4a9e-be6c-033979e75fff" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_34dbb599-2000-4a9e-be6c-033979e75fff_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_cc53de3f-19ed-4710-a3b6-88222037000b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_34dbb599-2000-4a9e-be6c-033979e75fff" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_cc53de3f-19ed-4710-a3b6-88222037000b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_PRAHealthSciencesIncMember_6c9283c0-f43c-450b-87c4-4d5b348b7782" xlink:href="iclr-20211231.xsd#iclr_PRAHealthSciencesIncMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_cc53de3f-19ed-4710-a3b6-88222037000b" xlink:to="loc_iclr_PRAHealthSciencesIncMember_6c9283c0-f43c-450b-87c4-4d5b348b7782" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/BusinessSegmentandGeographicalInformationDistributionofNetRevenuebyGeographicalAreaDetails" xlink:type="simple" xlink:href="iclr-20211231.xsd#BusinessSegmentandGeographicalInformationDistributionofNetRevenuebyGeographicalAreaDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/BusinessSegmentandGeographicalInformationDistributionofNetRevenuebyGeographicalAreaDetails" xlink:type="extended" id="i6d5b27a7e4fb4463b054cbbc89ead172_BusinessSegmentandGeographicalInformationDistributionofNetRevenuebyGeographicalAreaDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_6bfa8a48-fafb-497a-917a-a2d7e3eacdd3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_34def085-09a4-4da2-9342-c39a1a8ddd37" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_6bfa8a48-fafb-497a-917a-a2d7e3eacdd3" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_34def085-09a4-4da2-9342-c39a1a8ddd37" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_fcdc4114-95f6-47a2-bad0-54c824e9f813" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_6bfa8a48-fafb-497a-917a-a2d7e3eacdd3" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_fcdc4114-95f6-47a2-bad0-54c824e9f813" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementGeographicalAxis_9ba31d16-334c-4b21-b722-bd6a918130f4" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_StatementGeographicalAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_fcdc4114-95f6-47a2-bad0-54c824e9f813" xlink:to="loc_srt_StatementGeographicalAxis_9ba31d16-334c-4b21-b722-bd6a918130f4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_9ba31d16-334c-4b21-b722-bd6a918130f4_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_9ba31d16-334c-4b21-b722-bd6a918130f4" xlink:to="loc_srt_SegmentGeographicalDomain_9ba31d16-334c-4b21-b722-bd6a918130f4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_3c51e7c3-6915-455d-940d-c8b87995d5fc" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_9ba31d16-334c-4b21-b722-bd6a918130f4" xlink:to="loc_srt_SegmentGeographicalDomain_3c51e7c3-6915-455d-940d-c8b87995d5fc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_IE_0d54669b-65d2-4228-88b0-9e065bfda032" xlink:href="https://xbrl.sec.gov/country/2021/country-2021.xsd#country_IE"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_3c51e7c3-6915-455d-940d-c8b87995d5fc" xlink:to="loc_country_IE_0d54669b-65d2-4228-88b0-9e065bfda032" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_RestOfEuropeMember_c5fade99-cbaa-4638-980d-95d448b47257" xlink:href="iclr-20211231.xsd#iclr_RestOfEuropeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_3c51e7c3-6915-455d-940d-c8b87995d5fc" xlink:to="loc_iclr_RestOfEuropeMember_c5fade99-cbaa-4638-980d-95d448b47257" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_US_b2a5e490-b202-4d01-864a-d1fed6708e77" xlink:href="https://xbrl.sec.gov/country/2021/country-2021.xsd#country_US"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_3c51e7c3-6915-455d-940d-c8b87995d5fc" xlink:to="loc_country_US_b2a5e490-b202-4d01-864a-d1fed6708e77" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_OtherCountriesMember_22c56737-0d0d-4382-a2cc-6126326eb6a1" xlink:href="iclr-20211231.xsd#iclr_OtherCountriesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_3c51e7c3-6915-455d-940d-c8b87995d5fc" xlink:to="loc_iclr_OtherCountriesMember_22c56737-0d0d-4382-a2cc-6126326eb6a1" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/BusinessSegmentandGeographicalInformationDistributionofIncomefromOperationsExcludingRestructuringbyGeographicalAreaDetails" xlink:type="simple" xlink:href="iclr-20211231.xsd#BusinessSegmentandGeographicalInformationDistributionofIncomefromOperationsExcludingRestructuringbyGeographicalAreaDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/BusinessSegmentandGeographicalInformationDistributionofIncomefromOperationsExcludingRestructuringbyGeographicalAreaDetails" xlink:type="extended" id="i5d59e64ceb4f4d00bf641fd04c4e78cd_BusinessSegmentandGeographicalInformationDistributionofIncomefromOperationsExcludingRestructuringbyGeographicalAreaDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_ad2e1174-4d50-47dd-a4d9-0994cfdce887" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_OperatingIncomeLossBeforeRestructuringChargesTransactionAndIntegrationExpenses_36b22d25-c499-4a3b-9e92-51a9dc72a7ac" xlink:href="iclr-20211231.xsd#iclr_OperatingIncomeLossBeforeRestructuringChargesTransactionAndIntegrationExpenses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_ad2e1174-4d50-47dd-a4d9-0994cfdce887" xlink:to="loc_iclr_OperatingIncomeLossBeforeRestructuringChargesTransactionAndIntegrationExpenses_36b22d25-c499-4a3b-9e92-51a9dc72a7ac" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_f791d190-2d44-448d-8a2a-bf1720a8efde" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_ad2e1174-4d50-47dd-a4d9-0994cfdce887" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_f791d190-2d44-448d-8a2a-bf1720a8efde" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementGeographicalAxis_b1d9ef6a-3073-4547-b579-93d748f19474" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_StatementGeographicalAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_f791d190-2d44-448d-8a2a-bf1720a8efde" xlink:to="loc_srt_StatementGeographicalAxis_b1d9ef6a-3073-4547-b579-93d748f19474" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_b1d9ef6a-3073-4547-b579-93d748f19474_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_b1d9ef6a-3073-4547-b579-93d748f19474" xlink:to="loc_srt_SegmentGeographicalDomain_b1d9ef6a-3073-4547-b579-93d748f19474_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_b7e70007-5d22-4261-9e27-20b038389808" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_b1d9ef6a-3073-4547-b579-93d748f19474" xlink:to="loc_srt_SegmentGeographicalDomain_b7e70007-5d22-4261-9e27-20b038389808" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_IE_389edd49-3148-4589-ab1c-83aa7f04718b" xlink:href="https://xbrl.sec.gov/country/2021/country-2021.xsd#country_IE"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_b7e70007-5d22-4261-9e27-20b038389808" xlink:to="loc_country_IE_389edd49-3148-4589-ab1c-83aa7f04718b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_RestOfEuropeMember_dcfb56b8-07b3-43b5-9344-4dce80a6bd10" xlink:href="iclr-20211231.xsd#iclr_RestOfEuropeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_b7e70007-5d22-4261-9e27-20b038389808" xlink:to="loc_iclr_RestOfEuropeMember_dcfb56b8-07b3-43b5-9344-4dce80a6bd10" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_US_a3b900fb-5e2a-4e91-8f86-409ec0ab3952" xlink:href="https://xbrl.sec.gov/country/2021/country-2021.xsd#country_US"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_b7e70007-5d22-4261-9e27-20b038389808" xlink:to="loc_country_US_a3b900fb-5e2a-4e91-8f86-409ec0ab3952" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_OtherCountriesMember_142fa8d0-ddd2-440d-a795-c80c6df658a1" xlink:href="iclr-20211231.xsd#iclr_OtherCountriesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_b7e70007-5d22-4261-9e27-20b038389808" xlink:to="loc_iclr_OtherCountriesMember_142fa8d0-ddd2-440d-a795-c80c6df658a1" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/BusinessSegmentandGeographicalInformationDistributionofLonglivedAssetsNetbyGeographicalAreaDetails" xlink:type="simple" xlink:href="iclr-20211231.xsd#BusinessSegmentandGeographicalInformationDistributionofLonglivedAssetsNetbyGeographicalAreaDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/BusinessSegmentandGeographicalInformationDistributionofLonglivedAssetsNetbyGeographicalAreaDetails" xlink:type="extended" id="i068353aeff00490d95d525033073f1e7_BusinessSegmentandGeographicalInformationDistributionofLonglivedAssetsNetbyGeographicalAreaDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_278b4c7a-2d2f-4f30-a887-814d6b531ce7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NoncurrentAssets_d89567b3-6225-4e16-8032-0058993a11d3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_NoncurrentAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_278b4c7a-2d2f-4f30-a887-814d6b531ce7" xlink:to="loc_us-gaap_NoncurrentAssets_d89567b3-6225-4e16-8032-0058993a11d3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_a23ef652-50ae-4f16-b1a0-24991ff84b31" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_278b4c7a-2d2f-4f30-a887-814d6b531ce7" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_a23ef652-50ae-4f16-b1a0-24991ff84b31" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementGeographicalAxis_8fd14c0e-4553-450d-ac0d-60845b0a22ce" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_StatementGeographicalAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_a23ef652-50ae-4f16-b1a0-24991ff84b31" xlink:to="loc_srt_StatementGeographicalAxis_8fd14c0e-4553-450d-ac0d-60845b0a22ce" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_8fd14c0e-4553-450d-ac0d-60845b0a22ce_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_8fd14c0e-4553-450d-ac0d-60845b0a22ce" xlink:to="loc_srt_SegmentGeographicalDomain_8fd14c0e-4553-450d-ac0d-60845b0a22ce_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_3813b5a9-717d-496a-9406-1a1748b18c99" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_8fd14c0e-4553-450d-ac0d-60845b0a22ce" xlink:to="loc_srt_SegmentGeographicalDomain_3813b5a9-717d-496a-9406-1a1748b18c99" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_IE_8ebb2399-cbfa-4b10-aa49-e3da31c40f92" xlink:href="https://xbrl.sec.gov/country/2021/country-2021.xsd#country_IE"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_3813b5a9-717d-496a-9406-1a1748b18c99" xlink:to="loc_country_IE_8ebb2399-cbfa-4b10-aa49-e3da31c40f92" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_RestOfEuropeMember_3e0b01e5-f2ae-469b-9b46-33400aba5afc" xlink:href="iclr-20211231.xsd#iclr_RestOfEuropeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_3813b5a9-717d-496a-9406-1a1748b18c99" xlink:to="loc_iclr_RestOfEuropeMember_3e0b01e5-f2ae-469b-9b46-33400aba5afc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_US_dd2a6f8e-3555-4895-93a5-cbb0331ad839" xlink:href="https://xbrl.sec.gov/country/2021/country-2021.xsd#country_US"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_3813b5a9-717d-496a-9406-1a1748b18c99" xlink:to="loc_country_US_dd2a6f8e-3555-4895-93a5-cbb0331ad839" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_OtherCountriesMember_78d244ff-16df-45f8-a6b4-f8e8e39975cc" xlink:href="iclr-20211231.xsd#iclr_OtherCountriesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_3813b5a9-717d-496a-9406-1a1748b18c99" xlink:to="loc_iclr_OtherCountriesMember_78d244ff-16df-45f8-a6b4-f8e8e39975cc" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/AccumulatedOtherComprehensiveIncomeDetails" xlink:type="simple" xlink:href="iclr-20211231.xsd#AccumulatedOtherComprehensiveIncomeDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/AccumulatedOtherComprehensiveIncomeDetails" xlink:type="extended" id="i11150d63e2e94cbfb84b6dcda1e094e6_AccumulatedOtherComprehensiveIncomeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_64d67a18-424a-40b6-baf7-7acd245ae516" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossForeignCurrencyTranslationAdjustmentNetOfTax_1e4d4922-39e0-4698-901b-d3c572665f20" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossForeignCurrencyTranslationAdjustmentNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_64d67a18-424a-40b6-baf7-7acd245ae516" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossForeignCurrencyTranslationAdjustmentNetOfTax_1e4d4922-39e0-4698-901b-d3c572665f20" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_AccumulatedOtherComprehensiveIncomeLossCumulativeChangesInCurrencyImpactOnLongTermFundingNetOfTax_462db7a1-3857-4bbe-b6ee-294c978ec1bd" xlink:href="iclr-20211231.xsd#iclr_AccumulatedOtherComprehensiveIncomeLossCumulativeChangesInCurrencyImpactOnLongTermFundingNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_64d67a18-424a-40b6-baf7-7acd245ae516" xlink:to="loc_iclr_AccumulatedOtherComprehensiveIncomeLossCumulativeChangesInCurrencyImpactOnLongTermFundingNetOfTax_462db7a1-3857-4bbe-b6ee-294c978ec1bd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossDefinedBenefitPensionAndOtherPostretirementPlansNetOfTax_e9f491d3-8f7b-4e01-b4fa-6ce3eda371ef" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossDefinedBenefitPensionAndOtherPostretirementPlansNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_64d67a18-424a-40b6-baf7-7acd245ae516" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossDefinedBenefitPensionAndOtherPostretirementPlansNetOfTax_e9f491d3-8f7b-4e01-b4fa-6ce3eda371ef" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AociLossCashFlowHedgeCumulativeGainLossAfterTax_69af2baa-9ea3-4641-8487-a07e82b91953" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AociLossCashFlowHedgeCumulativeGainLossAfterTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_64d67a18-424a-40b6-baf7-7acd245ae516" xlink:to="loc_us-gaap_AociLossCashFlowHedgeCumulativeGainLossAfterTax_69af2baa-9ea3-4641-8487-a07e82b91953" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_AccumulatedOtherComprehensiveIncomeLossAmortizationOfCashFlowHedgesEffectNetofTax_e6fe8eb5-8c54-4715-b565-bac7272976fe" xlink:href="iclr-20211231.xsd#iclr_AccumulatedOtherComprehensiveIncomeLossAmortizationOfCashFlowHedgesEffectNetofTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_64d67a18-424a-40b6-baf7-7acd245ae516" xlink:to="loc_iclr_AccumulatedOtherComprehensiveIncomeLossAmortizationOfCashFlowHedgesEffectNetofTax_e6fe8eb5-8c54-4715-b565-bac7272976fe" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_AcumulatedOtherComprehensiveLossCashFlowHedgeLossReclassificationAfterTax_fa74e961-4f32-4f92-9941-ab8e7c961f0c" xlink:href="iclr-20211231.xsd#iclr_AcumulatedOtherComprehensiveLossCashFlowHedgeLossReclassificationAfterTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_64d67a18-424a-40b6-baf7-7acd245ae516" xlink:to="loc_iclr_AcumulatedOtherComprehensiveLossCashFlowHedgeLossReclassificationAfterTax_fa74e961-4f32-4f92-9941-ab8e7c961f0c" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_AccumulatedOtherComprehensiveLossAmortizationOfCashFlowHedgesEffectNetOfTax_effc27e6-f805-4aa9-9ae7-0e11fbebfe90" xlink:href="iclr-20211231.xsd#iclr_AccumulatedOtherComprehensiveLossAmortizationOfCashFlowHedgesEffectNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_64d67a18-424a-40b6-baf7-7acd245ae516" xlink:to="loc_iclr_AccumulatedOtherComprehensiveLossAmortizationOfCashFlowHedgesEffectNetOfTax_effc27e6-f805-4aa9-9ae7-0e11fbebfe90" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax_82f04895-4715-45eb-8d1e-ae94e47f0b30" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_64d67a18-424a-40b6-baf7-7acd245ae516" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax_82f04895-4715-45eb-8d1e-ae94e47f0b30" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_f91d0431-15b4-42e5-8877-2f76dc073410" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_64d67a18-424a-40b6-baf7-7acd245ae516" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_f91d0431-15b4-42e5-8877-2f76dc073410" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeInstrumentRiskAxis_5e1a7acb-4b74-435c-a5f2-f02c42e9b1de" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DerivativeInstrumentRiskAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_f91d0431-15b4-42e5-8877-2f76dc073410" xlink:to="loc_us-gaap_DerivativeInstrumentRiskAxis_5e1a7acb-4b74-435c-a5f2-f02c42e9b1de" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_5e1a7acb-4b74-435c-a5f2-f02c42e9b1de_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_5e1a7acb-4b74-435c-a5f2-f02c42e9b1de" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_5e1a7acb-4b74-435c-a5f2-f02c42e9b1de_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_63d46ee3-9b59-4838-b195-a6515a76da9c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_5e1a7acb-4b74-435c-a5f2-f02c42e9b1de" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_63d46ee3-9b59-4838-b195-a6515a76da9c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestRateContractMember_04a6cc5a-7a8d-4d64-988b-74abe697c21e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_InterestRateContractMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_63d46ee3-9b59-4838-b195-a6515a76da9c" xlink:to="loc_us-gaap_InterestRateContractMember_04a6cc5a-7a8d-4d64-988b-74abe697c21e" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/OperatingLeasesOperatingLeasesNarrativeDetails" xlink:type="simple" xlink:href="iclr-20211231.xsd#OperatingLeasesOperatingLeasesNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/OperatingLeasesOperatingLeasesNarrativeDetails" xlink:type="extended" id="i91530f8d132745fab039b2c2c67a930f_OperatingLeasesOperatingLeasesNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_343d9c0d-1217-4940-b5ef-ce7df4a2f486" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LeaseCost_103b6fbb-10ca-4903-a9b0-17cd45797169" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LeaseCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_343d9c0d-1217-4940-b5ef-ce7df4a2f486" xlink:to="loc_us-gaap_LeaseCost_103b6fbb-10ca-4903-a9b0-17cd45797169" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableLeaseCost_a269f952-7f23-4cca-890e-6a24ebc95bc1" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_VariableLeaseCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_343d9c0d-1217-4940-b5ef-ce7df4a2f486" xlink:to="loc_us-gaap_VariableLeaseCost_a269f952-7f23-4cca-890e-6a24ebc95bc1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RightOfUseAssetObtainedInExchangeForOperatingLeaseLiability_8a9a318a-c1ab-4afd-9dab-61c3a074aa47" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RightOfUseAssetObtainedInExchangeForOperatingLeaseLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_343d9c0d-1217-4940-b5ef-ce7df4a2f486" xlink:to="loc_us-gaap_RightOfUseAssetObtainedInExchangeForOperatingLeaseLiability_8a9a318a-c1ab-4afd-9dab-61c3a074aa47" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseImpairmentLoss_0d4148af-f9c1-4116-bdf7-c02c3fbb10a9" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OperatingLeaseImpairmentLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_343d9c0d-1217-4940-b5ef-ce7df4a2f486" xlink:to="loc_us-gaap_OperatingLeaseImpairmentLoss_0d4148af-f9c1-4116-bdf7-c02c3fbb10a9" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseWeightedAverageRemainingLeaseTerm1_26c3c7a6-e121-4991-aec2-2205b90e41b8" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OperatingLeaseWeightedAverageRemainingLeaseTerm1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_343d9c0d-1217-4940-b5ef-ce7df4a2f486" xlink:to="loc_us-gaap_OperatingLeaseWeightedAverageRemainingLeaseTerm1_26c3c7a6-e121-4991-aec2-2205b90e41b8" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseWeightedAverageDiscountRatePercent_fc8c2597-40a4-4726-b85d-ee1bccd221e7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OperatingLeaseWeightedAverageDiscountRatePercent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_343d9c0d-1217-4940-b5ef-ce7df4a2f486" xlink:to="loc_us-gaap_OperatingLeaseWeightedAverageDiscountRatePercent_fc8c2597-40a4-4726-b85d-ee1bccd221e7" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseLiabilityCurrent_68991803-04d4-45cb-a645-7d1baa61eb48" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OperatingLeaseLiabilityCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_343d9c0d-1217-4940-b5ef-ce7df4a2f486" xlink:to="loc_us-gaap_OperatingLeaseLiabilityCurrent_68991803-04d4-45cb-a645-7d1baa61eb48" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseLiabilityStatementOfFinancialPositionExtensibleList_29b9427b-c08c-46ac-a30a-e018afbddfc9" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OperatingLeaseLiabilityStatementOfFinancialPositionExtensibleList"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_343d9c0d-1217-4940-b5ef-ce7df4a2f486" xlink:to="loc_us-gaap_OperatingLeaseLiabilityStatementOfFinancialPositionExtensibleList_29b9427b-c08c-46ac-a30a-e018afbddfc9" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable_7f65d05b-759f-4e69-9504-b7472494a29c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_343d9c0d-1217-4940-b5ef-ce7df4a2f486" xlink:to="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable_7f65d05b-759f-4e69-9504-b7472494a29c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_afdf2020-3eea-433a-9500-a0dcdc6e4992" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeStatementLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable_7f65d05b-759f-4e69-9504-b7472494a29c" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_afdf2020-3eea-433a-9500-a0dcdc6e4992" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_afdf2020-3eea-433a-9500-a0dcdc6e4992_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_afdf2020-3eea-433a-9500-a0dcdc6e4992" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_afdf2020-3eea-433a-9500-a0dcdc6e4992_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_2415b499-33fd-432d-a965-beba23658dbb" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_afdf2020-3eea-433a-9500-a0dcdc6e4992" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_2415b499-33fd-432d-a965-beba23658dbb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SellingGeneralAndAdministrativeExpensesMember_3a53e4a4-2b38-44b9-a0b2-cc4aa4f15b3b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SellingGeneralAndAdministrativeExpensesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_2415b499-33fd-432d-a965-beba23658dbb" xlink:to="loc_us-gaap_SellingGeneralAndAdministrativeExpensesMember_3a53e4a4-2b38-44b9-a0b2-cc4aa4f15b3b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostOfSalesMember_824960ca-964c-43c4-8fd5-b3ce46891ca9" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CostOfSalesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_2415b499-33fd-432d-a965-beba23658dbb" xlink:to="loc_us-gaap_CostOfSalesMember_824960ca-964c-43c4-8fd5-b3ce46891ca9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationAxis_787dc55f-f597-4fc7-b25e-cbf26d534ef4" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BalanceSheetLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable_7f65d05b-759f-4e69-9504-b7472494a29c" xlink:to="loc_us-gaap_BalanceSheetLocationAxis_787dc55f-f597-4fc7-b25e-cbf26d534ef4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_787dc55f-f597-4fc7-b25e-cbf26d534ef4_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BalanceSheetLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_787dc55f-f597-4fc7-b25e-cbf26d534ef4" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_787dc55f-f597-4fc7-b25e-cbf26d534ef4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_5b12e2e0-bffe-487f-8388-3fd8da75cd76" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BalanceSheetLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_787dc55f-f597-4fc7-b25e-cbf26d534ef4" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_5b12e2e0-bffe-487f-8388-3fd8da75cd76" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherCurrentLiabilitiesMember_316a2523-53a0-4c28-862a-9f0b1a3eb1bf" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OtherCurrentLiabilitiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_5b12e2e0-bffe-487f-8388-3fd8da75cd76" xlink:to="loc_us-gaap_OtherCurrentLiabilitiesMember_316a2523-53a0-4c28-862a-9f0b1a3eb1bf" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/NoncurrentbankcreditlinesandloanfacilitiesScheduleofLongtermDebtDetails" xlink:type="simple" xlink:href="iclr-20211231.xsd#NoncurrentbankcreditlinesandloanfacilitiesScheduleofLongtermDebtDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/NoncurrentbankcreditlinesandloanfacilitiesScheduleofLongtermDebtDetails" xlink:type="extended" id="if86f8ffd31d34398badc9e243ab7f6bb_NoncurrentbankcreditlinesandloanfacilitiesScheduleofLongtermDebtDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_5cff42d1-cabc-472e-9751-69e31869f619" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_af8fa9d6-ba67-4e72-b3e7-ed3d668a6609" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_5cff42d1-cabc-472e-9751-69e31869f619" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_af8fa9d6-ba67-4e72-b3e7-ed3d668a6609" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentCarryingAmount_856f4d18-9205-4806-8eae-4b6e9e9dc0e4" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentCarryingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_5cff42d1-cabc-472e-9751-69e31869f619" xlink:to="loc_us-gaap_DebtInstrumentCarryingAmount_856f4d18-9205-4806-8eae-4b6e9e9dc0e4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_LongTermDebtCurrentMaturitiesGross_759618e3-c61d-4871-85a4-0a3ce13876e0" xlink:href="iclr-20211231.xsd#iclr_LongTermDebtCurrentMaturitiesGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_5cff42d1-cabc-472e-9751-69e31869f619" xlink:to="loc_iclr_LongTermDebtCurrentMaturitiesGross_759618e3-c61d-4871-85a4-0a3ce13876e0" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_LongTermDebtExcludingCurrentMaturitiesGross_b36fbdad-50c6-470b-9ad7-ae3c44473965" xlink:href="iclr-20211231.xsd#iclr_LongTermDebtExcludingCurrentMaturitiesGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_5cff42d1-cabc-472e-9751-69e31869f619" xlink:to="loc_iclr_LongTermDebtExcludingCurrentMaturitiesGross_b36fbdad-50c6-470b-9ad7-ae3c44473965" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentUnamortizedDiscountPremiumAndDebtIssuanceCostsNet_42c28922-4fc6-43f7-a4c0-b9ebfe2cd1a1" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentUnamortizedDiscountPremiumAndDebtIssuanceCostsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_5cff42d1-cabc-472e-9751-69e31869f619" xlink:to="loc_us-gaap_DebtInstrumentUnamortizedDiscountPremiumAndDebtIssuanceCostsNet_42c28922-4fc6-43f7-a4c0-b9ebfe2cd1a1" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtNoncurrent_e06e0fc3-0de5-40b1-9682-dc3ccbe550cd" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LongTermDebtNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_5cff42d1-cabc-472e-9751-69e31869f619" xlink:to="loc_us-gaap_LongTermDebtNoncurrent_e06e0fc3-0de5-40b1-9682-dc3ccbe550cd" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_eb216a20-52ef-4916-b246-1d72d2ae0e7d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_5cff42d1-cabc-472e-9751-69e31869f619" xlink:to="loc_us-gaap_DebtInstrumentTable_eb216a20-52ef-4916-b246-1d72d2ae0e7d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_c46f97b1-c2dc-4739-af9a-ff1e8f13e3aa" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_eb216a20-52ef-4916-b246-1d72d2ae0e7d" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_c46f97b1-c2dc-4739-af9a-ff1e8f13e3aa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_c46f97b1-c2dc-4739-af9a-ff1e8f13e3aa_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_c46f97b1-c2dc-4739-af9a-ff1e8f13e3aa" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_c46f97b1-c2dc-4739-af9a-ff1e8f13e3aa_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_c26040fa-804d-4507-92dc-86f322b594f3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_c46f97b1-c2dc-4739-af9a-ff1e8f13e3aa" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_c26040fa-804d-4507-92dc-86f322b594f3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredDebtMember_4e4607c9-40bd-4329-96b1-64de54cad9d4" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_c26040fa-804d-4507-92dc-86f322b594f3" xlink:to="loc_us-gaap_SecuredDebtMember_4e4607c9-40bd-4329-96b1-64de54cad9d4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SeniorSubordinatedNotesMember_db21605e-078f-4ec4-825b-25b342a8aa3a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SeniorSubordinatedNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_c26040fa-804d-4507-92dc-86f322b594f3" xlink:to="loc_us-gaap_SeniorSubordinatedNotesMember_db21605e-078f-4ec4-825b-25b342a8aa3a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SeniorNotesMember_0b10f726-1980-42b7-81a0-b3e8541f1429" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SeniorNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_c26040fa-804d-4507-92dc-86f322b594f3" xlink:to="loc_us-gaap_SeniorNotesMember_0b10f726-1980-42b7-81a0-b3e8541f1429" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_4b53337e-e6ee-4743-be52-9d35eaf3f456" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_eb216a20-52ef-4916-b246-1d72d2ae0e7d" xlink:to="loc_us-gaap_DebtInstrumentAxis_4b53337e-e6ee-4743-be52-9d35eaf3f456" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_4b53337e-e6ee-4743-be52-9d35eaf3f456_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_4b53337e-e6ee-4743-be52-9d35eaf3f456" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_4b53337e-e6ee-4743-be52-9d35eaf3f456_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_9a75d4b0-b6c8-4d2d-9943-9cb463cf0de6" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_4b53337e-e6ee-4743-be52-9d35eaf3f456" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_9a75d4b0-b6c8-4d2d-9943-9cb463cf0de6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_TermLoanFacilityMember_44a048e2-d2e2-4abe-aef3-27c94ea8c65c" xlink:href="iclr-20211231.xsd#iclr_TermLoanFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_9a75d4b0-b6c8-4d2d-9943-9cb463cf0de6" xlink:to="loc_iclr_TermLoanFacilityMember_44a048e2-d2e2-4abe-aef3-27c94ea8c65c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_SeniorANotesMember_39855a04-c6ed-441b-904c-1c91637791df" xlink:href="iclr-20211231.xsd#iclr_SeniorANotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_9a75d4b0-b6c8-4d2d-9943-9cb463cf0de6" xlink:to="loc_iclr_SeniorANotesMember_39855a04-c6ed-441b-904c-1c91637791df" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_SeniorBNotesMember_ada6ae49-e866-41f7-a572-5179d2756a5d" xlink:href="iclr-20211231.xsd#iclr_SeniorBNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_9a75d4b0-b6c8-4d2d-9943-9cb463cf0de6" xlink:to="loc_iclr_SeniorBNotesMember_ada6ae49-e866-41f7-a572-5179d2756a5d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_SeniorSecuredNotesMember_dfa70097-1549-407c-9926-41a774047096" xlink:href="iclr-20211231.xsd#iclr_SeniorSecuredNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_9a75d4b0-b6c8-4d2d-9943-9cb463cf0de6" xlink:to="loc_iclr_SeniorSecuredNotesMember_dfa70097-1549-407c-9926-41a774047096" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/NoncurrentbankcreditlinesandloanfacilitiesNarrativeDetails" xlink:type="simple" xlink:href="iclr-20211231.xsd#NoncurrentbankcreditlinesandloanfacilitiesNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/NoncurrentbankcreditlinesandloanfacilitiesNarrativeDetails" xlink:type="extended" id="i4da49a20ee5044b78be6dbb37dc279b3_NoncurrentbankcreditlinesandloanfacilitiesNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_ccebfde2-a21f-4363-90e0-43d08e907f76" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentUnamortizedDiscount_5ac38099-fd6d-49d3-a05b-d7c8cb8e7a1c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentUnamortizedDiscount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ccebfde2-a21f-4363-90e0-43d08e907f76" xlink:to="loc_us-gaap_DebtInstrumentUnamortizedDiscount_5ac38099-fd6d-49d3-a05b-d7c8cb8e7a1c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpense_5ca3d6a1-cd2c-4b42-9e08-7092a8e4807a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_InterestExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ccebfde2-a21f-4363-90e0-43d08e907f76" xlink:to="loc_us-gaap_InterestExpense_5ca3d6a1-cd2c-4b42-9e08-7092a8e4807a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentFaceAmount_4fbeb34b-0699-4ed5-9587-e9a2680e87a8" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentFaceAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ccebfde2-a21f-4363-90e0-43d08e907f76" xlink:to="loc_us-gaap_DebtInstrumentFaceAmount_4fbeb34b-0699-4ed5-9587-e9a2680e87a8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_02eef8d8-ff99-4f65-aeb5-09ae7cbfbbe5" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ccebfde2-a21f-4363-90e0-43d08e907f76" xlink:to="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_02eef8d8-ff99-4f65-aeb5-09ae7cbfbbe5" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_e5e05738-450a-4947-a117-c227fd20fb3c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ccebfde2-a21f-4363-90e0-43d08e907f76" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_e5e05738-450a-4947-a117-c227fd20fb3c" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_c08daaaf-a421-4583-b858-12402056e1a7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentBasisSpreadOnVariableRate1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ccebfde2-a21f-4363-90e0-43d08e907f76" xlink:to="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_c08daaaf-a421-4583-b858-12402056e1a7" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_DebtInstrumentBasisSpreadOnVariableRateStepDownPercentage_226b00af-76d7-48b8-990e-b951dc0bcc12" xlink:href="iclr-20211231.xsd#iclr_DebtInstrumentBasisSpreadOnVariableRateStepDownPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ccebfde2-a21f-4363-90e0-43d08e907f76" xlink:to="loc_iclr_DebtInstrumentBasisSpreadOnVariableRateStepDownPercentage_226b00af-76d7-48b8-990e-b951dc0bcc12" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_DebtInstrumentCovenantFirstLienNetLeverageRatio_8bfdaacb-9a68-4384-809c-8bc16cfc8df9" xlink:href="iclr-20211231.xsd#iclr_DebtInstrumentCovenantFirstLienNetLeverageRatio"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ccebfde2-a21f-4363-90e0-43d08e907f76" xlink:to="loc_iclr_DebtInstrumentCovenantFirstLienNetLeverageRatio_8bfdaacb-9a68-4384-809c-8bc16cfc8df9" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_DebtInstrumentIncreaseDecreaseBasisSpreadOnVariableRate_09781c5e-65e0-4911-91e8-0308a67e56fb" xlink:href="iclr-20211231.xsd#iclr_DebtInstrumentIncreaseDecreaseBasisSpreadOnVariableRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ccebfde2-a21f-4363-90e0-43d08e907f76" xlink:to="loc_iclr_DebtInstrumentIncreaseDecreaseBasisSpreadOnVariableRate_09781c5e-65e0-4911-91e8-0308a67e56fb" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCredit_a708e046-0a49-4146-bbba-08ff96d4778a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LineOfCredit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ccebfde2-a21f-4363-90e0-43d08e907f76" xlink:to="loc_us-gaap_LineOfCredit_a708e046-0a49-4146-bbba-08ff96d4778a" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LettersOfCreditOutstandingAmount_c7d006db-a00b-419c-8b9e-9dea7948ecb1" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LettersOfCreditOutstandingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ccebfde2-a21f-4363-90e0-43d08e907f76" xlink:to="loc_us-gaap_LettersOfCreditOutstandingAmount_c7d006db-a00b-419c-8b9e-9dea7948ecb1" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RepaymentsOfSeniorDebt_80b64712-c871-40a1-a19a-28ac849459aa" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RepaymentsOfSeniorDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ccebfde2-a21f-4363-90e0-43d08e907f76" xlink:to="loc_us-gaap_RepaymentsOfSeniorDebt_80b64712-c871-40a1-a19a-28ac849459aa" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestPaidNet_f536d4fb-54cd-45e7-88d9-0de3df936468" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_InterestPaidNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ccebfde2-a21f-4363-90e0-43d08e907f76" xlink:to="loc_us-gaap_InterestPaidNet_f536d4fb-54cd-45e7-88d9-0de3df936468" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsOfDebtRestructuringCosts_a85788da-03a0-4e1b-83e6-c5b1cbcf1301" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PaymentsOfDebtRestructuringCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ccebfde2-a21f-4363-90e0-43d08e907f76" xlink:to="loc_us-gaap_PaymentsOfDebtRestructuringCosts_a85788da-03a0-4e1b-83e6-c5b1cbcf1301" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromIssuanceOfLongTermDebt_15b18093-51c5-405b-9a54-5ce7db2dd743" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ProceedsFromIssuanceOfLongTermDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ccebfde2-a21f-4363-90e0-43d08e907f76" xlink:to="loc_us-gaap_ProceedsFromIssuanceOfLongTermDebt_15b18093-51c5-405b-9a54-5ce7db2dd743" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_IssueOfSeniorNotesNumberOfTranches_effb9e23-d24d-4c28-b1e8-a84524082406" xlink:href="iclr-20211231.xsd#iclr_IssueOfSeniorNotesNumberOfTranches"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ccebfde2-a21f-4363-90e0-43d08e907f76" xlink:to="loc_iclr_IssueOfSeniorNotesNumberOfTranches_effb9e23-d24d-4c28-b1e8-a84524082406" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateDuringPeriod_37a17ea6-74da-4a32-8a75-d2cb711d3351" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentInterestRateDuringPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ccebfde2-a21f-4363-90e0-43d08e907f76" xlink:to="loc_us-gaap_DebtInstrumentInterestRateDuringPeriod_37a17ea6-74da-4a32-8a75-d2cb711d3351" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RepaymentsOfDebt_aef78467-7e06-44be-9b36-d045612b1aab" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RepaymentsOfDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ccebfde2-a21f-4363-90e0-43d08e907f76" xlink:to="loc_us-gaap_RepaymentsOfDebt_aef78467-7e06-44be-9b36-d045612b1aab" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtFairValue_083a2987-58e5-4af7-a244-46f8c5f60ded" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LongTermDebtFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ccebfde2-a21f-4363-90e0-43d08e907f76" xlink:to="loc_us-gaap_LongTermDebtFairValue_083a2987-58e5-4af7-a244-46f8c5f60ded" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_5dee5fa7-95e6-4560-9d9a-5e687a09832a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ccebfde2-a21f-4363-90e0-43d08e907f76" xlink:to="loc_us-gaap_DebtInstrumentTable_5dee5fa7-95e6-4560-9d9a-5e687a09832a" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_e8a09fce-9348-45c8-a4c4-e3089b79a6da" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_5dee5fa7-95e6-4560-9d9a-5e687a09832a" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_e8a09fce-9348-45c8-a4c4-e3089b79a6da" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_e8a09fce-9348-45c8-a4c4-e3089b79a6da_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_e8a09fce-9348-45c8-a4c4-e3089b79a6da" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_e8a09fce-9348-45c8-a4c4-e3089b79a6da_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_aeb340d8-3f90-467d-90f3-e318bcb590ca" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_e8a09fce-9348-45c8-a4c4-e3089b79a6da" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_aeb340d8-3f90-467d-90f3-e318bcb590ca" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredDebtMember_353d4180-8b6f-4403-b2db-d7bef800e8da" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_aeb340d8-3f90-467d-90f3-e318bcb590ca" xlink:to="loc_us-gaap_SecuredDebtMember_353d4180-8b6f-4403-b2db-d7bef800e8da" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditMember_d13ae572-f2c6-4c3c-8bdd-d62e250115b6" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LineOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_aeb340d8-3f90-467d-90f3-e318bcb590ca" xlink:to="loc_us-gaap_LineOfCreditMember_d13ae572-f2c6-4c3c-8bdd-d62e250115b6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LetterOfCreditMember_9658f595-634d-4fcb-8edd-062e8902270a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LetterOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_aeb340d8-3f90-467d-90f3-e318bcb590ca" xlink:to="loc_us-gaap_LetterOfCreditMember_9658f595-634d-4fcb-8edd-062e8902270a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SeniorSubordinatedNotesMember_814ba4ce-e35b-4deb-9240-d2df6c3b2048" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SeniorSubordinatedNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_aeb340d8-3f90-467d-90f3-e318bcb590ca" xlink:to="loc_us-gaap_SeniorSubordinatedNotesMember_814ba4ce-e35b-4deb-9240-d2df6c3b2048" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SeniorNotesMember_2c7f37ea-d5bb-46f8-93e4-eeceaac58114" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SeniorNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_aeb340d8-3f90-467d-90f3-e318bcb590ca" xlink:to="loc_us-gaap_SeniorNotesMember_2c7f37ea-d5bb-46f8-93e4-eeceaac58114" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_2c7d1c5f-4a3a-4f1e-acbf-a05883b94a1e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_5dee5fa7-95e6-4560-9d9a-5e687a09832a" xlink:to="loc_us-gaap_DebtInstrumentAxis_2c7d1c5f-4a3a-4f1e-acbf-a05883b94a1e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_2c7d1c5f-4a3a-4f1e-acbf-a05883b94a1e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_2c7d1c5f-4a3a-4f1e-acbf-a05883b94a1e" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_2c7d1c5f-4a3a-4f1e-acbf-a05883b94a1e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_f0c7eef9-5451-4abb-a905-0a83bdd8ef73" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_2c7d1c5f-4a3a-4f1e-acbf-a05883b94a1e" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_f0c7eef9-5451-4abb-a905-0a83bdd8ef73" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_SeniorSecuredCreditFacilityAndSeniorSecuredNotesMember_5bbb3b58-b65d-4771-af55-bcdeee82c03a" xlink:href="iclr-20211231.xsd#iclr_SeniorSecuredCreditFacilityAndSeniorSecuredNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_f0c7eef9-5451-4abb-a905-0a83bdd8ef73" xlink:to="loc_iclr_SeniorSecuredCreditFacilityAndSeniorSecuredNotesMember_5bbb3b58-b65d-4771-af55-bcdeee82c03a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_TermLoanFacilityMember_991e42b0-12ef-4bee-8b68-6ec376ebdb61" xlink:href="iclr-20211231.xsd#iclr_TermLoanFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_f0c7eef9-5451-4abb-a905-0a83bdd8ef73" xlink:to="loc_iclr_TermLoanFacilityMember_991e42b0-12ef-4bee-8b68-6ec376ebdb61" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_SeniorSecuredRevolvingLoanFacilityMember_3630b999-19ac-43fe-8e76-086475418d8a" xlink:href="iclr-20211231.xsd#iclr_SeniorSecuredRevolvingLoanFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_f0c7eef9-5451-4abb-a905-0a83bdd8ef73" xlink:to="loc_iclr_SeniorSecuredRevolvingLoanFacilityMember_3630b999-19ac-43fe-8e76-086475418d8a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_SeniorSecuredNotesMember_f3467378-f724-4600-bdec-ddeb42224549" xlink:href="iclr-20211231.xsd#iclr_SeniorSecuredNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_f0c7eef9-5451-4abb-a905-0a83bdd8ef73" xlink:to="loc_iclr_SeniorSecuredNotesMember_f3467378-f724-4600-bdec-ddeb42224549" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_SeniorNotes2020Member_104c8319-f924-481f-a653-ced3d60ac919" xlink:href="iclr-20211231.xsd#iclr_SeniorNotes2020Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_f0c7eef9-5451-4abb-a905-0a83bdd8ef73" xlink:to="loc_iclr_SeniorNotes2020Member_104c8319-f924-481f-a653-ced3d60ac919" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_SeniorANotesMember_8199671b-6eef-4a42-bd7d-5890f2d2f68d" xlink:href="iclr-20211231.xsd#iclr_SeniorANotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_f0c7eef9-5451-4abb-a905-0a83bdd8ef73" xlink:to="loc_iclr_SeniorANotesMember_8199671b-6eef-4a42-bd7d-5890f2d2f68d" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_SeniorBNotesMember_55b7c14d-a993-47f4-889f-286e21798ae0" xlink:href="iclr-20211231.xsd#iclr_SeniorBNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_f0c7eef9-5451-4abb-a905-0a83bdd8ef73" xlink:to="loc_iclr_SeniorBNotesMember_55b7c14d-a993-47f4-889f-286e21798ae0" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityAxis_f0cffa91-8333-4705-83eb-6150a94097e4" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CreditFacilityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_5dee5fa7-95e6-4560-9d9a-5e687a09832a" xlink:to="loc_us-gaap_CreditFacilityAxis_f0cffa91-8333-4705-83eb-6150a94097e4" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_f0cffa91-8333-4705-83eb-6150a94097e4_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_f0cffa91-8333-4705-83eb-6150a94097e4" xlink:to="loc_us-gaap_CreditFacilityDomain_f0cffa91-8333-4705-83eb-6150a94097e4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_3ff57f3f-e96e-4549-a6f9-62d30342fa74" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_f0cffa91-8333-4705-83eb-6150a94097e4" xlink:to="loc_us-gaap_CreditFacilityDomain_3ff57f3f-e96e-4549-a6f9-62d30342fa74" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevolvingCreditFacilityMember_818c2d7a-0319-49a5-a040-33c79d9f8d03" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RevolvingCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_3ff57f3f-e96e-4549-a6f9-62d30342fa74" xlink:to="loc_us-gaap_RevolvingCreditFacilityMember_818c2d7a-0319-49a5-a040-33c79d9f8d03" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_VariableRateComponentAxis_788fda0f-4e2a-4416-bbf1-f5933ad94002" xlink:href="iclr-20211231.xsd#iclr_VariableRateComponentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_5dee5fa7-95e6-4560-9d9a-5e687a09832a" xlink:to="loc_iclr_VariableRateComponentAxis_788fda0f-4e2a-4416-bbf1-f5933ad94002" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_VariableRateComponentDomain_788fda0f-4e2a-4416-bbf1-f5933ad94002_default" xlink:href="iclr-20211231.xsd#iclr_VariableRateComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_iclr_VariableRateComponentAxis_788fda0f-4e2a-4416-bbf1-f5933ad94002" xlink:to="loc_iclr_VariableRateComponentDomain_788fda0f-4e2a-4416-bbf1-f5933ad94002_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_VariableRateComponentDomain_375fcc8d-e810-42cd-bb8a-e71aa7d400f1" xlink:href="iclr-20211231.xsd#iclr_VariableRateComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_iclr_VariableRateComponentAxis_788fda0f-4e2a-4416-bbf1-f5933ad94002" xlink:to="loc_iclr_VariableRateComponentDomain_375fcc8d-e810-42cd-bb8a-e71aa7d400f1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_VariableRateComponentOneMember_7f5ea0ce-6f44-4a37-b74f-686430f5c771" xlink:href="iclr-20211231.xsd#iclr_VariableRateComponentOneMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iclr_VariableRateComponentDomain_375fcc8d-e810-42cd-bb8a-e71aa7d400f1" xlink:to="loc_iclr_VariableRateComponentOneMember_7f5ea0ce-6f44-4a37-b74f-686430f5c771" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_VariableRateComponentTwoMember_60019700-267a-4896-b91c-f82324718ca4" xlink:href="iclr-20211231.xsd#iclr_VariableRateComponentTwoMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iclr_VariableRateComponentDomain_375fcc8d-e810-42cd-bb8a-e71aa7d400f1" xlink:to="loc_iclr_VariableRateComponentTwoMember_60019700-267a-4896-b91c-f82324718ca4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateAxis_d5ebbc9e-bf83-4a12-a709-6cc039a9ce0a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_VariableRateAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_5dee5fa7-95e6-4560-9d9a-5e687a09832a" xlink:to="loc_us-gaap_VariableRateAxis_d5ebbc9e-bf83-4a12-a709-6cc039a9ce0a" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateDomain_d5ebbc9e-bf83-4a12-a709-6cc039a9ce0a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_VariableRateDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_VariableRateAxis_d5ebbc9e-bf83-4a12-a709-6cc039a9ce0a" xlink:to="loc_us-gaap_VariableRateDomain_d5ebbc9e-bf83-4a12-a709-6cc039a9ce0a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateDomain_e6b28bf5-498e-4405-aaab-4f45f734ff9f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_VariableRateDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_VariableRateAxis_d5ebbc9e-bf83-4a12-a709-6cc039a9ce0a" xlink:to="loc_us-gaap_VariableRateDomain_e6b28bf5-498e-4405-aaab-4f45f734ff9f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LondonInterbankOfferedRateLIBORMember_c9b213bb-f9a9-4c6c-8c43-6b4f98216ae7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LondonInterbankOfferedRateLIBORMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableRateDomain_e6b28bf5-498e-4405-aaab-4f45f734ff9f" xlink:to="loc_us-gaap_LondonInterbankOfferedRateLIBORMember_c9b213bb-f9a9-4c6c-8c43-6b4f98216ae7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BaseRateMember_01715ed7-4e45-4b0b-b44b-1e418479e441" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BaseRateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableRateDomain_e6b28bf5-498e-4405-aaab-4f45f734ff9f" xlink:to="loc_us-gaap_BaseRateMember_01715ed7-4e45-4b0b-b44b-1e418479e441" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_9af2e9ab-a19d-45a1-9a59-64ac8fc12e40" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_5dee5fa7-95e6-4560-9d9a-5e687a09832a" xlink:to="loc_srt_RangeAxis_9af2e9ab-a19d-45a1-9a59-64ac8fc12e40" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_9af2e9ab-a19d-45a1-9a59-64ac8fc12e40_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_9af2e9ab-a19d-45a1-9a59-64ac8fc12e40" xlink:to="loc_srt_RangeMember_9af2e9ab-a19d-45a1-9a59-64ac8fc12e40_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_3460a660-3092-4745-bd8a-acd54c1cd598" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_9af2e9ab-a19d-45a1-9a59-64ac8fc12e40" xlink:to="loc_srt_RangeMember_3460a660-3092-4745-bd8a-acd54c1cd598" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_82603ff7-ea69-4060-b6d0-36c43a707eb0" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_3460a660-3092-4745-bd8a-acd54c1cd598" xlink:to="loc_srt_MaximumMember_82603ff7-ea69-4060-b6d0-36c43a707eb0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_b0410d76-8748-4499-a6d5-be144b223be6" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_3460a660-3092-4745-bd8a-acd54c1cd598" xlink:to="loc_srt_MinimumMember_b0410d76-8748-4499-a6d5-be144b223be6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_e64a983a-cdf4-44af-a869-84185afb8d21" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_5dee5fa7-95e6-4560-9d9a-5e687a09832a" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_e64a983a-cdf4-44af-a869-84185afb8d21" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_e64a983a-cdf4-44af-a869-84185afb8d21_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_e64a983a-cdf4-44af-a869-84185afb8d21" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_e64a983a-cdf4-44af-a869-84185afb8d21_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_227ac2e8-e7a4-473c-9359-d63534b0fdd7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_e64a983a-cdf4-44af-a869-84185afb8d21" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_227ac2e8-e7a4-473c-9359-d63534b0fdd7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_PRAHealthSciencesIncMember_91613663-3074-4434-ab43-16d8fa20d6e9" xlink:href="iclr-20211231.xsd#iclr_PRAHealthSciencesIncMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_227ac2e8-e7a4-473c-9359-d63534b0fdd7" xlink:to="loc_iclr_PRAHealthSciencesIncMember_91613663-3074-4434-ab43-16d8fa20d6e9" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/FairvalueDetails" xlink:type="simple" xlink:href="iclr-20211231.xsd#FairvalueDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/FairvalueDetails" xlink:type="extended" id="if622d428342a4a14902947429eee1fbd_FairvalueDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_2d7dce7e-72b4-4c9b-a188-c7c5b2d39d2b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesFvNiGainLoss_0996263d-e39b-4d74-89c7-6cae2e5bfd23" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EquitySecuritiesFvNiGainLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_2d7dce7e-72b4-4c9b-a188-c7c5b2d39d2b" xlink:to="loc_us-gaap_EquitySecuritiesFvNiGainLoss_0996263d-e39b-4d74-89c7-6cae2e5bfd23" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesFvNi_24e1e308-3610-4e5d-aaac-a96aecd86ecf" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EquitySecuritiesFvNi"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_2d7dce7e-72b4-4c9b-a188-c7c5b2d39d2b" xlink:to="loc_us-gaap_EquitySecuritiesFvNi_24e1e308-3610-4e5d-aaac-a96aecd86ecf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsFairValueDisclosure_d4c0df37-b45f-4211-8b0b-cb722ba158d9" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AssetsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_2d7dce7e-72b4-4c9b-a188-c7c5b2d39d2b" xlink:to="loc_us-gaap_AssetsFairValueDisclosure_d4c0df37-b45f-4211-8b0b-cb722ba158d9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillFairValueDisclosure_d8c29b05-6fa2-4017-bbc5-8ac87d836520" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_GoodwillFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_2d7dce7e-72b4-4c9b-a188-c7c5b2d39d2b" xlink:to="loc_us-gaap_GoodwillFairValueDisclosure_d8c29b05-6fa2-4017-bbc5-8ac87d836520" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsFairValueDisclosure_719e77a0-23d6-4d98-a248-680499555234" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_2d7dce7e-72b4-4c9b-a188-c7c5b2d39d2b" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsFairValueDisclosure_719e77a0-23d6-4d98-a248-680499555234" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_18e0885c-260a-4f36-b82e-987b241d0bde" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_2d7dce7e-72b4-4c9b-a188-c7c5b2d39d2b" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_18e0885c-260a-4f36-b82e-987b241d0bde" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByMeasurementFrequencyAxis_9c9ddd46-11ab-48ab-a342-4b8a3daae33a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueByMeasurementFrequencyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_18e0885c-260a-4f36-b82e-987b241d0bde" xlink:to="loc_us-gaap_FairValueByMeasurementFrequencyAxis_9c9ddd46-11ab-48ab-a342-4b8a3daae33a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_9c9ddd46-11ab-48ab-a342-4b8a3daae33a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueMeasurementFrequencyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_9c9ddd46-11ab-48ab-a342-4b8a3daae33a" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_9c9ddd46-11ab-48ab-a342-4b8a3daae33a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_10ed7b38-1e06-422f-a564-9e298c3562ec" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueMeasurementFrequencyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_9c9ddd46-11ab-48ab-a342-4b8a3daae33a" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_10ed7b38-1e06-422f-a564-9e298c3562ec" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsNonrecurringMember_7713758b-5535-41da-b211-4e3c6d5b7c94" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueMeasurementsNonrecurringMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementFrequencyDomain_10ed7b38-1e06-422f-a564-9e298c3562ec" xlink:to="loc_us-gaap_FairValueMeasurementsNonrecurringMember_7713758b-5535-41da-b211-4e3c6d5b7c94" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/ImpactofNewAccountingPronouncementsDetails" xlink:type="simple" xlink:href="iclr-20211231.xsd#ImpactofNewAccountingPronouncementsDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/ImpactofNewAccountingPronouncementsDetails" xlink:type="extended" id="ifc1e00fdd0244efeb9cd45b8b12c8f81_ImpactofNewAccountingPronouncementsDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_05e0c4c1-c43f-40ad-8f00-c6b99926783e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_7efaea1d-cc01-4fad-8778-0f13931cb7eb" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_Goodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_05e0c4c1-c43f-40ad-8f00-c6b99926783e" xlink:to="loc_us-gaap_Goodwill_7efaea1d-cc01-4fad-8778-0f13931cb7eb" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerLiability_69703d5e-55fa-4aa8-8372-6f4d3940dc90" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ContractWithCustomerLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_05e0c4c1-c43f-40ad-8f00-c6b99926783e" xlink:to="loc_us-gaap_ContractWithCustomerLiability_69703d5e-55fa-4aa8-8372-6f4d3940dc90" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerLiabilityRevenueRecognized_1f3889b3-de04-4227-aecd-db7831e58cd7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ContractWithCustomerLiabilityRevenueRecognized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_05e0c4c1-c43f-40ad-8f00-c6b99926783e" xlink:to="loc_us-gaap_ContractWithCustomerLiabilityRevenueRecognized_1f3889b3-de04-4227-aecd-db7831e58cd7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_IncreaseDecreaseRevenueFromContractsWithCustomers_635ab7b3-1a97-4f91-81ec-99bfc1271aeb" xlink:href="iclr-20211231.xsd#iclr_IncreaseDecreaseRevenueFromContractsWithCustomers"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_05e0c4c1-c43f-40ad-8f00-c6b99926783e" xlink:to="loc_iclr_IncreaseDecreaseRevenueFromContractsWithCustomers_635ab7b3-1a97-4f91-81ec-99bfc1271aeb" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable_3565ebdb-d41d-4e66-9312-f659b35f1306" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_05e0c4c1-c43f-40ad-8f00-c6b99926783e" xlink:to="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable_3565ebdb-d41d-4e66-9312-f659b35f1306" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsForNewAccountingPronouncementsAxis_5e1c2a1c-45bd-44dc-995a-1398c2c85248" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AdjustmentsForNewAccountingPronouncementsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable_3565ebdb-d41d-4e66-9312-f659b35f1306" xlink:to="loc_us-gaap_AdjustmentsForNewAccountingPronouncementsAxis_5e1c2a1c-45bd-44dc-995a-1398c2c85248" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfAdoptionMember_5e1c2a1c-45bd-44dc-995a-1398c2c85248_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TypeOfAdoptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AdjustmentsForNewAccountingPronouncementsAxis_5e1c2a1c-45bd-44dc-995a-1398c2c85248" xlink:to="loc_us-gaap_TypeOfAdoptionMember_5e1c2a1c-45bd-44dc-995a-1398c2c85248_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfAdoptionMember_30b42111-2b58-474c-8e12-a9d2e96df472" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TypeOfAdoptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AdjustmentsForNewAccountingPronouncementsAxis_5e1c2a1c-45bd-44dc-995a-1398c2c85248" xlink:to="loc_us-gaap_TypeOfAdoptionMember_30b42111-2b58-474c-8e12-a9d2e96df472" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_AccountingStandardsUpdate202108Member_dbe45a58-fc3c-449a-8017-641e23059751" xlink:href="iclr-20211231.xsd#iclr_AccountingStandardsUpdate202108Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfAdoptionMember_30b42111-2b58-474c-8e12-a9d2e96df472" xlink:to="loc_iclr_AccountingStandardsUpdate202108Member_dbe45a58-fc3c-449a-8017-641e23059751" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/RelatedPartiesDetails" xlink:type="simple" xlink:href="iclr-20211231.xsd#RelatedPartiesDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/RelatedPartiesDetails" xlink:type="extended" id="ifefd7e609c47487891d203ab55bc4b83_RelatedPartiesDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionLineItems_10d2cd4e-587d-46ca-975d-90dc8abf11ee" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RelatedPartyTransactionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromRelatedParties_09a5ad00-47b1-4fa5-9ea3-64d9f18ea24a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RevenueFromRelatedParties"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_10d2cd4e-587d-46ca-975d-90dc8abf11ee" xlink:to="loc_us-gaap_RevenueFromRelatedParties_09a5ad00-47b1-4fa5-9ea3-64d9f18ea24a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DueFromRelatedParties_32f0d275-aaad-43a9-af5f-a13d953ed63a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DueFromRelatedParties"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_10d2cd4e-587d-46ca-975d-90dc8abf11ee" xlink:to="loc_us-gaap_DueFromRelatedParties_32f0d275-aaad-43a9-af5f-a13d953ed63a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityMethodInvestmentAggregateCost_e58e3537-15dc-4f25-80c6-01bbb8ee2879" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EquityMethodInvestmentAggregateCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_10d2cd4e-587d-46ca-975d-90dc8abf11ee" xlink:to="loc_us-gaap_EquityMethodInvestmentAggregateCost_e58e3537-15dc-4f25-80c6-01bbb8ee2879" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityMethodInvestmentOwnershipPercentage_7dc4572e-cbd0-4794-95ed-2c930f7a552f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EquityMethodInvestmentOwnershipPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_10d2cd4e-587d-46ca-975d-90dc8abf11ee" xlink:to="loc_us-gaap_EquityMethodInvestmentOwnershipPercentage_7dc4572e-cbd0-4794-95ed-2c930f7a552f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionExpensesFromTransactionsWithRelatedParty_0f8ba22c-d65d-4824-98ef-12ee56494708" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RelatedPartyTransactionExpensesFromTransactionsWithRelatedParty"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_10d2cd4e-587d-46ca-975d-90dc8abf11ee" xlink:to="loc_us-gaap_RelatedPartyTransactionExpensesFromTransactionsWithRelatedParty_0f8ba22c-d65d-4824-98ef-12ee56494708" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LoansAndLeasesReceivableRelatedPartiesAdditions_a92c8a1e-3b46-4b2d-bc18-bb9078365181" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LoansAndLeasesReceivableRelatedPartiesAdditions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_10d2cd4e-587d-46ca-975d-90dc8abf11ee" xlink:to="loc_us-gaap_LoansAndLeasesReceivableRelatedPartiesAdditions_a92c8a1e-3b46-4b2d-bc18-bb9078365181" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionRate_7c630cb5-67f5-482f-906a-4cd893707d93" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RelatedPartyTransactionRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_10d2cd4e-587d-46ca-975d-90dc8abf11ee" xlink:to="loc_us-gaap_RelatedPartyTransactionRate_7c630cb5-67f5-482f-906a-4cd893707d93" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_InterestReceivableRelatedPartyLoans_46d0d666-da67-4040-8da0-57604c312cd1" xlink:href="iclr-20211231.xsd#iclr_InterestReceivableRelatedPartyLoans"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_10d2cd4e-587d-46ca-975d-90dc8abf11ee" xlink:to="loc_iclr_InterestReceivableRelatedPartyLoans_46d0d666-da67-4040-8da0-57604c312cd1" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_EquityMethodInvestmentMajorityOwnershipPercentage_878cb494-1092-47ba-9c6b-fd921f5edd8d" xlink:href="iclr-20211231.xsd#iclr_EquityMethodInvestmentMajorityOwnershipPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_10d2cd4e-587d-46ca-975d-90dc8abf11ee" xlink:to="loc_iclr_EquityMethodInvestmentMajorityOwnershipPercentage_878cb494-1092-47ba-9c6b-fd921f5edd8d" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_7db1deb2-a9b3-4116-b413-7f244f5e586b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_10d2cd4e-587d-46ca-975d-90dc8abf11ee" xlink:to="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_7db1deb2-a9b3-4116-b413-7f244f5e586b" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionAxis_a7610941-d5ff-4dd6-a72e-ad26471fda8e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RelatedPartyTransactionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_7db1deb2-a9b3-4116-b413-7f244f5e586b" xlink:to="loc_us-gaap_RelatedPartyTransactionAxis_a7610941-d5ff-4dd6-a72e-ad26471fda8e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionDomain_a7610941-d5ff-4dd6-a72e-ad26471fda8e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RelatedPartyTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RelatedPartyTransactionAxis_a7610941-d5ff-4dd6-a72e-ad26471fda8e" xlink:to="loc_us-gaap_RelatedPartyTransactionDomain_a7610941-d5ff-4dd6-a72e-ad26471fda8e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionDomain_3501013d-e4df-4655-8fbc-ab1e1ad97e1a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RelatedPartyTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RelatedPartyTransactionAxis_a7610941-d5ff-4dd6-a72e-ad26471fda8e" xlink:to="loc_us-gaap_RelatedPartyTransactionDomain_3501013d-e4df-4655-8fbc-ab1e1ad97e1a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_DSBiopharmaLimitedMember_418f33f5-b33e-43b4-aa23-cc221db0d8f0" xlink:href="iclr-20211231.xsd#iclr_DSBiopharmaLimitedMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionDomain_3501013d-e4df-4655-8fbc-ab1e1ad97e1a" xlink:to="loc_iclr_DSBiopharmaLimitedMember_418f33f5-b33e-43b4-aa23-cc221db0d8f0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_AfimmuneLimitedMember_09b62cc2-8b1c-4ea6-b897-865f7489d642" xlink:href="iclr-20211231.xsd#iclr_AfimmuneLimitedMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionDomain_3501013d-e4df-4655-8fbc-ab1e1ad97e1a" xlink:to="loc_iclr_AfimmuneLimitedMember_09b62cc2-8b1c-4ea6-b897-865f7489d642" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_OncacareMember_94494403-5045-4bd1-ba7b-9070ce3cf68c" xlink:href="iclr-20211231.xsd#iclr_OncacareMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionDomain_3501013d-e4df-4655-8fbc-ab1e1ad97e1a" xlink:to="loc_iclr_OncacareMember_94494403-5045-4bd1-ba7b-9070ce3cf68c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_7c2311b0-9191-44b4-8eac-f7982d371198" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_7db1deb2-a9b3-4116-b413-7f244f5e586b" xlink:to="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_7c2311b0-9191-44b4-8eac-f7982d371198" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_7c2311b0-9191-44b4-8eac-f7982d371198_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_7c2311b0-9191-44b4-8eac-f7982d371198" xlink:to="loc_us-gaap_RelatedPartyDomain_7c2311b0-9191-44b4-8eac-f7982d371198_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_c55ca080-a8ab-4f49-9725-2f24b430a958" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_7c2311b0-9191-44b4-8eac-f7982d371198" xlink:to="loc_us-gaap_RelatedPartyDomain_c55ca080-a8ab-4f49-9725-2f24b430a958" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsidiaryOfCommonParentMember_164be0b0-e73f-4e03-b165-ce93cc928ddf" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SubsidiaryOfCommonParentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_c55ca080-a8ab-4f49-9725-2f24b430a958" xlink:to="loc_us-gaap_SubsidiaryOfCommonParentMember_164be0b0-e73f-4e03-b165-ce93cc928ddf" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_f2148156-af5f-4894-bdfd-1c6ee06e3259" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_7db1deb2-a9b3-4116-b413-7f244f5e586b" xlink:to="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_f2148156-af5f-4894-bdfd-1c6ee06e3259" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_EquityMethodInvesteeNameDomain_f2148156-af5f-4894-bdfd-1c6ee06e3259_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_EquityMethodInvesteeNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_f2148156-af5f-4894-bdfd-1c6ee06e3259" xlink:to="loc_srt_EquityMethodInvesteeNameDomain_f2148156-af5f-4894-bdfd-1c6ee06e3259_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_EquityMethodInvesteeNameDomain_45ada2b3-4c21-4a25-b3a7-00aa41798e12" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_EquityMethodInvesteeNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_f2148156-af5f-4894-bdfd-1c6ee06e3259" xlink:to="loc_srt_EquityMethodInvesteeNameDomain_45ada2b3-4c21-4a25-b3a7-00aa41798e12" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_OncacareMember_6256a646-47c5-4c77-954c-9cdd25813d75" xlink:href="iclr-20211231.xsd#iclr_OncacareMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_EquityMethodInvesteeNameDomain_45ada2b3-4c21-4a25-b3a7-00aa41798e12" xlink:to="loc_iclr_OncacareMember_6256a646-47c5-4c77-954c-9cdd25813d75" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/SubsequentEventsDetails" xlink:type="simple" xlink:href="iclr-20211231.xsd#SubsequentEventsDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/SubsequentEventsDetails" xlink:type="extended" id="i81be2d09b7a74720a93bc89ab343f381_SubsequentEventsDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventLineItems_187f9769-41aa-4a0c-8515-c26c6528f540" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SubsequentEventLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchaseProgramAuthorizedAmount1_f0c7f041-e9c4-4cdd-906b-d2e78e7a66cd" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockRepurchaseProgramAuthorizedAmount1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_187f9769-41aa-4a0c-8515-c26c6528f540" xlink:to="loc_us-gaap_StockRepurchaseProgramAuthorizedAmount1_f0c7f041-e9c4-4cdd-906b-d2e78e7a66cd" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodValue_6350509b-b075-4fdd-93c2-8e50afd95bf1" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockRepurchasedAndRetiredDuringPeriodValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_187f9769-41aa-4a0c-8515-c26c6528f540" xlink:to="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodValue_6350509b-b075-4fdd-93c2-8e50afd95bf1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTable_5b769548-61dd-4e21-9d0e-fec2942ce5e4" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SubsequentEventTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SubsequentEventLineItems_187f9769-41aa-4a0c-8515-c26c6528f540" xlink:to="loc_us-gaap_SubsequentEventTable_5b769548-61dd-4e21-9d0e-fec2942ce5e4" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeAxis_1d71eae8-52e0-4ae9-830f-b207b81421bb" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SubsequentEventTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_5b769548-61dd-4e21-9d0e-fec2942ce5e4" xlink:to="loc_us-gaap_SubsequentEventTypeAxis_1d71eae8-52e0-4ae9-830f-b207b81421bb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_1d71eae8-52e0-4ae9-830f-b207b81421bb_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_1d71eae8-52e0-4ae9-830f-b207b81421bb" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_1d71eae8-52e0-4ae9-830f-b207b81421bb_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_d4a67005-83c8-41ff-a419-078e06bf0fc7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_1d71eae8-52e0-4ae9-830f-b207b81421bb" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_d4a67005-83c8-41ff-a419-078e06bf0fc7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventMember_c375c92b-156b-4859-8176-7f915659c6f7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SubsequentEventMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventTypeDomain_d4a67005-83c8-41ff-a419-078e06bf0fc7" xlink:to="loc_us-gaap_SubsequentEventMember_c375c92b-156b-4859-8176-7f915659c6f7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareRepurchaseProgramAxis_f2f3df26-86fd-4210-8ac9-4c12aa11fb55" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareRepurchaseProgramAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_5b769548-61dd-4e21-9d0e-fec2942ce5e4" xlink:to="loc_us-gaap_ShareRepurchaseProgramAxis_f2f3df26-86fd-4210-8ac9-4c12aa11fb55" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareRepurchaseProgramDomain_f2f3df26-86fd-4210-8ac9-4c12aa11fb55_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareRepurchaseProgramDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ShareRepurchaseProgramAxis_f2f3df26-86fd-4210-8ac9-4c12aa11fb55" xlink:to="loc_us-gaap_ShareRepurchaseProgramDomain_f2f3df26-86fd-4210-8ac9-4c12aa11fb55_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareRepurchaseProgramDomain_1470c73c-7b7c-43cd-87b2-b7036f9af79e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareRepurchaseProgramDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ShareRepurchaseProgramAxis_f2f3df26-86fd-4210-8ac9-4c12aa11fb55" xlink:to="loc_us-gaap_ShareRepurchaseProgramDomain_1470c73c-7b7c-43cd-87b2-b7036f9af79e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BuybackProgramMember_be4bdbf0-1e1d-4e41-bfbf-83b6df954429" xlink:href="iclr-20211231.xsd#iclr_BuybackProgramMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareRepurchaseProgramDomain_1470c73c-7b7c-43cd-87b2-b7036f9af79e" xlink:to="loc_iclr_BuybackProgramMember_be4bdbf0-1e1d-4e41-bfbf-83b6df954429" xlink:type="arc" order="0"/>
  </link:definitionLink>
</link:linkbase>
