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

<!--XBRL Document Created with Wdesk from Workiva-->
<!--Copyright 2022 Workiva-->
<!--r:83028fc6-4e4e-41ac-b7ba-779deead058e,g:0b06fb89-96e3-4896-891d-5b6cf862fe34-->
<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/CONDENSEDCONSOLIDATEDSTATEMENTSOFSHAREHOLDERSEQUITYUNAUDITED" xlink:type="simple" xlink:href="iclr-20220930.xsd#CONDENSEDCONSOLIDATEDSTATEMENTSOFSHAREHOLDERSEQUITYUNAUDITED"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFSHAREHOLDERSEQUITYUNAUDITED" xlink:type="extended" id="id27ff6aa10e040de8f47d5b55f2c5101_CONDENSEDCONSOLIDATEDSTATEMENTSOFSHAREHOLDERSEQUITYUNAUDITED">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_ef1cf901-0a23-4ae3-92cf-c9628cd0015c" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_4aa0916b-218b-4a83-9243-29cca823612e" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_ef1cf901-0a23-4ae3-92cf-c9628cd0015c" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_4aa0916b-218b-4a83-9243-29cca823612e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesIssued_5dc6c0cf-63b6-46ff-a664-974f7a799056" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_CommonStockSharesIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_4aa0916b-218b-4a83-9243-29cca823612e" xlink:to="loc_us-gaap_CommonStockSharesIssued_5dc6c0cf-63b6-46ff-a664-974f7a799056" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_2604f527-9cd4-48f2-b293-e75a1426b4f3" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_4aa0916b-218b-4a83-9243-29cca823612e" xlink:to="loc_us-gaap_StockholdersEquity_2604f527-9cd4-48f2-b293-e75a1426b4f3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_f29d7a25-8caf-4d4f-9b9b-6f2e8af1240f" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_4aa0916b-218b-4a83-9243-29cca823612e" xlink:to="loc_us-gaap_NetIncomeLoss_f29d7a25-8caf-4d4f-9b9b-6f2e8af1240f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_ef236ab0-b1ae-4b77-b01c-de8a05eb58a9" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_4aa0916b-218b-4a83-9243-29cca823612e" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_ef236ab0-b1ae-4b77-b01c-de8a05eb58a9" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_f179ef02-376e-43e7-8e2a-2ca9866a3cd0" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_4aa0916b-218b-4a83-9243-29cca823612e" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_f179ef02-376e-43e7-8e2a-2ca9866a3cd0" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation_06df6b80-bf8a-4895-8d1b-567999a77520" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_4aa0916b-218b-4a83-9243-29cca823612e" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation_06df6b80-bf8a-4895-8d1b-567999a77520" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation_d6e77104-99c5-4599-879f-5b066bca42ea" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_4aa0916b-218b-4a83-9243-29cca823612e" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation_d6e77104-99c5-4599-879f-5b066bca42ea" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesAcquisitions_cd6940e3-6b6d-4155-a548-fd1ff8791c69" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StockIssuedDuringPeriodSharesAcquisitions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_4aa0916b-218b-4a83-9243-29cca823612e" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesAcquisitions_cd6940e3-6b6d-4155-a548-fd1ff8791c69" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueAcquisitions_f8754a9f-386d-4860-8ca3-afbff1c3c8b3" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StockIssuedDuringPeriodValueAcquisitions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_4aa0916b-218b-4a83-9243-29cca823612e" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueAcquisitions_f8754a9f-386d-4860-8ca3-afbff1c3c8b3" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_SharesIssuedSharesShareBasedPaymentArrangementAcquisitions_dba354a4-8406-463c-bf9c-cbeaa863350a" xlink:href="iclr-20220930.xsd#iclr_SharesIssuedSharesShareBasedPaymentArrangementAcquisitions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_4aa0916b-218b-4a83-9243-29cca823612e" xlink:to="loc_iclr_SharesIssuedSharesShareBasedPaymentArrangementAcquisitions_dba354a4-8406-463c-bf9c-cbeaa863350a" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_0eddcc99-5daf-4c0a-a032-aa3e79fb3814" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_4aa0916b-218b-4a83-9243-29cca823612e" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_0eddcc99-5daf-4c0a-a032-aa3e79fb3814" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalStockIssuedIssuanceCosts_912f0e8d-f047-45ae-a225-0e97c4e40bc0" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalStockIssuedIssuanceCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_4aa0916b-218b-4a83-9243-29cca823612e" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalStockIssuedIssuanceCosts_912f0e8d-f047-45ae-a225-0e97c4e40bc0" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodShares_ebc606bd-95dc-4ad8-8518-a20f150569ec" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StockRepurchasedAndRetiredDuringPeriodShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_4aa0916b-218b-4a83-9243-29cca823612e" xlink:to="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodShares_ebc606bd-95dc-4ad8-8518-a20f150569ec" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodValue_3fcbf1bd-b94c-4285-8028-fd8387574a2a" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StockRepurchasedAndRetiredDuringPeriodValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_4aa0916b-218b-4a83-9243-29cca823612e" xlink:to="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodValue_3fcbf1bd-b94c-4285-8028-fd8387574a2a" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_AdjustmentToRetainedEarningShareRepurchaseCosts_d0847bbf-a847-4db8-ac19-cf8feefea82a" xlink:href="iclr-20220930.xsd#iclr_AdjustmentToRetainedEarningShareRepurchaseCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_4aa0916b-218b-4a83-9243-29cca823612e" xlink:to="loc_iclr_AdjustmentToRetainedEarningShareRepurchaseCosts_d0847bbf-a847-4db8-ac19-cf8feefea82a" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_c3dfcdca-4bbf-4e6a-a1e9-629040b616bd" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_4aa0916b-218b-4a83-9243-29cca823612e" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_c3dfcdca-4bbf-4e6a-a1e9-629040b616bd" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesIssued_33001dc8-6d1b-4bf0-afda-9d3f0a786010" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_CommonStockSharesIssued"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_20f98807-3b0c-40d9-8c31-e4280c639a80" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StockholdersEquity"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_3f3f6851-0b4e-4f75-a1d8-fdb2735bfe4d" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_ef1cf901-0a23-4ae3-92cf-c9628cd0015c" xlink:to="loc_us-gaap_StatementTable_3f3f6851-0b4e-4f75-a1d8-fdb2735bfe4d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_abbb6c1c-b4dd-4a95-92e8-e24e10559e0b" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_3f3f6851-0b4e-4f75-a1d8-fdb2735bfe4d" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_abbb6c1c-b4dd-4a95-92e8-e24e10559e0b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_abbb6c1c-b4dd-4a95-92e8-e24e10559e0b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_abbb6c1c-b4dd-4a95-92e8-e24e10559e0b" xlink:to="loc_us-gaap_EquityComponentDomain_abbb6c1c-b4dd-4a95-92e8-e24e10559e0b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_b36d40c6-6ec3-4ebd-87b9-c3646e2c421e" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_abbb6c1c-b4dd-4a95-92e8-e24e10559e0b" xlink:to="loc_us-gaap_EquityComponentDomain_b36d40c6-6ec3-4ebd-87b9-c3646e2c421e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_2484a6bd-2805-426f-ad90-6ce3afde0259" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_b36d40c6-6ec3-4ebd-87b9-c3646e2c421e" xlink:to="loc_us-gaap_CommonStockMember_2484a6bd-2805-426f-ad90-6ce3afde0259" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_7dad2733-21a6-4a60-bd44-09b83ed662a3" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AdditionalPaidInCapitalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_b36d40c6-6ec3-4ebd-87b9-c3646e2c421e" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_7dad2733-21a6-4a60-bd44-09b83ed662a3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_OtherUndenominatedCapitalMember_a81120fc-4c34-4020-84ea-1ab85db85199" xlink:href="iclr-20220930.xsd#iclr_OtherUndenominatedCapitalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_b36d40c6-6ec3-4ebd-87b9-c3646e2c421e" xlink:to="loc_iclr_OtherUndenominatedCapitalMember_a81120fc-4c34-4020-84ea-1ab85db85199" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_8fa5bb11-6d71-4b73-8724-bcac92c43960" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_b36d40c6-6ec3-4ebd-87b9-c3646e2c421e" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_8fa5bb11-6d71-4b73-8724-bcac92c43960" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsMember_29d9f3f0-797b-46a7-9081-2ed73e065bc1" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RetainedEarningsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_b36d40c6-6ec3-4ebd-87b9-c3646e2c421e" xlink:to="loc_us-gaap_RetainedEarningsMember_29d9f3f0-797b-46a7-9081-2ed73e065bc1" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/RevenueDetails" xlink:type="simple" xlink:href="iclr-20220930.xsd#RevenueDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/RevenueDetails" xlink:type="extended" id="i297e5401c2f64aae92cb555027f84768_RevenueDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueLineItems_e1af901b-d3de-40e7-8d53-07edae3c4b1b" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DisaggregationOfRevenueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_710abfd8-2b62-46a4-8769-d151ab437165" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_e1af901b-d3de-40e7-8d53-07edae3c4b1b" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_710abfd8-2b62-46a4-8769-d151ab437165" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueTable_1afa330b-3c30-41a3-803b-da911b07c6c1" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DisaggregationOfRevenueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_e1af901b-d3de-40e7-8d53-07edae3c4b1b" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_1afa330b-3c30-41a3-803b-da911b07c6c1" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MajorCustomersAxis_15a0a9fa-a8bc-46e4-b33b-3836cb52447c" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_MajorCustomersAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_1afa330b-3c30-41a3-803b-da911b07c6c1" xlink:to="loc_srt_MajorCustomersAxis_15a0a9fa-a8bc-46e4-b33b-3836cb52447c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_NameOfMajorCustomerDomain_15a0a9fa-a8bc-46e4-b33b-3836cb52447c_default" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_NameOfMajorCustomerDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_MajorCustomersAxis_15a0a9fa-a8bc-46e4-b33b-3836cb52447c" xlink:to="loc_srt_NameOfMajorCustomerDomain_15a0a9fa-a8bc-46e4-b33b-3836cb52447c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_NameOfMajorCustomerDomain_35b4b657-5bf4-4d40-9085-b6972705f657" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_NameOfMajorCustomerDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_MajorCustomersAxis_15a0a9fa-a8bc-46e4-b33b-3836cb52447c" xlink:to="loc_srt_NameOfMajorCustomerDomain_35b4b657-5bf4-4d40-9085-b6972705f657" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_Clients15Member_732a8d7b-52b2-4949-b62a-135a2d22b251" xlink:href="iclr-20220930.xsd#iclr_Clients15Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_35b4b657-5bf4-4d40-9085-b6972705f657" xlink:to="loc_iclr_Clients15Member_732a8d7b-52b2-4949-b62a-135a2d22b251" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_Clients610Member_5ea8227a-6199-48d8-a447-75faae3c9614" xlink:href="iclr-20220930.xsd#iclr_Clients610Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_35b4b657-5bf4-4d40-9085-b6972705f657" xlink:to="loc_iclr_Clients610Member_5ea8227a-6199-48d8-a447-75faae3c9614" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_Clients1125Member_3f2a14a6-4c55-4c98-a5ef-e80ecbd87a23" xlink:href="iclr-20220930.xsd#iclr_Clients1125Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_35b4b657-5bf4-4d40-9085-b6972705f657" xlink:to="loc_iclr_Clients1125Member_3f2a14a6-4c55-4c98-a5ef-e80ecbd87a23" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_OtherCustomersMember_31eab3a0-d947-4c0d-bbc4-afcc19ba101f" xlink:href="iclr-20220930.xsd#iclr_OtherCustomersMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_35b4b657-5bf4-4d40-9085-b6972705f657" xlink:to="loc_iclr_OtherCustomersMember_31eab3a0-d947-4c0d-bbc4-afcc19ba101f" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/AccountsreceivableunbilledrevenuecontractassetsandunearnedrevenueorpaymentsonaccountcontractliabilitiesNarrativeDetails" xlink:type="simple" xlink:href="iclr-20220930.xsd#AccountsreceivableunbilledrevenuecontractassetsandunearnedrevenueorpaymentsonaccountcontractliabilitiesNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/AccountsreceivableunbilledrevenuecontractassetsandunearnedrevenueorpaymentsonaccountcontractliabilitiesNarrativeDetails" xlink:type="extended" id="i5485d2c508fd46cb85f17336aa36af5c_AccountsreceivableunbilledrevenuecontractassetsandunearnedrevenueorpaymentsonaccountcontractliabilitiesNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_9e3ce894-41ec-4d1c-9108-ed90c96f5ced" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_IncreaseDecreaseInContractWithCustomerAssetGrossCurrent_858520e4-5dec-40ef-a7e9-ec0ee3d21ead" xlink:href="iclr-20220930.xsd#iclr_IncreaseDecreaseInContractWithCustomerAssetGrossCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_9e3ce894-41ec-4d1c-9108-ed90c96f5ced" xlink:to="loc_iclr_IncreaseDecreaseInContractWithCustomerAssetGrossCurrent_858520e4-5dec-40ef-a7e9-ec0ee3d21ead" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_IncreaseDecreaseInContractWithCustomerLiabilityGrossCurrent_62f0c4f3-86b8-4407-9e20-43e392414152" xlink:href="iclr-20220930.xsd#iclr_IncreaseDecreaseInContractWithCustomerLiabilityGrossCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_9e3ce894-41ec-4d1c-9108-ed90c96f5ced" xlink:to="loc_iclr_IncreaseDecreaseInContractWithCustomerLiabilityGrossCurrent_62f0c4f3-86b8-4407-9e20-43e392414152" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_ChangeInAdvancePaymentsNettedAgainstUnbilledContractsReceivable_592c9558-2c30-4bac-9b54-d430a493aa86" xlink:href="iclr-20220930.xsd#iclr_ChangeInAdvancePaymentsNettedAgainstUnbilledContractsReceivable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_9e3ce894-41ec-4d1c-9108-ed90c96f5ced" xlink:to="loc_iclr_ChangeInAdvancePaymentsNettedAgainstUnbilledContractsReceivable_592c9558-2c30-4bac-9b54-d430a493aa86" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligation_95dd1ee4-2733-4723-bf97-0a9984fc52cb" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RevenueRemainingPerformanceObligation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_9e3ce894-41ec-4d1c-9108-ed90c96f5ced" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligation_95dd1ee4-2733-4723-bf97-0a9984fc52cb" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationPercentage_b3ef1b3b-adc9-451e-b3a4-60597e2c8492" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RevenueRemainingPerformanceObligationPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_9e3ce894-41ec-4d1c-9108-ed90c96f5ced" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationPercentage_b3ef1b3b-adc9-451e-b3a4-60597e2c8492" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_c3773713-81c6-4e8f-b5cb-bedf25666f93" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_9e3ce894-41ec-4d1c-9108-ed90c96f5ced" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_c3773713-81c6-4e8f-b5cb-bedf25666f93" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_f8845c3a-b806-4fcd-b601-5ab8582aad76" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_9e3ce894-41ec-4d1c-9108-ed90c96f5ced" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_f8845c3a-b806-4fcd-b601-5ab8582aad76" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionStartDateAxis_fefd9e4f-3dd3-4aac-8ae9-1c01c79d6404" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionStartDateAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_f8845c3a-b806-4fcd-b601-5ab8582aad76" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionStartDateAxis_fefd9e4f-3dd3-4aac-8ae9-1c01c79d6404" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/AccountsreceivableunbilledrevenuecontractassetsandunearnedrevenueorpaymentsonaccountcontractliabilitiesNarrativeDetails_1" xlink:type="simple" xlink:href="iclr-20220930.xsd#AccountsreceivableunbilledrevenuecontractassetsandunearnedrevenueorpaymentsonaccountcontractliabilitiesNarrativeDetails_1"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/AccountsreceivableunbilledrevenuecontractassetsandunearnedrevenueorpaymentsonaccountcontractliabilitiesNarrativeDetails_1" xlink:type="extended" id="i61d4b6d076fa415aa6575d762e8ec046_AccountsreceivableunbilledrevenuecontractassetsandunearnedrevenueorpaymentsonaccountcontractliabilitiesNarrativeDetails_1">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_9e3ce894-41ec-4d1c-9108-ed90c96f5ced" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_IncreaseDecreaseInContractWithCustomerAssetGrossCurrent_858520e4-5dec-40ef-a7e9-ec0ee3d21ead" xlink:href="iclr-20220930.xsd#iclr_IncreaseDecreaseInContractWithCustomerAssetGrossCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_9e3ce894-41ec-4d1c-9108-ed90c96f5ced" xlink:to="loc_iclr_IncreaseDecreaseInContractWithCustomerAssetGrossCurrent_858520e4-5dec-40ef-a7e9-ec0ee3d21ead" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_IncreaseDecreaseInContractWithCustomerLiabilityGrossCurrent_62f0c4f3-86b8-4407-9e20-43e392414152" xlink:href="iclr-20220930.xsd#iclr_IncreaseDecreaseInContractWithCustomerLiabilityGrossCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_9e3ce894-41ec-4d1c-9108-ed90c96f5ced" xlink:to="loc_iclr_IncreaseDecreaseInContractWithCustomerLiabilityGrossCurrent_62f0c4f3-86b8-4407-9e20-43e392414152" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_ChangeInAdvancePaymentsNettedAgainstUnbilledContractsReceivable_592c9558-2c30-4bac-9b54-d430a493aa86" xlink:href="iclr-20220930.xsd#iclr_ChangeInAdvancePaymentsNettedAgainstUnbilledContractsReceivable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_9e3ce894-41ec-4d1c-9108-ed90c96f5ced" xlink:to="loc_iclr_ChangeInAdvancePaymentsNettedAgainstUnbilledContractsReceivable_592c9558-2c30-4bac-9b54-d430a493aa86" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligation_95dd1ee4-2733-4723-bf97-0a9984fc52cb" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RevenueRemainingPerformanceObligation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_9e3ce894-41ec-4d1c-9108-ed90c96f5ced" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligation_95dd1ee4-2733-4723-bf97-0a9984fc52cb" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationPercentage_b3ef1b3b-adc9-451e-b3a4-60597e2c8492" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RevenueRemainingPerformanceObligationPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_9e3ce894-41ec-4d1c-9108-ed90c96f5ced" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationPercentage_b3ef1b3b-adc9-451e-b3a4-60597e2c8492" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_c3773713-81c6-4e8f-b5cb-bedf25666f93" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_9e3ce894-41ec-4d1c-9108-ed90c96f5ced" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_c3773713-81c6-4e8f-b5cb-bedf25666f93" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_f8845c3a-b806-4fcd-b601-5ab8582aad76" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_9e3ce894-41ec-4d1c-9108-ed90c96f5ced" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_f8845c3a-b806-4fcd-b601-5ab8582aad76" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/IntangibleassetsDetails" xlink:type="simple" xlink:href="iclr-20220930.xsd#IntangibleassetsDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/IntangibleassetsDetails" xlink:type="extended" id="i3c84934eef2d4b2984ad4d3ed598dc62_IntangibleassetsDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_d482949e-9664-42c4-b22a-2ffbf716aff2" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FiniteLivedIntangibleAssetsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsGross_c7c470bf-6352-4953-a9cc-993be9f3596a" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FiniteLivedIntangibleAssetsGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_d482949e-9664-42c4-b22a-2ffbf716aff2" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsGross_c7c470bf-6352-4953-a9cc-993be9f3596a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_5d4a467e-662f-45b8-9426-9eae5a7be100" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_d482949e-9664-42c4-b22a-2ffbf716aff2" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_5d4a467e-662f-45b8-9426-9eae5a7be100" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsNet_a4ce1d6d-a8fc-4a88-ba43-29f12a0fddef" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FiniteLivedIntangibleAssetsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_d482949e-9664-42c4-b22a-2ffbf716aff2" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsNet_a4ce1d6d-a8fc-4a88-ba43-29f12a0fddef" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_06d141ba-962b-467b-9146-4c76c5f36292" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_d482949e-9664-42c4-b22a-2ffbf716aff2" xlink:to="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_06d141ba-962b-467b-9146-4c76c5f36292" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_f10cc050-53d2-47b9-8080-4f22a8f543ed" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_06d141ba-962b-467b-9146-4c76c5f36292" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_f10cc050-53d2-47b9-8080-4f22a8f543ed" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_f10cc050-53d2-47b9-8080-4f22a8f543ed_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_f10cc050-53d2-47b9-8080-4f22a8f543ed" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_f10cc050-53d2-47b9-8080-4f22a8f543ed_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_bdde73f4-f563-4d06-b857-2be7ee8ccccb" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_f10cc050-53d2-47b9-8080-4f22a8f543ed" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_bdde73f4-f563-4d06-b857-2be7ee8ccccb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerRelationshipsMember_74c5212d-9542-4ef7-86e4-14fe9a8f844b" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_CustomerRelationshipsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_bdde73f4-f563-4d06-b857-2be7ee8ccccb" xlink:to="loc_us-gaap_CustomerRelationshipsMember_74c5212d-9542-4ef7-86e4-14fe9a8f844b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OrderOrProductionBacklogMember_a46db415-9714-4650-82d2-20a874c8f10f" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_OrderOrProductionBacklogMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_bdde73f4-f563-4d06-b857-2be7ee8ccccb" xlink:to="loc_us-gaap_OrderOrProductionBacklogMember_a46db415-9714-4650-82d2-20a874c8f10f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TradeNamesMember_c3194954-7e4d-46a6-88b7-574e1556a118" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_TradeNamesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_bdde73f4-f563-4d06-b857-2be7ee8ccccb" xlink:to="loc_us-gaap_TradeNamesMember_c3194954-7e4d-46a6-88b7-574e1556a118" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DatabasesMember_86c7808a-6540-49fa-9581-348e15178504" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DatabasesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_bdde73f4-f563-4d06-b857-2be7ee8ccccb" xlink:to="loc_us-gaap_DatabasesMember_86c7808a-6540-49fa-9581-348e15178504" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TechnologyBasedIntangibleAssetsMember_50ed7d61-d631-4000-a231-e642cba90b2e" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_TechnologyBasedIntangibleAssetsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_bdde73f4-f563-4d06-b857-2be7ee8ccccb" xlink:to="loc_us-gaap_TechnologyBasedIntangibleAssetsMember_50ed7d61-d631-4000-a231-e642cba90b2e" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/BusinesscombinationsNarrativeDetails" xlink:type="simple" xlink:href="iclr-20220930.xsd#BusinesscombinationsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/BusinesscombinationsNarrativeDetails" xlink:type="extended" id="i02be0f38967946d7a0cfa44b85c03cc0_BusinesscombinationsNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_066f9bea-4c18-48ee-9eb6-da7b89260f00" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityNumberOfEmployees_b7743768-4780-48be-9099-b63c65c0e3f4" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_EntityNumberOfEmployees"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_066f9bea-4c18-48ee-9eb6-da7b89260f00" xlink:to="loc_dei_EntityNumberOfEmployees_b7743768-4780-48be-9099-b63c65c0e3f4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BusinessCombinationConsiderationTransferredCashPaymentToShareholdersPerShareHeld_0bbdc7a2-f3d8-4b5f-986d-dd0b6629a640" xlink:href="iclr-20220930.xsd#iclr_BusinessCombinationConsiderationTransferredCashPaymentToShareholdersPerShareHeld"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_066f9bea-4c18-48ee-9eb6-da7b89260f00" xlink:to="loc_iclr_BusinessCombinationConsiderationTransferredCashPaymentToShareholdersPerShareHeld_0bbdc7a2-f3d8-4b5f-986d-dd0b6629a640" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BusinessCombinationConsiderationTransferredSharesIssuedToShareholdersPerCurrentShareHeld_a964f188-544c-48a6-a50f-9653bc9f48dd" xlink:href="iclr-20220930.xsd#iclr_BusinessCombinationConsiderationTransferredSharesIssuedToShareholdersPerCurrentShareHeld"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_066f9bea-4c18-48ee-9eb6-da7b89260f00" xlink:to="loc_iclr_BusinessCombinationConsiderationTransferredSharesIssuedToShareholdersPerCurrentShareHeld_a964f188-544c-48a6-a50f-9653bc9f48dd" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationIntegrationRelatedCosts_e34f6753-d701-457a-9de8-9c03c3a237fc" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessCombinationIntegrationRelatedCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_066f9bea-4c18-48ee-9eb6-da7b89260f00" xlink:to="loc_us-gaap_BusinessCombinationIntegrationRelatedCosts_e34f6753-d701-457a-9de8-9c03c3a237fc" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationAcquisitionRelatedCosts_86fd23ec-f4eb-4e88-b03d-43ae20f0ce15" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessCombinationAcquisitionRelatedCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_066f9bea-4c18-48ee-9eb6-da7b89260f00" xlink:to="loc_us-gaap_BusinessCombinationAcquisitionRelatedCosts_86fd23ec-f4eb-4e88-b03d-43ae20f0ce15" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_56a69b39-b95c-4e3a-a670-a4770f0b7cb9" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_066f9bea-4c18-48ee-9eb6-da7b89260f00" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_56a69b39-b95c-4e3a-a670-a4770f0b7cb9" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_ba1360e5-25fc-4133-a037-16e5764016ea" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_56a69b39-b95c-4e3a-a670-a4770f0b7cb9" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_ba1360e5-25fc-4133-a037-16e5764016ea" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_ba1360e5-25fc-4133-a037-16e5764016ea_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_ba1360e5-25fc-4133-a037-16e5764016ea" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_ba1360e5-25fc-4133-a037-16e5764016ea_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_73bc2cf2-2e9c-4a2a-976a-e2d27c260035" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_ba1360e5-25fc-4133-a037-16e5764016ea" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_73bc2cf2-2e9c-4a2a-976a-e2d27c260035" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_PRAHealthSciencesIncMember_34a702e4-6634-4a8b-94a7-ba9f33931c4e" xlink:href="iclr-20220930.xsd#iclr_PRAHealthSciencesIncMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_73bc2cf2-2e9c-4a2a-976a-e2d27c260035" xlink:to="loc_iclr_PRAHealthSciencesIncMember_34a702e4-6634-4a8b-94a7-ba9f33931c4e" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/BusinesscombinationsScheduleofFairValueofPRAHealthSciencesIncAcquisitionConsiderationDetails" xlink:type="simple" xlink:href="iclr-20220930.xsd#BusinesscombinationsScheduleofFairValueofPRAHealthSciencesIncAcquisitionConsiderationDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/BusinesscombinationsScheduleofFairValueofPRAHealthSciencesIncAcquisitionConsiderationDetails" xlink:type="extended" id="i651e716019de4bc78bf62946cc89af7a_BusinesscombinationsScheduleofFairValueofPRAHealthSciencesIncAcquisitionConsiderationDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_5360c895-135b-4905-a1b0-1519d58ba0c2" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquireBusinessesGross_0107d3f5-1ab4-48a0-a39a-3aaaa719a2b2" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_PaymentsToAcquireBusinessesGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_5360c895-135b-4905-a1b0-1519d58ba0c2" xlink:to="loc_us-gaap_PaymentsToAcquireBusinessesGross_0107d3f5-1ab4-48a0-a39a-3aaaa719a2b2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationConsiderationTransferredEquityInterestsIssuedAndIssuable_2fe9cea7-1c10-4df8-a72f-c5ebb7d12e67" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessCombinationConsiderationTransferredEquityInterestsIssuedAndIssuable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_5360c895-135b-4905-a1b0-1519d58ba0c2" xlink:to="loc_us-gaap_BusinessCombinationConsiderationTransferredEquityInterestsIssuedAndIssuable_2fe9cea7-1c10-4df8-a72f-c5ebb7d12e67" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationConsiderationTransferredOther1_d8eb66bf-aaf8-44fa-af59-7cfafcca29f5" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessCombinationConsiderationTransferredOther1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_5360c895-135b-4905-a1b0-1519d58ba0c2" xlink:to="loc_us-gaap_BusinessCombinationConsiderationTransferredOther1_d8eb66bf-aaf8-44fa-af59-7cfafcca29f5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationConsiderationTransferredLiabilitiesIncurred_0de0d07b-16f0-4ae0-acbf-b0636b3c46fb" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessCombinationConsiderationTransferredLiabilitiesIncurred"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_5360c895-135b-4905-a1b0-1519d58ba0c2" xlink:to="loc_us-gaap_BusinessCombinationConsiderationTransferredLiabilitiesIncurred_0de0d07b-16f0-4ae0-acbf-b0636b3c46fb" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationConsiderationTransferred1_ddd77076-fd55-429d-8999-08f7961a05b8" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessCombinationConsiderationTransferred1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_5360c895-135b-4905-a1b0-1519d58ba0c2" xlink:to="loc_us-gaap_BusinessCombinationConsiderationTransferred1_ddd77076-fd55-429d-8999-08f7961a05b8" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BusinessCombinationConsiderationTransferredPaymentsToSettleDebtObligationsThatExistedPriorToAcquisitionDate_f2b268a8-a974-48ef-be8d-ed82417b9343" xlink:href="iclr-20220930.xsd#iclr_BusinessCombinationConsiderationTransferredPaymentsToSettleDebtObligationsThatExistedPriorToAcquisitionDate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_5360c895-135b-4905-a1b0-1519d58ba0c2" xlink:to="loc_iclr_BusinessCombinationConsiderationTransferredPaymentsToSettleDebtObligationsThatExistedPriorToAcquisitionDate_f2b268a8-a974-48ef-be8d-ed82417b9343" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_7c02151a-6ab2-4e32-9b5f-a75fb5734453" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_5360c895-135b-4905-a1b0-1519d58ba0c2" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_7c02151a-6ab2-4e32-9b5f-a75fb5734453" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_1e4ae655-8475-4fb0-bc3f-4cc665488651" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_7c02151a-6ab2-4e32-9b5f-a75fb5734453" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_1e4ae655-8475-4fb0-bc3f-4cc665488651" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_1e4ae655-8475-4fb0-bc3f-4cc665488651_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_1e4ae655-8475-4fb0-bc3f-4cc665488651" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_1e4ae655-8475-4fb0-bc3f-4cc665488651_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_0b3a9fc8-aeab-407d-960d-4b311bd25310" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_1e4ae655-8475-4fb0-bc3f-4cc665488651" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_0b3a9fc8-aeab-407d-960d-4b311bd25310" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_PRAHealthSciencesIncMember_a486b06b-005e-45be-9cba-e4343e70d1fe" xlink:href="iclr-20220930.xsd#iclr_PRAHealthSciencesIncMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_0b3a9fc8-aeab-407d-960d-4b311bd25310" xlink:to="loc_iclr_PRAHealthSciencesIncMember_a486b06b-005e-45be-9cba-e4343e70d1fe" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/BusinesscombinationsScheduleofPRAHealthSciencesIncAcquisitionDetails" xlink:type="simple" xlink:href="iclr-20220930.xsd#BusinesscombinationsScheduleofPRAHealthSciencesIncAcquisitionDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/BusinesscombinationsScheduleofPRAHealthSciencesIncAcquisitionDetails" xlink:type="extended" id="i29a3b112a05248ba9398cb4779d9fee1_BusinesscombinationsScheduleofPRAHealthSciencesIncAcquisitionDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_7e0c0e1e-cfc9-4ce6-a75c-e1a93b9a5104" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents_65900b0c-8054-4f94-aa0d-ff081b15c837" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_7e0c0e1e-cfc9-4ce6-a75c-e1a93b9a5104" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents_65900b0c-8054-4f94-aa0d-ff081b15c837" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsReceivables_fabc365f-52f2-470b-bc63-c3767e6ce9d6" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsReceivables"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_7e0c0e1e-cfc9-4ce6-a75c-e1a93b9a5104" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsReceivables_fabc365f-52f2-470b-bc63-c3767e6ce9d6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsOther_b51d1f43-1f16-4206-bcd4-badb24687071" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsOther"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_7e0c0e1e-cfc9-4ce6-a75c-e1a93b9a5104" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsOther_b51d1f43-1f16-4206-bcd4-badb24687071" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment_eabded45-014a-42a3-8431-9e3d5d673011" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_7e0c0e1e-cfc9-4ce6-a75c-e1a93b9a5104" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment_eabded45-014a-42a3-8431-9e3d5d673011" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOperatingLeaseRightOfUseAsset_1fa9e1df-6fa6-4aa7-829a-06c52c4ce454" xlink:href="iclr-20220930.xsd#iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOperatingLeaseRightOfUseAsset"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_7e0c0e1e-cfc9-4ce6-a75c-e1a93b9a5104" xlink:to="loc_iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOperatingLeaseRightOfUseAsset_1fa9e1df-6fa6-4aa7-829a-06c52c4ce454" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_c6e50ebf-c545-41cf-ac96-3ae7e7246d89" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_Goodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_7e0c0e1e-cfc9-4ce6-a75c-e1a93b9a5104" xlink:to="loc_us-gaap_Goodwill_c6e50ebf-c545-41cf-ac96-3ae7e7246d89" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles_2db5de06-8434-40f2-b382-cb42c720ca3f" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_7e0c0e1e-cfc9-4ce6-a75c-e1a93b9a5104" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles_2db5de06-8434-40f2-b382-cb42c720ca3f" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxAssets_41244ad5-daff-4d3d-b4d4-bc9a4aba57a2" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_7e0c0e1e-cfc9-4ce6-a75c-e1a93b9a5104" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxAssets_41244ad5-daff-4d3d-b4d4-bc9a4aba57a2" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOtherNoncurrentAssets_a0b1b595-529b-44e9-9f87-1a2405b8887d" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOtherNoncurrentAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_7e0c0e1e-cfc9-4ce6-a75c-e1a93b9a5104" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOtherNoncurrentAssets_a0b1b595-529b-44e9-9f87-1a2405b8887d" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesAccountsPayable_e85e2fba-40a1-4e16-85dc-a95861be21b4" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesAccountsPayable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_7e0c0e1e-cfc9-4ce6-a75c-e1a93b9a5104" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesAccountsPayable_e85e2fba-40a1-4e16-85dc-a95861be21b4" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesOther_58175dc7-e1ee-4504-a901-130fd96061bb" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesOther"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_7e0c0e1e-cfc9-4ce6-a75c-e1a93b9a5104" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesOther_58175dc7-e1ee-4504-a901-130fd96061bb" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedCurrentOperatingLeaseLiability_362961c8-a91c-4060-ad2d-981d812fb765" xlink:href="iclr-20220930.xsd#iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedCurrentOperatingLeaseLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_7e0c0e1e-cfc9-4ce6-a75c-e1a93b9a5104" xlink:to="loc_iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedCurrentOperatingLeaseLiability_362961c8-a91c-4060-ad2d-981d812fb765" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedContractWithCustomerLiabilityCurrent_2a431aa4-44bf-4cc2-bee8-3e505052cdc2" xlink:href="iclr-20220930.xsd#iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedContractWithCustomerLiabilityCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_7e0c0e1e-cfc9-4ce6-a75c-e1a93b9a5104" xlink:to="loc_iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedContractWithCustomerLiabilityCurrent_2a431aa4-44bf-4cc2-bee8-3e505052cdc2" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedNoncurrentOperatingLeaseLiability_f5fe718b-d42f-4e73-ad4a-fd10300886ed" xlink:href="iclr-20220930.xsd#iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedNoncurrentOperatingLeaseLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_7e0c0e1e-cfc9-4ce6-a75c-e1a93b9a5104" xlink:to="loc_iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedNoncurrentOperatingLeaseLiability_f5fe718b-d42f-4e73-ad4a-fd10300886ed" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxLiabilities_892d1dbc-21fd-45e2-80a1-6d7bd6339067" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_7e0c0e1e-cfc9-4ce6-a75c-e1a93b9a5104" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxLiabilities_892d1dbc-21fd-45e2-80a1-6d7bd6339067" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilitiesOther_0fa49661-67f3-4a74-a08e-4be969f11b2f" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilitiesOther"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_7e0c0e1e-cfc9-4ce6-a75c-e1a93b9a5104" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilitiesOther_0fa49661-67f3-4a74-a08e-4be969f11b2f" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet_07203da2-9b5d-4cdc-80c3-fffc41b06cae" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_7e0c0e1e-cfc9-4ce6-a75c-e1a93b9a5104" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet_07203da2-9b5d-4cdc-80c3-fffc41b06cae" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionPurchasePriceAllocationGoodwillExpectedTaxDeductibleAmount_724d404e-07aa-4079-8332-847568b59775" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionPurchasePriceAllocationGoodwillExpectedTaxDeductibleAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_7e0c0e1e-cfc9-4ce6-a75c-e1a93b9a5104" xlink:to="loc_us-gaap_BusinessAcquisitionPurchasePriceAllocationGoodwillExpectedTaxDeductibleAmount_724d404e-07aa-4079-8332-847568b59775" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_990e1620-bde9-44d8-abeb-748a1e43205f" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_7e0c0e1e-cfc9-4ce6-a75c-e1a93b9a5104" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_990e1620-bde9-44d8-abeb-748a1e43205f" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_413a903d-0f98-4404-803b-ab85b91cfca2" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_990e1620-bde9-44d8-abeb-748a1e43205f" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_413a903d-0f98-4404-803b-ab85b91cfca2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_413a903d-0f98-4404-803b-ab85b91cfca2_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_413a903d-0f98-4404-803b-ab85b91cfca2" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_413a903d-0f98-4404-803b-ab85b91cfca2_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_e5c0b854-34fb-40bc-9606-291af542b8db" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_413a903d-0f98-4404-803b-ab85b91cfca2" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_e5c0b854-34fb-40bc-9606-291af542b8db" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_PRAHealthSciencesIncMember_70a37f2a-9330-435e-92f0-306f62423669" xlink:href="iclr-20220930.xsd#iclr_PRAHealthSciencesIncMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_e5c0b854-34fb-40bc-9606-291af542b8db" xlink:to="loc_iclr_PRAHealthSciencesIncMember_70a37f2a-9330-435e-92f0-306f62423669" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/BusinesscombinationsScheduleofFairValueofIntangibleAssetsPRAHealthSciencesIncAcquisitionDetails" xlink:type="simple" xlink:href="iclr-20220930.xsd#BusinesscombinationsScheduleofFairValueofIntangibleAssetsPRAHealthSciencesIncAcquisitionDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/BusinesscombinationsScheduleofFairValueofIntangibleAssetsPRAHealthSciencesIncAcquisitionDetails" xlink:type="extended" id="iafa75702d4284a71b3a8f5fdebf3d904_BusinesscombinationsScheduleofFairValueofIntangibleAssetsPRAHealthSciencesIncAcquisitionDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_194519f7-42ab-45f8-9ead-c95955de9e1a" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles_3c922d00-470a-4284-b2b1-a1756e634fca" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_194519f7-42ab-45f8-9ead-c95955de9e1a" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles_3c922d00-470a-4284-b2b1-a1756e634fca" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetUsefulLife_14ea180e-03f1-4132-b88a-997efea2a8e3" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FiniteLivedIntangibleAssetUsefulLife"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_194519f7-42ab-45f8-9ead-c95955de9e1a" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetUsefulLife_14ea180e-03f1-4132-b88a-997efea2a8e3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_679d128a-ddb7-43f0-a094-4296acb3290b" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_194519f7-42ab-45f8-9ead-c95955de9e1a" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_679d128a-ddb7-43f0-a094-4296acb3290b" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_93538b6b-54d8-4ce2-911a-e4f2271fb7d9" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_679d128a-ddb7-43f0-a094-4296acb3290b" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_93538b6b-54d8-4ce2-911a-e4f2271fb7d9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_93538b6b-54d8-4ce2-911a-e4f2271fb7d9_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_93538b6b-54d8-4ce2-911a-e4f2271fb7d9" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_93538b6b-54d8-4ce2-911a-e4f2271fb7d9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_f944c031-0157-4b92-bd7d-efdab96a25f6" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_93538b6b-54d8-4ce2-911a-e4f2271fb7d9" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_f944c031-0157-4b92-bd7d-efdab96a25f6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_PRAHealthSciencesIncMember_cf7696f0-8d19-4176-9792-92a6ced815e8" xlink:href="iclr-20220930.xsd#iclr_PRAHealthSciencesIncMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_f944c031-0157-4b92-bd7d-efdab96a25f6" xlink:to="loc_iclr_PRAHealthSciencesIncMember_cf7696f0-8d19-4176-9792-92a6ced815e8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_28800fbe-ec51-47ef-ad58-3f991a71677b" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_679d128a-ddb7-43f0-a094-4296acb3290b" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_28800fbe-ec51-47ef-ad58-3f991a71677b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_28800fbe-ec51-47ef-ad58-3f991a71677b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_28800fbe-ec51-47ef-ad58-3f991a71677b" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_28800fbe-ec51-47ef-ad58-3f991a71677b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_5eb209b4-1571-41ff-b1ec-1aced3cad94a" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_28800fbe-ec51-47ef-ad58-3f991a71677b" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_5eb209b4-1571-41ff-b1ec-1aced3cad94a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerRelationshipsMember_5e6c3b87-04ac-4355-b110-21dbb1ebd665" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_CustomerRelationshipsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_5eb209b4-1571-41ff-b1ec-1aced3cad94a" xlink:to="loc_us-gaap_CustomerRelationshipsMember_5e6c3b87-04ac-4355-b110-21dbb1ebd665" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OrderOrProductionBacklogMember_6490adff-391c-4a45-b4a6-57f22a542601" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_OrderOrProductionBacklogMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_5eb209b4-1571-41ff-b1ec-1aced3cad94a" xlink:to="loc_us-gaap_OrderOrProductionBacklogMember_6490adff-391c-4a45-b4a6-57f22a542601" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TradeNamesMember_851bb8c5-34e6-443e-8248-d9c35ee5da0b" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_TradeNamesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_5eb209b4-1571-41ff-b1ec-1aced3cad94a" xlink:to="loc_us-gaap_TradeNamesMember_851bb8c5-34e6-443e-8248-d9c35ee5da0b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DatabasesMember_25d7c0ca-254c-4caa-b074-ec654a5c8c30" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DatabasesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_5eb209b4-1571-41ff-b1ec-1aced3cad94a" xlink:to="loc_us-gaap_DatabasesMember_25d7c0ca-254c-4caa-b074-ec654a5c8c30" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TechnologyBasedIntangibleAssetsMember_f73b6a56-3751-4d1f-a646-c6043ce0bb21" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_TechnologyBasedIntangibleAssetsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_5eb209b4-1571-41ff-b1ec-1aced3cad94a" xlink:to="loc_us-gaap_TechnologyBasedIntangibleAssetsMember_f73b6a56-3751-4d1f-a646-c6043ce0bb21" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/ScheduleofMeasurementPeriodAdjustmentsofPRAHealthSciencesIncAcquisitionDetails" xlink:type="simple" xlink:href="iclr-20220930.xsd#ScheduleofMeasurementPeriodAdjustmentsofPRAHealthSciencesIncAcquisitionDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/ScheduleofMeasurementPeriodAdjustmentsofPRAHealthSciencesIncAcquisitionDetails" xlink:type="extended" id="ice52268fa7d54303bf6954bc03e4cb22_ScheduleofMeasurementPeriodAdjustmentsofPRAHealthSciencesIncAcquisitionDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionProFormaInformationNonrecurringAdjustmentLineItems_ae683c16-023d-46d8-a22f-4f9f7208a7c3" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionProFormaInformationNonrecurringAdjustmentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentCashAndCashEquivalents_86eb07d7-5df9-4aff-a277-17eda14ec36a" xlink:href="iclr-20220930.xsd#iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentCashAndCashEquivalents"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionProFormaInformationNonrecurringAdjustmentLineItems_ae683c16-023d-46d8-a22f-4f9f7208a7c3" xlink:to="loc_iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentCashAndCashEquivalents_86eb07d7-5df9-4aff-a277-17eda14ec36a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentAccountsReceivableAndUnbilledRevenue_5bc04326-c9e7-4c83-a331-28bbca441575" xlink:href="iclr-20220930.xsd#iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentAccountsReceivableAndUnbilledRevenue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionProFormaInformationNonrecurringAdjustmentLineItems_ae683c16-023d-46d8-a22f-4f9f7208a7c3" xlink:to="loc_iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentAccountsReceivableAndUnbilledRevenue_5bc04326-c9e7-4c83-a331-28bbca441575" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentOtherCurrentAssets_edbfac8b-a5ec-4cdc-b12a-17e88731d551" xlink:href="iclr-20220930.xsd#iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentOtherCurrentAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionProFormaInformationNonrecurringAdjustmentLineItems_ae683c16-023d-46d8-a22f-4f9f7208a7c3" xlink:to="loc_iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentOtherCurrentAssets_edbfac8b-a5ec-4cdc-b12a-17e88731d551" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentPropertyPlantAndEquipment_62e3fc56-dfeb-4c25-b2f7-deb44f77a3ae" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentPropertyPlantAndEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionProFormaInformationNonrecurringAdjustmentLineItems_ae683c16-023d-46d8-a22f-4f9f7208a7c3" xlink:to="loc_us-gaap_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentPropertyPlantAndEquipment_62e3fc56-dfeb-4c25-b2f7-deb44f77a3ae" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentOperatingLeaseRightOfUseAssets_7f53bc45-5ac0-4a02-8b94-f07f65dd26fc" xlink:href="iclr-20220930.xsd#iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentOperatingLeaseRightOfUseAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionProFormaInformationNonrecurringAdjustmentLineItems_ae683c16-023d-46d8-a22f-4f9f7208a7c3" xlink:to="loc_iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentOperatingLeaseRightOfUseAssets_7f53bc45-5ac0-4a02-8b94-f07f65dd26fc" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentGoodwill_d19f1748-ca02-4daf-a340-452ed9343575" xlink:href="iclr-20220930.xsd#iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentGoodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionProFormaInformationNonrecurringAdjustmentLineItems_ae683c16-023d-46d8-a22f-4f9f7208a7c3" xlink:to="loc_iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentGoodwill_d19f1748-ca02-4daf-a340-452ed9343575" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentIntangibles_960245ab-6bc7-48eb-8560-ed23ebebbc1d" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentIntangibles"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionProFormaInformationNonrecurringAdjustmentLineItems_ae683c16-023d-46d8-a22f-4f9f7208a7c3" xlink:to="loc_us-gaap_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentIntangibles_960245ab-6bc7-48eb-8560-ed23ebebbc1d" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentDeferredTaxAssets_cad21865-374f-40ab-8d4d-d3cb54c85b71" xlink:href="iclr-20220930.xsd#iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentDeferredTaxAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionProFormaInformationNonrecurringAdjustmentLineItems_ae683c16-023d-46d8-a22f-4f9f7208a7c3" xlink:to="loc_iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentDeferredTaxAssets_cad21865-374f-40ab-8d4d-d3cb54c85b71" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentOtherAssets_6af8a283-d641-4eb0-9c87-df18d3df870c" xlink:href="iclr-20220930.xsd#iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentOtherAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionProFormaInformationNonrecurringAdjustmentLineItems_ae683c16-023d-46d8-a22f-4f9f7208a7c3" xlink:to="loc_iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentOtherAssets_6af8a283-d641-4eb0-9c87-df18d3df870c" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentAccountsPayable_0ff0b57c-3bc5-42e5-9def-d92b667dbc87" xlink:href="iclr-20220930.xsd#iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentAccountsPayable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionProFormaInformationNonrecurringAdjustmentLineItems_ae683c16-023d-46d8-a22f-4f9f7208a7c3" xlink:to="loc_iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentAccountsPayable_0ff0b57c-3bc5-42e5-9def-d92b667dbc87" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentAccruedExpensesAndOtherCurrentLiabilities_e41bf8de-9a7e-4a64-aaa4-741fd7752b97" xlink:href="iclr-20220930.xsd#iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentAccruedExpensesAndOtherCurrentLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionProFormaInformationNonrecurringAdjustmentLineItems_ae683c16-023d-46d8-a22f-4f9f7208a7c3" xlink:to="loc_iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentAccruedExpensesAndOtherCurrentLiabilities_e41bf8de-9a7e-4a64-aaa4-741fd7752b97" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentCurrentPortionOfOperatingLeaseLiabilities_4c65cda8-1a41-4fa9-be2b-cbd4e99da4ba" xlink:href="iclr-20220930.xsd#iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentCurrentPortionOfOperatingLeaseLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionProFormaInformationNonrecurringAdjustmentLineItems_ae683c16-023d-46d8-a22f-4f9f7208a7c3" xlink:to="loc_iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentCurrentPortionOfOperatingLeaseLiabilities_4c65cda8-1a41-4fa9-be2b-cbd4e99da4ba" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentUnearnedRevenue_9cc4f165-009f-4335-aab9-7b06561248bd" xlink:href="iclr-20220930.xsd#iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentUnearnedRevenue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionProFormaInformationNonrecurringAdjustmentLineItems_ae683c16-023d-46d8-a22f-4f9f7208a7c3" xlink:to="loc_iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentUnearnedRevenue_9cc4f165-009f-4335-aab9-7b06561248bd" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentNonCurrentPortionOfOperatingLeaseLiabilities_dc3f5a12-5c1f-43c2-a4d0-e21c877eaac5" xlink:href="iclr-20220930.xsd#iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentNonCurrentPortionOfOperatingLeaseLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionProFormaInformationNonrecurringAdjustmentLineItems_ae683c16-023d-46d8-a22f-4f9f7208a7c3" xlink:to="loc_iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentNonCurrentPortionOfOperatingLeaseLiabilities_dc3f5a12-5c1f-43c2-a4d0-e21c877eaac5" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentNonCurrentDeferredTaxLiabilities_28b49c35-b0a5-4076-aa0d-b86a1859df5c" xlink:href="iclr-20220930.xsd#iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentNonCurrentDeferredTaxLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionProFormaInformationNonrecurringAdjustmentLineItems_ae683c16-023d-46d8-a22f-4f9f7208a7c3" xlink:to="loc_iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentNonCurrentDeferredTaxLiabilities_28b49c35-b0a5-4076-aa0d-b86a1859df5c" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentOtherNonCurrentLiabilities_388c920c-d95d-4b9c-a022-4a8800f615ae" xlink:href="iclr-20220930.xsd#iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentOtherNonCurrentLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionProFormaInformationNonrecurringAdjustmentLineItems_ae683c16-023d-46d8-a22f-4f9f7208a7c3" xlink:to="loc_iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentOtherNonCurrentLiabilities_388c920c-d95d-4b9c-a022-4a8800f615ae" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentsRelatedToPreviousPeriod_95795924-8433-44e4-abcf-4741681e52f8" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentsRelatedToPreviousPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionProFormaInformationNonrecurringAdjustmentLineItems_ae683c16-023d-46d8-a22f-4f9f7208a7c3" xlink:to="loc_us-gaap_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentsRelatedToPreviousPeriod_95795924-8433-44e4-abcf-4741681e52f8" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionProFormaInformationNonrecurringAdjustmentsTable_2cebbc56-ce2b-421a-b898-5ed37f94832a" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionProFormaInformationNonrecurringAdjustmentsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionProFormaInformationNonrecurringAdjustmentLineItems_ae683c16-023d-46d8-a22f-4f9f7208a7c3" xlink:to="loc_us-gaap_BusinessAcquisitionProFormaInformationNonrecurringAdjustmentsTable_2cebbc56-ce2b-421a-b898-5ed37f94832a" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_90f3d30d-a674-40e6-9f10-d7d275047b47" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_BusinessAcquisitionProFormaInformationNonrecurringAdjustmentsTable_2cebbc56-ce2b-421a-b898-5ed37f94832a" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_90f3d30d-a674-40e6-9f10-d7d275047b47" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_90f3d30d-a674-40e6-9f10-d7d275047b47_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_90f3d30d-a674-40e6-9f10-d7d275047b47" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_90f3d30d-a674-40e6-9f10-d7d275047b47_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_e35abcf6-f076-42f4-8409-33e07c0e67af" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_90f3d30d-a674-40e6-9f10-d7d275047b47" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_e35abcf6-f076-42f4-8409-33e07c0e67af" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_PRAHealthSciencesIncMember_3e818318-9195-43a3-843a-1bb6a564956d" xlink:href="iclr-20220930.xsd#iclr_PRAHealthSciencesIncMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_e35abcf6-f076-42f4-8409-33e07c0e67af" xlink:to="loc_iclr_PRAHealthSciencesIncMember_3e818318-9195-43a3-843a-1bb6a564956d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsForNewAccountingPronouncementsAxis_49cc0a2f-0afb-4262-9603-61d4234d88b9" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AdjustmentsForNewAccountingPronouncementsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_BusinessAcquisitionProFormaInformationNonrecurringAdjustmentsTable_2cebbc56-ce2b-421a-b898-5ed37f94832a" xlink:to="loc_us-gaap_AdjustmentsForNewAccountingPronouncementsAxis_49cc0a2f-0afb-4262-9603-61d4234d88b9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfAdoptionMember_49cc0a2f-0afb-4262-9603-61d4234d88b9_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_TypeOfAdoptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AdjustmentsForNewAccountingPronouncementsAxis_49cc0a2f-0afb-4262-9603-61d4234d88b9" xlink:to="loc_us-gaap_TypeOfAdoptionMember_49cc0a2f-0afb-4262-9603-61d4234d88b9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfAdoptionMember_b478efb0-3136-4a41-8c2d-a2e46185047b" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_TypeOfAdoptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AdjustmentsForNewAccountingPronouncementsAxis_49cc0a2f-0afb-4262-9603-61d4234d88b9" xlink:to="loc_us-gaap_TypeOfAdoptionMember_b478efb0-3136-4a41-8c2d-a2e46185047b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountingStandardsUpdate202108Member_1225206f-1f2c-4f18-9505-71abbfd8a4d0" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AccountingStandardsUpdate202108Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfAdoptionMember_b478efb0-3136-4a41-8c2d-a2e46185047b" xlink:to="loc_us-gaap_AccountingStandardsUpdate202108Member_1225206f-1f2c-4f18-9505-71abbfd8a4d0" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/BusinesscombinationsScheduleofProformaInformationofPRAHealthSciencesIncAcquisitionDetails" xlink:type="simple" xlink:href="iclr-20220930.xsd#BusinesscombinationsScheduleofProformaInformationofPRAHealthSciencesIncAcquisitionDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/BusinesscombinationsScheduleofProformaInformationofPRAHealthSciencesIncAcquisitionDetails" xlink:type="extended" id="i85e2849d54c949e382cc034faa70aec2_BusinesscombinationsScheduleofProformaInformationofPRAHealthSciencesIncAcquisitionDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_a0468474-4614-4224-b91b-455ac134e4bf" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionsProFormaRevenue_26a1db00-b95b-4294-827b-d39a905b7b66" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionsProFormaRevenue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_a0468474-4614-4224-b91b-455ac134e4bf" xlink:to="loc_us-gaap_BusinessAcquisitionsProFormaRevenue_26a1db00-b95b-4294-827b-d39a905b7b66" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionsProFormaNetIncomeLoss_fbb8f256-3cd4-4e3a-8ff6-8ae1a91123b6" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionsProFormaNetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_a0468474-4614-4224-b91b-455ac134e4bf" xlink:to="loc_us-gaap_BusinessAcquisitionsProFormaNetIncomeLoss_fbb8f256-3cd4-4e3a-8ff6-8ae1a91123b6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_cff2658e-bdd7-4e9e-aa44-2a13bd416ed7" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_a0468474-4614-4224-b91b-455ac134e4bf" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_cff2658e-bdd7-4e9e-aa44-2a13bd416ed7" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_c822e45e-5991-4ab1-93f2-fbaad306672d" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_cff2658e-bdd7-4e9e-aa44-2a13bd416ed7" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_c822e45e-5991-4ab1-93f2-fbaad306672d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_c822e45e-5991-4ab1-93f2-fbaad306672d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_c822e45e-5991-4ab1-93f2-fbaad306672d" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_c822e45e-5991-4ab1-93f2-fbaad306672d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_2cbd9b59-19cc-4243-a70b-22ce20ba21d5" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_c822e45e-5991-4ab1-93f2-fbaad306672d" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_2cbd9b59-19cc-4243-a70b-22ce20ba21d5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_PRAHealthSciencesIncMember_9ca9bf3a-f33b-43de-afb4-91db1317a751" xlink:href="iclr-20220930.xsd#iclr_PRAHealthSciencesIncMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_2cbd9b59-19cc-4243-a70b-22ce20ba21d5" xlink:to="loc_iclr_PRAHealthSciencesIncMember_9ca9bf3a-f33b-43de-afb4-91db1317a751" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/FairvaluemeasurementsDetails" xlink:type="simple" xlink:href="iclr-20220930.xsd#FairvaluemeasurementsDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/FairvaluemeasurementsDetails" xlink:type="extended" id="i02acc73441704cba9d0207b97453c615_FairvaluemeasurementsDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_e9068806-652d-4a73-bbf0-cf6849d38348" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInEquitySecuritiesFvNi_2a1e21df-7e5a-4417-a223-8b1ce3568795" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_IncreaseDecreaseInEquitySecuritiesFvNi"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_e9068806-652d-4a73-bbf0-cf6849d38348" xlink:to="loc_us-gaap_IncreaseDecreaseInEquitySecuritiesFvNi_2a1e21df-7e5a-4417-a223-8b1ce3568795" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesFvNi_3c8b7f87-412c-4835-8904-076e4faa5fe3" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_EquitySecuritiesFvNi"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_e9068806-652d-4a73-bbf0-cf6849d38348" xlink:to="loc_us-gaap_EquitySecuritiesFvNi_3c8b7f87-412c-4835-8904-076e4faa5fe3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsFairValueDisclosure_c33044f1-5de5-489c-9c29-ee172d0c960d" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AssetsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_e9068806-652d-4a73-bbf0-cf6849d38348" xlink:to="loc_us-gaap_AssetsFairValueDisclosure_c33044f1-5de5-489c-9c29-ee172d0c960d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillFairValueDisclosure_a240d1d1-d4ec-4eef-80f5-9c66dd352554" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_GoodwillFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_e9068806-652d-4a73-bbf0-cf6849d38348" xlink:to="loc_us-gaap_GoodwillFairValueDisclosure_a240d1d1-d4ec-4eef-80f5-9c66dd352554" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsFairValueDisclosure_e4358d5f-07ff-401d-be8b-cda1e32f8175" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FiniteLivedIntangibleAssetsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_e9068806-652d-4a73-bbf0-cf6849d38348" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsFairValueDisclosure_e4358d5f-07ff-401d-be8b-cda1e32f8175" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_6eb90c92-dbea-4bfb-9ebf-80f979c08f8a" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_e9068806-652d-4a73-bbf0-cf6849d38348" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_6eb90c92-dbea-4bfb-9ebf-80f979c08f8a" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByMeasurementFrequencyAxis_d1b3f369-f357-4a8d-bd78-d89cd1688223" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FairValueByMeasurementFrequencyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_6eb90c92-dbea-4bfb-9ebf-80f979c08f8a" xlink:to="loc_us-gaap_FairValueByMeasurementFrequencyAxis_d1b3f369-f357-4a8d-bd78-d89cd1688223" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_d1b3f369-f357-4a8d-bd78-d89cd1688223_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FairValueMeasurementFrequencyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_d1b3f369-f357-4a8d-bd78-d89cd1688223" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_d1b3f369-f357-4a8d-bd78-d89cd1688223_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_e62419ba-898e-41c6-a718-bbb1c2bda75f" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FairValueMeasurementFrequencyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_d1b3f369-f357-4a8d-bd78-d89cd1688223" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_e62419ba-898e-41c6-a718-bbb1c2bda75f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsNonrecurringMember_18c41fb1-c305-4cb0-bfa7-118a3f857bbf" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FairValueMeasurementsNonrecurringMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementFrequencyDomain_e62419ba-898e-41c6-a718-bbb1c2bda75f" xlink:to="loc_us-gaap_FairValueMeasurementsNonrecurringMember_18c41fb1-c305-4cb0-bfa7-118a3f857bbf" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_88d18863-5d78-4fd1-acff-899d5405c310" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_6eb90c92-dbea-4bfb-9ebf-80f979c08f8a" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_88d18863-5d78-4fd1-acff-899d5405c310" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_88d18863-5d78-4fd1-acff-899d5405c310_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_88d18863-5d78-4fd1-acff-899d5405c310" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_88d18863-5d78-4fd1-acff-899d5405c310_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_a24f4c7b-1f61-435b-858f-f7864b524dd5" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_88d18863-5d78-4fd1-acff-899d5405c310" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_a24f4c7b-1f61-435b-858f-f7864b524dd5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_3dc8b1d2-bb9e-48b2-a9f2-956084ee98bf" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FairValueInputsLevel3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_a24f4c7b-1f61-435b-858f-f7864b524dd5" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_3dc8b1d2-bb9e-48b2-a9f2-956084ee98bf" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/RestructuringNarrativeDetails" xlink:type="simple" xlink:href="iclr-20220930.xsd#RestructuringNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/RestructuringNarrativeDetails" xlink:type="extended" id="i6b1d7a824ba74c95a58c15d8bc0ee508_RestructuringNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCostAndReserveLineItems_5cdb44f2-fded-49bd-a1d7-618248a7b145" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RestructuringCostAndReserveLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCosts_a430bda5-1f2a-47a6-928f-6e7985a488c6" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RestructuringCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_5cdb44f2-fded-49bd-a1d7-618248a7b145" xlink:to="loc_us-gaap_RestructuringCosts_a430bda5-1f2a-47a6-928f-6e7985a488c6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringReserve_93f4bfe2-74b3-490b-a46a-0f0f3ad0fe7e" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RestructuringReserve"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_5cdb44f2-fded-49bd-a1d7-618248a7b145" xlink:to="loc_us-gaap_RestructuringReserve_93f4bfe2-74b3-490b-a46a-0f0f3ad0fe7e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_e5dd4fee-b3ae-4b31-8e2f-73c7d05344fe" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ScheduleOfRestructuringAndRelatedCostsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_5cdb44f2-fded-49bd-a1d7-618248a7b145" xlink:to="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_e5dd4fee-b3ae-4b31-8e2f-73c7d05344fe" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCostAndReserveAxis_4a7bc309-d552-4939-b797-7b1def6cfb2e" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RestructuringCostAndReserveAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_e5dd4fee-b3ae-4b31-8e2f-73c7d05344fe" xlink:to="loc_us-gaap_RestructuringCostAndReserveAxis_4a7bc309-d552-4939-b797-7b1def6cfb2e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfRestructuringDomain_4a7bc309-d552-4939-b797-7b1def6cfb2e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_TypeOfRestructuringDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RestructuringCostAndReserveAxis_4a7bc309-d552-4939-b797-7b1def6cfb2e" xlink:to="loc_us-gaap_TypeOfRestructuringDomain_4a7bc309-d552-4939-b797-7b1def6cfb2e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfRestructuringDomain_2c3faf8e-7f4a-4034-b4f7-f20d136d2e3f" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_TypeOfRestructuringDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RestructuringCostAndReserveAxis_4a7bc309-d552-4939-b797-7b1def6cfb2e" xlink:to="loc_us-gaap_TypeOfRestructuringDomain_2c3faf8e-7f4a-4034-b4f7-f20d136d2e3f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeSeveranceMember_4c5d5dac-d842-4c92-924b-d233fb944019" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_EmployeeSeveranceMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_2c3faf8e-7f4a-4034-b4f7-f20d136d2e3f" xlink:to="loc_us-gaap_EmployeeSeveranceMember_4c5d5dac-d842-4c92-924b-d233fb944019" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_OtherLiabilityMember_2b991611-a650-47d4-ac25-c0741a092394" xlink:href="iclr-20220930.xsd#iclr_OtherLiabilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_2c3faf8e-7f4a-4034-b4f7-f20d136d2e3f" xlink:to="loc_iclr_OtherLiabilityMember_2b991611-a650-47d4-ac25-c0741a092394" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_FacilityLiabilityMember_87aac8e7-ca5b-49ad-a2e3-cd3a1ec4b64e" xlink:href="iclr-20220930.xsd#iclr_FacilityLiabilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_2c3faf8e-7f4a-4034-b4f7-f20d136d2e3f" xlink:to="loc_iclr_FacilityLiabilityMember_87aac8e7-ca5b-49ad-a2e3-cd3a1ec4b64e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationAxis_0ead0aef-89f0-41dc-b69f-e3130f3a74d6" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BalanceSheetLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_e5dd4fee-b3ae-4b31-8e2f-73c7d05344fe" xlink:to="loc_us-gaap_BalanceSheetLocationAxis_0ead0aef-89f0-41dc-b69f-e3130f3a74d6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_0ead0aef-89f0-41dc-b69f-e3130f3a74d6_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BalanceSheetLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_0ead0aef-89f0-41dc-b69f-e3130f3a74d6" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_0ead0aef-89f0-41dc-b69f-e3130f3a74d6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_a1dc9062-d3d1-4301-a6eb-bc76bd560c25" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BalanceSheetLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_0ead0aef-89f0-41dc-b69f-e3130f3a74d6" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_a1dc9062-d3d1-4301-a6eb-bc76bd560c25" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherCurrentLiabilitiesMember_b4f90d40-5676-4177-8026-3caee8af3190" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_OtherCurrentLiabilitiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_a1dc9062-d3d1-4301-a6eb-bc76bd560c25" xlink:to="loc_us-gaap_OtherCurrentLiabilitiesMember_b4f90d40-5676-4177-8026-3caee8af3190" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherNoncurrentLiabilitiesMember_a9c458af-99f8-466d-93c9-5ee7b9a9dd7b" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_OtherNoncurrentLiabilitiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_a1dc9062-d3d1-4301-a6eb-bc76bd560c25" xlink:to="loc_us-gaap_OtherNoncurrentLiabilitiesMember_a9c458af-99f8-466d-93c9-5ee7b9a9dd7b" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/OperatingleasesNarrativeDetails" xlink:type="simple" xlink:href="iclr-20220930.xsd#OperatingleasesNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/OperatingleasesNarrativeDetails" xlink:type="extended" id="id5d517b4a5c94241a0a625b6ff38f8d9_OperatingleasesNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeLeaseDescriptionLineItems_915d4b28-1143-4392-8497-8c86d3e2f0da" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LesseeLeaseDescriptionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LeaseCost_c074b884-36c0-4101-b78b-58ff0799a267" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LeaseCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_915d4b28-1143-4392-8497-8c86d3e2f0da" xlink:to="loc_us-gaap_LeaseCost_c074b884-36c0-4101-b78b-58ff0799a267" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_EarlyTerminationOptionsReasonablyCertainToBeExercisedRightOfUseAssetObtainedInExchangeForOperatingLeaseLiability_fc1b69ce-ca1f-43d6-b7be-0199d51e06b1" xlink:href="iclr-20220930.xsd#iclr_EarlyTerminationOptionsReasonablyCertainToBeExercisedRightOfUseAssetObtainedInExchangeForOperatingLeaseLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_915d4b28-1143-4392-8497-8c86d3e2f0da" xlink:to="loc_iclr_EarlyTerminationOptionsReasonablyCertainToBeExercisedRightOfUseAssetObtainedInExchangeForOperatingLeaseLiability_fc1b69ce-ca1f-43d6-b7be-0199d51e06b1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RightOfUseAssetObtainedInExchangeForOperatingLeaseLiability_a9736fd7-3759-4ab4-a3ae-eee448ee029c" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RightOfUseAssetObtainedInExchangeForOperatingLeaseLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_915d4b28-1143-4392-8497-8c86d3e2f0da" xlink:to="loc_us-gaap_RightOfUseAssetObtainedInExchangeForOperatingLeaseLiability_a9736fd7-3759-4ab4-a3ae-eee448ee029c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseWeightedAverageRemainingLeaseTerm1_15150c3b-f32f-40bd-b6dd-06fb7a8ec021" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_OperatingLeaseWeightedAverageRemainingLeaseTerm1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_915d4b28-1143-4392-8497-8c86d3e2f0da" xlink:to="loc_us-gaap_OperatingLeaseWeightedAverageRemainingLeaseTerm1_15150c3b-f32f-40bd-b6dd-06fb7a8ec021" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseWeightedAverageDiscountRatePercent_7e12c8d1-98a5-4862-894d-14be038e3087" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_OperatingLeaseWeightedAverageDiscountRatePercent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_915d4b28-1143-4392-8497-8c86d3e2f0da" xlink:to="loc_us-gaap_OperatingLeaseWeightedAverageDiscountRatePercent_7e12c8d1-98a5-4862-894d-14be038e3087" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseLiabilityCurrent_2d87b15c-a551-4083-8b68-cf287980bd9c" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_OperatingLeaseLiabilityCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_915d4b28-1143-4392-8497-8c86d3e2f0da" xlink:to="loc_us-gaap_OperatingLeaseLiabilityCurrent_2d87b15c-a551-4083-8b68-cf287980bd9c" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseLiabilityStatementOfFinancialPositionExtensibleList_82fc8cf5-bdef-484a-9e48-0fb437c0b3cf" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_OperatingLeaseLiabilityStatementOfFinancialPositionExtensibleList"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_915d4b28-1143-4392-8497-8c86d3e2f0da" xlink:to="loc_us-gaap_OperatingLeaseLiabilityStatementOfFinancialPositionExtensibleList_82fc8cf5-bdef-484a-9e48-0fb437c0b3cf" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeLeaseDescriptionTable_f282a609-e48e-44b1-9a93-de04a37a0d29" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LesseeLeaseDescriptionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_915d4b28-1143-4392-8497-8c86d3e2f0da" xlink:to="loc_us-gaap_LesseeLeaseDescriptionTable_f282a609-e48e-44b1-9a93-de04a37a0d29" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_b900fce3-3977-4653-8a85-8ad02288fcf5" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_IncomeStatementLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LesseeLeaseDescriptionTable_f282a609-e48e-44b1-9a93-de04a37a0d29" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_b900fce3-3977-4653-8a85-8ad02288fcf5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_b900fce3-3977-4653-8a85-8ad02288fcf5_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_b900fce3-3977-4653-8a85-8ad02288fcf5" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_b900fce3-3977-4653-8a85-8ad02288fcf5_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_9fe79fa4-5d7b-4b7b-9ae3-dc8038e048a1" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_b900fce3-3977-4653-8a85-8ad02288fcf5" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_9fe79fa4-5d7b-4b7b-9ae3-dc8038e048a1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SellingGeneralAndAdministrativeExpensesMember_fbaf4c04-85b6-4314-95f6-f81ebc972b3b" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_SellingGeneralAndAdministrativeExpensesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_9fe79fa4-5d7b-4b7b-9ae3-dc8038e048a1" xlink:to="loc_us-gaap_SellingGeneralAndAdministrativeExpensesMember_fbaf4c04-85b6-4314-95f6-f81ebc972b3b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostOfSalesMember_55f04db4-cabc-45cd-9670-0aec6f835b05" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_CostOfSalesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_9fe79fa4-5d7b-4b7b-9ae3-dc8038e048a1" xlink:to="loc_us-gaap_CostOfSalesMember_55f04db4-cabc-45cd-9670-0aec6f835b05" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationAxis_1c41ed7e-8efb-4994-bdf4-02a8f6f9704a" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BalanceSheetLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LesseeLeaseDescriptionTable_f282a609-e48e-44b1-9a93-de04a37a0d29" xlink:to="loc_us-gaap_BalanceSheetLocationAxis_1c41ed7e-8efb-4994-bdf4-02a8f6f9704a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_1c41ed7e-8efb-4994-bdf4-02a8f6f9704a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BalanceSheetLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_1c41ed7e-8efb-4994-bdf4-02a8f6f9704a" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_1c41ed7e-8efb-4994-bdf4-02a8f6f9704a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_8ab4e071-8aa5-43b4-978e-8fda78c2150b" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BalanceSheetLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_1c41ed7e-8efb-4994-bdf4-02a8f6f9704a" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_8ab4e071-8aa5-43b4-978e-8fda78c2150b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherCurrentLiabilitiesMember_bc92debc-072d-4b4e-876d-f7ecd14739c8" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_OtherCurrentLiabilitiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_8ab4e071-8aa5-43b4-978e-8fda78c2150b" xlink:to="loc_us-gaap_OtherCurrentLiabilitiesMember_bc92debc-072d-4b4e-876d-f7ecd14739c8" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/BankcreditlinesandloanfacilitiesScheduleofOutstandingDebtDetails" xlink:type="simple" xlink:href="iclr-20220930.xsd#BankcreditlinesandloanfacilitiesScheduleofOutstandingDebtDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/BankcreditlinesandloanfacilitiesScheduleofOutstandingDebtDetails" xlink:type="extended" id="i6a5b2c1cee944ddbabe7b0ab3a32e39a_BankcreditlinesandloanfacilitiesScheduleofOutstandingDebtDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_9564900e-7237-4117-9b3f-66fcf3190203" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_294ec093-a898-4527-adad-c9076093215b" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_9564900e-7237-4117-9b3f-66fcf3190203" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_294ec093-a898-4527-adad-c9076093215b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentCarryingAmount_b136e8c0-f2bb-4097-bed4-370dca1869d0" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentCarryingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_9564900e-7237-4117-9b3f-66fcf3190203" xlink:to="loc_us-gaap_DebtInstrumentCarryingAmount_b136e8c0-f2bb-4097-bed4-370dca1869d0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_LongTermDebtCurrentMaturitiesGross_25494c67-4919-47bd-a618-5818acdffb16" xlink:href="iclr-20220930.xsd#iclr_LongTermDebtCurrentMaturitiesGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_9564900e-7237-4117-9b3f-66fcf3190203" xlink:to="loc_iclr_LongTermDebtCurrentMaturitiesGross_25494c67-4919-47bd-a618-5818acdffb16" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_LongTermDebtExcludingCurrentMaturitiesGross_61e7eb62-7c73-4806-a479-53fb2c0ffc8d" xlink:href="iclr-20220930.xsd#iclr_LongTermDebtExcludingCurrentMaturitiesGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_9564900e-7237-4117-9b3f-66fcf3190203" xlink:to="loc_iclr_LongTermDebtExcludingCurrentMaturitiesGross_61e7eb62-7c73-4806-a479-53fb2c0ffc8d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentUnamortizedDiscountPremiumAndDebtIssuanceCostsNet_feef1d70-5c89-4703-ae28-013887e296c8" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentUnamortizedDiscountPremiumAndDebtIssuanceCostsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_9564900e-7237-4117-9b3f-66fcf3190203" xlink:to="loc_us-gaap_DebtInstrumentUnamortizedDiscountPremiumAndDebtIssuanceCostsNet_feef1d70-5c89-4703-ae28-013887e296c8" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtNoncurrent_e8e090d2-72ec-41c8-af69-d0d3cbca7b14" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LongTermDebtNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_9564900e-7237-4117-9b3f-66fcf3190203" xlink:to="loc_us-gaap_LongTermDebtNoncurrent_e8e090d2-72ec-41c8-af69-d0d3cbca7b14" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_8214d8e3-fd6d-4882-9655-84b1b343da67" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_9564900e-7237-4117-9b3f-66fcf3190203" xlink:to="loc_us-gaap_DebtInstrumentTable_8214d8e3-fd6d-4882-9655-84b1b343da67" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_22b48f90-e901-43ca-8a9e-ad62f9a89290" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_8214d8e3-fd6d-4882-9655-84b1b343da67" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_22b48f90-e901-43ca-8a9e-ad62f9a89290" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_22b48f90-e901-43ca-8a9e-ad62f9a89290_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_22b48f90-e901-43ca-8a9e-ad62f9a89290" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_22b48f90-e901-43ca-8a9e-ad62f9a89290_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_2da28b99-f1ab-414d-97ac-4c248772b300" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_22b48f90-e901-43ca-8a9e-ad62f9a89290" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_2da28b99-f1ab-414d-97ac-4c248772b300" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredDebtMember_ff270afe-f2e3-4109-9519-c819e265aebc" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_SecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_2da28b99-f1ab-414d-97ac-4c248772b300" xlink:to="loc_us-gaap_SecuredDebtMember_ff270afe-f2e3-4109-9519-c819e265aebc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SeniorSubordinatedNotesMember_4df2cdab-aacf-4999-928e-529188e1200e" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_SeniorSubordinatedNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_2da28b99-f1ab-414d-97ac-4c248772b300" xlink:to="loc_us-gaap_SeniorSubordinatedNotesMember_4df2cdab-aacf-4999-928e-529188e1200e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_db42e7b5-d791-43db-a9ba-b1aa16570299" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_8214d8e3-fd6d-4882-9655-84b1b343da67" xlink:to="loc_us-gaap_DebtInstrumentAxis_db42e7b5-d791-43db-a9ba-b1aa16570299" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_db42e7b5-d791-43db-a9ba-b1aa16570299_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_db42e7b5-d791-43db-a9ba-b1aa16570299" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_db42e7b5-d791-43db-a9ba-b1aa16570299_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_83d78d09-036f-4dfb-a5ad-75eb906b9b3c" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_db42e7b5-d791-43db-a9ba-b1aa16570299" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_83d78d09-036f-4dfb-a5ad-75eb906b9b3c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_TermLoanFacilityMember_e4354f77-10ad-43df-986a-89aec8478d7e" xlink:href="iclr-20220930.xsd#iclr_TermLoanFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_83d78d09-036f-4dfb-a5ad-75eb906b9b3c" xlink:to="loc_iclr_TermLoanFacilityMember_e4354f77-10ad-43df-986a-89aec8478d7e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_SeniorSecuredNotesMember_db38c772-c086-4f22-bf17-98d2647e2499" xlink:href="iclr-20220930.xsd#iclr_SeniorSecuredNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_83d78d09-036f-4dfb-a5ad-75eb906b9b3c" xlink:to="loc_iclr_SeniorSecuredNotesMember_db38c772-c086-4f22-bf17-98d2647e2499" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/BankcreditlinesandloanfacilitiesNarrativeDetails" xlink:type="simple" xlink:href="iclr-20220930.xsd#BankcreditlinesandloanfacilitiesNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/BankcreditlinesandloanfacilitiesNarrativeDetails" xlink:type="extended" id="i63e96a67cb774aada9088a42afaedc21_BankcreditlinesandloanfacilitiesNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_d492c648-e1bc-4ec7-8845-943db9096551" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentUnamortizedDiscount_5636ba2a-05b9-4c89-9128-4e4eaf3748b4" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentUnamortizedDiscount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_d492c648-e1bc-4ec7-8845-943db9096551" xlink:to="loc_us-gaap_DebtInstrumentUnamortizedDiscount_5636ba2a-05b9-4c89-9128-4e4eaf3748b4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentFaceAmount_239a794e-5168-43ce-b47e-3575a311b731" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentFaceAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_d492c648-e1bc-4ec7-8845-943db9096551" xlink:to="loc_us-gaap_DebtInstrumentFaceAmount_239a794e-5168-43ce-b47e-3575a311b731" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_e5ce525d-13b8-47ac-adb1-8f3f8f5b158a" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_d492c648-e1bc-4ec7-8845-943db9096551" xlink:to="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_e5ce525d-13b8-47ac-adb1-8f3f8f5b158a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_85e88b8f-711b-4592-8a00-ff2f1fe21e8a" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_d492c648-e1bc-4ec7-8845-943db9096551" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_85e88b8f-711b-4592-8a00-ff2f1fe21e8a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_54f12f59-b928-4ad5-8c5b-fb5e0a065259" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentBasisSpreadOnVariableRate1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_d492c648-e1bc-4ec7-8845-943db9096551" xlink:to="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_54f12f59-b928-4ad5-8c5b-fb5e0a065259" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_DebtInstrumentBasisSpreadOnVariableRateStepDownPercentage_11a6ce45-4634-42cf-b85c-7cb33c0487f9" xlink:href="iclr-20220930.xsd#iclr_DebtInstrumentBasisSpreadOnVariableRateStepDownPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_d492c648-e1bc-4ec7-8845-943db9096551" xlink:to="loc_iclr_DebtInstrumentBasisSpreadOnVariableRateStepDownPercentage_11a6ce45-4634-42cf-b85c-7cb33c0487f9" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_DebtInstrumentCovenantFirstLienNetLeverageRatio_837d7892-82f4-4021-9f3d-e9f1ed011e3e" xlink:href="iclr-20220930.xsd#iclr_DebtInstrumentCovenantFirstLienNetLeverageRatio"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_d492c648-e1bc-4ec7-8845-943db9096551" xlink:to="loc_iclr_DebtInstrumentCovenantFirstLienNetLeverageRatio_837d7892-82f4-4021-9f3d-e9f1ed011e3e" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_DebtInstrumentIncreaseDecreaseBasisSpreadOnVariableRate_d303ff0d-a633-4932-82a5-81177da96469" xlink:href="iclr-20220930.xsd#iclr_DebtInstrumentIncreaseDecreaseBasisSpreadOnVariableRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_d492c648-e1bc-4ec7-8845-943db9096551" xlink:to="loc_iclr_DebtInstrumentIncreaseDecreaseBasisSpreadOnVariableRate_d303ff0d-a633-4932-82a5-81177da96469" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtNoncurrent_27c7a405-94cd-4ecc-bfdf-071fb30242ca" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LongTermDebtNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_d492c648-e1bc-4ec7-8845-943db9096551" xlink:to="loc_us-gaap_LongTermDebtNoncurrent_27c7a405-94cd-4ecc-bfdf-071fb30242ca" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_LongTermDebtFloatingRateDuration_661b398b-a3dc-4f23-aae3-2f2919c1b6c9" xlink:href="iclr-20220930.xsd#iclr_LongTermDebtFloatingRateDuration"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_d492c648-e1bc-4ec7-8845-943db9096551" xlink:to="loc_iclr_LongTermDebtFloatingRateDuration_661b398b-a3dc-4f23-aae3-2f2919c1b6c9" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity_a24151c6-689a-4e4b-ac56-3a5088645ded" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_d492c648-e1bc-4ec7-8845-943db9096551" xlink:to="loc_us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity_a24151c6-689a-4e4b-ac56-3a5088645ded" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromLinesOfCredit_4d5b2a8c-e041-49e6-9512-a1a4c1b57902" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ProceedsFromLinesOfCredit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_d492c648-e1bc-4ec7-8845-943db9096551" xlink:to="loc_us-gaap_ProceedsFromLinesOfCredit_4d5b2a8c-e041-49e6-9512-a1a4c1b57902" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityInterestRateDuringPeriod_15204dd0-5f82-4024-9ed6-99f9209232cb" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LineOfCreditFacilityInterestRateDuringPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_d492c648-e1bc-4ec7-8845-943db9096551" xlink:to="loc_us-gaap_LineOfCreditFacilityInterestRateDuringPeriod_15204dd0-5f82-4024-9ed6-99f9209232cb" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RepaymentsOfSeniorDebt_9e178572-f480-4ce1-932e-7efe79ff98d2" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RepaymentsOfSeniorDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_d492c648-e1bc-4ec7-8845-943db9096551" xlink:to="loc_us-gaap_RepaymentsOfSeniorDebt_9e178572-f480-4ce1-932e-7efe79ff98d2" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestPaidNet_71d3b5b7-274b-43e2-abc9-97c9b8e572ab" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_InterestPaidNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_d492c648-e1bc-4ec7-8845-943db9096551" xlink:to="loc_us-gaap_InterestPaidNet_71d3b5b7-274b-43e2-abc9-97c9b8e572ab" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsOfDebtRestructuringCosts_37b27458-467c-404c-b5f2-f600028ddc9d" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_PaymentsOfDebtRestructuringCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_d492c648-e1bc-4ec7-8845-943db9096551" xlink:to="loc_us-gaap_PaymentsOfDebtRestructuringCosts_37b27458-467c-404c-b5f2-f600028ddc9d" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtFairValue_89feaa2d-9682-48d6-bcc1-7453c5b54069" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LongTermDebtFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_d492c648-e1bc-4ec7-8845-943db9096551" xlink:to="loc_us-gaap_LongTermDebtFairValue_89feaa2d-9682-48d6-bcc1-7453c5b54069" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_1b47ce81-dc38-4e7d-85e9-4635a50222c9" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_d492c648-e1bc-4ec7-8845-943db9096551" xlink:to="loc_us-gaap_DebtInstrumentTable_1b47ce81-dc38-4e7d-85e9-4635a50222c9" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_536ad046-426f-4d25-a7ca-101323f29af2" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_1b47ce81-dc38-4e7d-85e9-4635a50222c9" xlink:to="loc_us-gaap_DebtInstrumentAxis_536ad046-426f-4d25-a7ca-101323f29af2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_536ad046-426f-4d25-a7ca-101323f29af2_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_536ad046-426f-4d25-a7ca-101323f29af2" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_536ad046-426f-4d25-a7ca-101323f29af2_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_003fdbdc-aac8-44e2-83bc-b8515f078ed2" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_536ad046-426f-4d25-a7ca-101323f29af2" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_003fdbdc-aac8-44e2-83bc-b8515f078ed2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_SeniorSecuredCreditFacilityAndSeniorSecuredNotesMember_67b6112d-6975-41b3-b367-b10a0d535299" xlink:href="iclr-20220930.xsd#iclr_SeniorSecuredCreditFacilityAndSeniorSecuredNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_003fdbdc-aac8-44e2-83bc-b8515f078ed2" xlink:to="loc_iclr_SeniorSecuredCreditFacilityAndSeniorSecuredNotesMember_67b6112d-6975-41b3-b367-b10a0d535299" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_TermLoanFacilityMember_29b82e17-179f-4d6b-b3a8-d840fd656941" xlink:href="iclr-20220930.xsd#iclr_TermLoanFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_003fdbdc-aac8-44e2-83bc-b8515f078ed2" xlink:to="loc_iclr_TermLoanFacilityMember_29b82e17-179f-4d6b-b3a8-d840fd656941" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_SeniorSecuredRevolvingCreditFacilityMember_05150023-60ad-4c9e-81e5-5a0da34698b6" xlink:href="iclr-20220930.xsd#iclr_SeniorSecuredRevolvingCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_003fdbdc-aac8-44e2-83bc-b8515f078ed2" xlink:to="loc_iclr_SeniorSecuredRevolvingCreditFacilityMember_05150023-60ad-4c9e-81e5-5a0da34698b6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_SeniorSecuredNotesMember_5b4d244f-d4ae-4ff0-9948-e60bf455ec8a" xlink:href="iclr-20220930.xsd#iclr_SeniorSecuredNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_003fdbdc-aac8-44e2-83bc-b8515f078ed2" xlink:to="loc_iclr_SeniorSecuredNotesMember_5b4d244f-d4ae-4ff0-9948-e60bf455ec8a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_4248fa23-d209-433b-84de-684af38aaa1f" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_1b47ce81-dc38-4e7d-85e9-4635a50222c9" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_4248fa23-d209-433b-84de-684af38aaa1f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_4248fa23-d209-433b-84de-684af38aaa1f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_4248fa23-d209-433b-84de-684af38aaa1f" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_4248fa23-d209-433b-84de-684af38aaa1f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_9d838e99-6ae2-40b0-9b26-a0202b769ef7" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_4248fa23-d209-433b-84de-684af38aaa1f" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_9d838e99-6ae2-40b0-9b26-a0202b769ef7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredDebtMember_83b71222-5a22-409d-8209-52c7d9e659f5" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_SecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_9d838e99-6ae2-40b0-9b26-a0202b769ef7" xlink:to="loc_us-gaap_SecuredDebtMember_83b71222-5a22-409d-8209-52c7d9e659f5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditMember_a9853b50-24e2-440f-9082-2b50b40bb5ac" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LineOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_9d838e99-6ae2-40b0-9b26-a0202b769ef7" xlink:to="loc_us-gaap_LineOfCreditMember_a9853b50-24e2-440f-9082-2b50b40bb5ac" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SeniorSubordinatedNotesMember_b10fb907-8fa7-42d8-b7c9-ddcb514e8361" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_SeniorSubordinatedNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_9d838e99-6ae2-40b0-9b26-a0202b769ef7" xlink:to="loc_us-gaap_SeniorSubordinatedNotesMember_b10fb907-8fa7-42d8-b7c9-ddcb514e8361" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityAxis_ad23d2aa-0e8a-4d8a-a7be-ee1733165733" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_CreditFacilityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_1b47ce81-dc38-4e7d-85e9-4635a50222c9" xlink:to="loc_us-gaap_CreditFacilityAxis_ad23d2aa-0e8a-4d8a-a7be-ee1733165733" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_ad23d2aa-0e8a-4d8a-a7be-ee1733165733_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_ad23d2aa-0e8a-4d8a-a7be-ee1733165733" xlink:to="loc_us-gaap_CreditFacilityDomain_ad23d2aa-0e8a-4d8a-a7be-ee1733165733_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_2ddc1f1e-2039-4ea1-ac32-9a8d51633ee8" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_ad23d2aa-0e8a-4d8a-a7be-ee1733165733" xlink:to="loc_us-gaap_CreditFacilityDomain_2ddc1f1e-2039-4ea1-ac32-9a8d51633ee8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevolvingCreditFacilityMember_a163d115-346f-4bfe-8211-8255e57961b3" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RevolvingCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_2ddc1f1e-2039-4ea1-ac32-9a8d51633ee8" xlink:to="loc_us-gaap_RevolvingCreditFacilityMember_a163d115-346f-4bfe-8211-8255e57961b3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateAxis_7a47b313-d745-4737-abf0-326b51c3ca43" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_VariableRateAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_1b47ce81-dc38-4e7d-85e9-4635a50222c9" xlink:to="loc_us-gaap_VariableRateAxis_7a47b313-d745-4737-abf0-326b51c3ca43" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateDomain_7a47b313-d745-4737-abf0-326b51c3ca43_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_VariableRateDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_VariableRateAxis_7a47b313-d745-4737-abf0-326b51c3ca43" xlink:to="loc_us-gaap_VariableRateDomain_7a47b313-d745-4737-abf0-326b51c3ca43_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateDomain_bbaa4615-9fd7-4e02-a486-f3ede26c03aa" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_VariableRateDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_VariableRateAxis_7a47b313-d745-4737-abf0-326b51c3ca43" xlink:to="loc_us-gaap_VariableRateDomain_bbaa4615-9fd7-4e02-a486-f3ede26c03aa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LondonInterbankOfferedRateLIBORMember_f399583f-4cda-4424-8584-8ceb431f1d82" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LondonInterbankOfferedRateLIBORMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableRateDomain_bbaa4615-9fd7-4e02-a486-f3ede26c03aa" xlink:to="loc_us-gaap_LondonInterbankOfferedRateLIBORMember_f399583f-4cda-4424-8584-8ceb431f1d82" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BaseRateMember_f7cbb44e-568f-494f-b8b0-9d080c301e9e" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BaseRateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableRateDomain_bbaa4615-9fd7-4e02-a486-f3ede26c03aa" xlink:to="loc_us-gaap_BaseRateMember_f7cbb44e-568f-494f-b8b0-9d080c301e9e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_d631ddf8-a7be-4513-b1d6-04ecba9bd01f" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_1b47ce81-dc38-4e7d-85e9-4635a50222c9" xlink:to="loc_srt_RangeAxis_d631ddf8-a7be-4513-b1d6-04ecba9bd01f" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_d631ddf8-a7be-4513-b1d6-04ecba9bd01f_default" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_d631ddf8-a7be-4513-b1d6-04ecba9bd01f" xlink:to="loc_srt_RangeMember_d631ddf8-a7be-4513-b1d6-04ecba9bd01f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_7fdbbe2d-22eb-4c05-9052-43fc55afddc5" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_d631ddf8-a7be-4513-b1d6-04ecba9bd01f" xlink:to="loc_srt_RangeMember_7fdbbe2d-22eb-4c05-9052-43fc55afddc5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_6c909ebd-c9ae-4381-9bcd-31c9a1141d2e" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_7fdbbe2d-22eb-4c05-9052-43fc55afddc5" xlink:to="loc_srt_MaximumMember_6c909ebd-c9ae-4381-9bcd-31c9a1141d2e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_b4f259a7-50ee-45bd-8021-7d795bc4687b" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_7fdbbe2d-22eb-4c05-9052-43fc55afddc5" xlink:to="loc_srt_MinimumMember_b4f259a7-50ee-45bd-8021-7d795bc4687b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_VariableRateComponentAxis_0fa6608b-1684-48ca-b2bb-557ec891d049" xlink:href="iclr-20220930.xsd#iclr_VariableRateComponentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_1b47ce81-dc38-4e7d-85e9-4635a50222c9" xlink:to="loc_iclr_VariableRateComponentAxis_0fa6608b-1684-48ca-b2bb-557ec891d049" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_VariableRateComponentDomain_0fa6608b-1684-48ca-b2bb-557ec891d049_default" xlink:href="iclr-20220930.xsd#iclr_VariableRateComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_iclr_VariableRateComponentAxis_0fa6608b-1684-48ca-b2bb-557ec891d049" xlink:to="loc_iclr_VariableRateComponentDomain_0fa6608b-1684-48ca-b2bb-557ec891d049_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_VariableRateComponentDomain_9e1526ec-d33d-44f7-9145-42d93458ddef" xlink:href="iclr-20220930.xsd#iclr_VariableRateComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_iclr_VariableRateComponentAxis_0fa6608b-1684-48ca-b2bb-557ec891d049" xlink:to="loc_iclr_VariableRateComponentDomain_9e1526ec-d33d-44f7-9145-42d93458ddef" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_VariableRateComponentOneMember_a0cc5376-265f-4974-9ff6-4cf8fe7ec166" xlink:href="iclr-20220930.xsd#iclr_VariableRateComponentOneMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iclr_VariableRateComponentDomain_9e1526ec-d33d-44f7-9145-42d93458ddef" xlink:to="loc_iclr_VariableRateComponentOneMember_a0cc5376-265f-4974-9ff6-4cf8fe7ec166" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_VariableRateComponentTwoMember_cbc9377e-66ec-455f-a390-490f5e846b58" xlink:href="iclr-20220930.xsd#iclr_VariableRateComponentTwoMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iclr_VariableRateComponentDomain_9e1526ec-d33d-44f7-9145-42d93458ddef" xlink:to="loc_iclr_VariableRateComponentTwoMember_cbc9377e-66ec-455f-a390-490f5e846b58" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeAxis_827620f1-6ccc-4013-8738-97679b1d8b56" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_SubsequentEventTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_1b47ce81-dc38-4e7d-85e9-4635a50222c9" xlink:to="loc_us-gaap_SubsequentEventTypeAxis_827620f1-6ccc-4013-8738-97679b1d8b56" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_827620f1-6ccc-4013-8738-97679b1d8b56_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_827620f1-6ccc-4013-8738-97679b1d8b56" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_827620f1-6ccc-4013-8738-97679b1d8b56_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_28284efc-4c22-40af-8ce0-f872160ec80f" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_827620f1-6ccc-4013-8738-97679b1d8b56" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_28284efc-4c22-40af-8ce0-f872160ec80f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventMember_c0e28c28-0530-48ba-9b0c-9f4906ca46bc" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_SubsequentEventMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventTypeDomain_28284efc-4c22-40af-8ce0-f872160ec80f" xlink:to="loc_us-gaap_SubsequentEventMember_c0e28c28-0530-48ba-9b0c-9f4906ca46bc" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/SharebasedawardsSummaryofStockOptionActivityDetails" xlink:type="simple" xlink:href="iclr-20220930.xsd#SharebasedawardsSummaryofStockOptionActivityDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/SharebasedawardsSummaryofStockOptionActivityDetails" xlink:type="extended" id="ifb0b1f3841804993abb5515e635ccdf8_SharebasedawardsSummaryofStockOptionActivityDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_083c014c-7ffe-4e26-803f-99d3ed942262" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_da48ebd6-a0c8-4e92-a8fc-e6e212a9b4e5" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_083c014c-7ffe-4e26-803f-99d3ed942262" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_da48ebd6-a0c8-4e92-a8fc-e6e212a9b4e5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_433af56c-d449-4a34-b94b-f300375438a3" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_da48ebd6-a0c8-4e92-a8fc-e6e212a9b4e5" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_433af56c-d449-4a34-b94b-f300375438a3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross_899f1672-50cc-4530-8cf9-d0df3e7de0b4" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_da48ebd6-a0c8-4e92-a8fc-e6e212a9b4e5" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross_899f1672-50cc-4530-8cf9-d0df3e7de0b4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_b3c9234b-bf3e-4a99-86a0-5cb3f941ed5a" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_da48ebd6-a0c8-4e92-a8fc-e6e212a9b4e5" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_b3c9234b-bf3e-4a99-86a0-5cb3f941ed5a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriod_e55712ca-94e8-4d85-b700-23b2680d824f" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_da48ebd6-a0c8-4e92-a8fc-e6e212a9b4e5" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriod_e55712ca-94e8-4d85-b700-23b2680d824f" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_ae281601-a5fd-4b25-b204-f3bfb0462c1c" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber_d755775a-fb3b-4a14-8e59-f02ff87fadaa" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_083c014c-7ffe-4e26-803f-99d3ed942262" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber_d755775a-fb3b-4a14-8e59-f02ff87fadaa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_48309a01-c960-4673-8cc7-9302581ca857" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_083c014c-7ffe-4e26-803f-99d3ed942262" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_48309a01-c960-4673-8cc7-9302581ca857" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice_257fdec4-c3b8-4632-8f9e-5d8e262ba18d" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_48309a01-c960-4673-8cc7-9302581ca857" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice_257fdec4-c3b8-4632-8f9e-5d8e262ba18d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice_31db7438-be69-4c6b-91c9-36de31ecd91e" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_48309a01-c960-4673-8cc7-9302581ca857" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice_31db7438-be69-4c6b-91c9-36de31ecd91e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice_fe0416cd-d68d-4f6c-85d0-d0d348f8fdea" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_48309a01-c960-4673-8cc7-9302581ca857" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice_fe0416cd-d68d-4f6c-85d0-d0d348f8fdea" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriodWeightedAverageExercisePrice_08fbea74-2dff-472a-a54a-dce396e6142b" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriodWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_48309a01-c960-4673-8cc7-9302581ca857" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriodWeightedAverageExercisePrice_08fbea74-2dff-472a-a54a-dce396e6142b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice_753b6da3-b75e-430a-9e2f-eeae86ded089" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice_7b371968-fc94-410f-9cfd-8128cb6bf1e9" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_083c014c-7ffe-4e26-803f-99d3ed942262" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice_7b371968-fc94-410f-9cfd-8128cb6bf1e9" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsWeightedAverageRemainingContractualTermAbstract_43ddccf1-726d-4be7-a5fb-8307c176bada" xlink:href="iclr-20220930.xsd#iclr_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsWeightedAverageRemainingContractualTermAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_083c014c-7ffe-4e26-803f-99d3ed942262" xlink:to="loc_iclr_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsWeightedAverageRemainingContractualTermAbstract_43ddccf1-726d-4be7-a5fb-8307c176bada" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2_b939a173-0025-431a-8f66-37897e5a650c" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iclr_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsWeightedAverageRemainingContractualTermAbstract_43ddccf1-726d-4be7-a5fb-8307c176bada" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2_b939a173-0025-431a-8f66-37897e5a650c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1_511a409a-79c7-4d65-a892-58cf8d60add7" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iclr_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsWeightedAverageRemainingContractualTermAbstract_43ddccf1-726d-4be7-a5fb-8307c176bada" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1_511a409a-79c7-4d65-a892-58cf8d60add7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_8ae9c171-c4a4-464c-a83e-69cd03038682" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_083c014c-7ffe-4e26-803f-99d3ed942262" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_8ae9c171-c4a4-464c-a83e-69cd03038682" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_88c8af63-7f28-4ba8-aea5-8d869246a023" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_8ae9c171-c4a4-464c-a83e-69cd03038682" xlink:to="loc_us-gaap_AwardTypeAxis_88c8af63-7f28-4ba8-aea5-8d869246a023" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_88c8af63-7f28-4ba8-aea5-8d869246a023_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_88c8af63-7f28-4ba8-aea5-8d869246a023" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_88c8af63-7f28-4ba8-aea5-8d869246a023_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_e09f54ff-c7cf-4475-9ccb-81bee061b50b" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_88c8af63-7f28-4ba8-aea5-8d869246a023" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_e09f54ff-c7cf-4475-9ccb-81bee061b50b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_14f5a0d7-bd03-4562-9e6b-a4c092e6ac16" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_e09f54ff-c7cf-4475-9ccb-81bee061b50b" xlink:to="loc_us-gaap_EmployeeStockOptionMember_14f5a0d7-bd03-4562-9e6b-a4c092e6ac16" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/SharebasedawardsNarrativeDetails" xlink:type="simple" xlink:href="iclr-20220930.xsd#SharebasedawardsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/SharebasedawardsNarrativeDetails" xlink:type="extended" id="i3f41f54d13574a41b6bc2c5e796ee3da_SharebasedawardsNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureLineItems_b0db99df-91a5-49bb-8498-63efc7f83cd6" xlink:href="iclr-20220930.xsd#iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber_c403df38-d620-47a5-8762-c1ea065ea445" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureLineItems_b0db99df-91a5-49bb-8498-63efc7f83cd6" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber_c403df38-d620-47a5-8762-c1ea065ea445" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1_f6b5cd39-ca5d-4380-8f02-091a8e0ce693" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureLineItems_b0db99df-91a5-49bb-8498-63efc7f83cd6" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1_f6b5cd39-ca5d-4380-8f02-091a8e0ce693" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice_c147fe6d-b5a6-46f9-8f67-d304ae10ecbd" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureLineItems_b0db99df-91a5-49bb-8498-63efc7f83cd6" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice_c147fe6d-b5a6-46f9-8f67-d304ae10ecbd" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized_9ddde7aa-c1aa-46ca-99d7-acdb3bb907c1" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureLineItems_b0db99df-91a5-49bb-8498-63efc7f83cd6" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized_9ddde7aa-c1aa-46ca-99d7-acdb3bb907c1" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_288fd76d-74f5-478a-b5e9-48efa4db355a" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureLineItems_b0db99df-91a5-49bb-8498-63efc7f83cd6" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_288fd76d-74f5-478a-b5e9-48efa4db355a" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue_0ea80368-a192-4151-9b00-311b793c97bd" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureLineItems_b0db99df-91a5-49bb-8498-63efc7f83cd6" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue_0ea80368-a192-4151-9b00-311b793c97bd" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_7150dfd1-abe6-4aae-9821-91cf8caf2ccc" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureLineItems_b0db99df-91a5-49bb-8498-63efc7f83cd6" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_7150dfd1-abe6-4aae-9821-91cf8caf2ccc" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureTable_c06e854f-a436-4b0f-b82f-974045e87415" xlink:href="iclr-20220930.xsd#iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureLineItems_b0db99df-91a5-49bb-8498-63efc7f83cd6" xlink:to="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureTable_c06e854f-a436-4b0f-b82f-974045e87415" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_b4576e21-f0a0-4f5e-b04c-ad7a85efc548" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureTable_c06e854f-a436-4b0f-b82f-974045e87415" xlink:to="loc_us-gaap_AwardTypeAxis_b4576e21-f0a0-4f5e-b04c-ad7a85efc548" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_b4576e21-f0a0-4f5e-b04c-ad7a85efc548_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_b4576e21-f0a0-4f5e-b04c-ad7a85efc548" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_b4576e21-f0a0-4f5e-b04c-ad7a85efc548_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_e6c7c125-758c-4d83-9185-1d16ac2992e6" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_b4576e21-f0a0-4f5e-b04c-ad7a85efc548" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_e6c7c125-758c-4d83-9185-1d16ac2992e6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_23a1e6fe-0ad6-48e0-ac95-7aa84321ce76" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_e6c7c125-758c-4d83-9185-1d16ac2992e6" xlink:to="loc_us-gaap_EmployeeStockOptionMember_23a1e6fe-0ad6-48e0-ac95-7aa84321ce76" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_ConsultantsRestrictedStockUnitsTwentyNineteenMember_9f709fd5-0834-4f0d-ac38-c907d010f5ae" xlink:href="iclr-20220930.xsd#iclr_ConsultantsRestrictedStockUnitsTwentyNineteenMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_e6c7c125-758c-4d83-9185-1d16ac2992e6" xlink:to="loc_iclr_ConsultantsRestrictedStockUnitsTwentyNineteenMember_9f709fd5-0834-4f0d-ac38-c907d010f5ae" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_PerformanceShareUnitMember_8b1bdf73-2ced-4b83-a255-72c2004226d7" xlink:href="iclr-20220930.xsd#iclr_PerformanceShareUnitMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_e6c7c125-758c-4d83-9185-1d16ac2992e6" xlink:to="loc_iclr_PerformanceShareUnitMember_8b1bdf73-2ced-4b83-a255-72c2004226d7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_7dfa646a-a51b-4b21-be25-6cc73f138a06" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_e6c7c125-758c-4d83-9185-1d16ac2992e6" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_7dfa646a-a51b-4b21-be25-6cc73f138a06" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_PerformanceBasedGrantsMember_dcc467d6-45dc-4171-82a6-6358f56d87b2" xlink:href="iclr-20220930.xsd#iclr_PerformanceBasedGrantsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_e6c7c125-758c-4d83-9185-1d16ac2992e6" xlink:to="loc_iclr_PerformanceBasedGrantsMember_dcc467d6-45dc-4171-82a6-6358f56d87b2" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualAxis_8ec80704-6793-4bd5-b4f2-154eac88a22e" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_TitleOfIndividualAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureTable_c06e854f-a436-4b0f-b82f-974045e87415" xlink:to="loc_srt_TitleOfIndividualAxis_8ec80704-6793-4bd5-b4f2-154eac88a22e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_8ec80704-6793-4bd5-b4f2-154eac88a22e_default" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_TitleOfIndividualWithRelationshipToEntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_TitleOfIndividualAxis_8ec80704-6793-4bd5-b4f2-154eac88a22e" xlink:to="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_8ec80704-6793-4bd5-b4f2-154eac88a22e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_d0c51e14-fb21-4cd3-ab35-8336cde0c9a1" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_TitleOfIndividualWithRelationshipToEntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_TitleOfIndividualAxis_8ec80704-6793-4bd5-b4f2-154eac88a22e" xlink:to="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_d0c51e14-fb21-4cd3-ab35-8336cde0c9a1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_NonExecutiveDirectorMember_4a665816-0842-43d9-b7b2-0116d5ae9bde" xlink:href="iclr-20220930.xsd#iclr_NonExecutiveDirectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_d0c51e14-fb21-4cd3-ab35-8336cde0c9a1" xlink:to="loc_iclr_NonExecutiveDirectorMember_4a665816-0842-43d9-b7b2-0116d5ae9bde" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_80a88b8d-6013-4b21-a333-ae72c1c97fcf" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureTable_c06e854f-a436-4b0f-b82f-974045e87415" xlink:to="loc_srt_RangeAxis_80a88b8d-6013-4b21-a333-ae72c1c97fcf" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_80a88b8d-6013-4b21-a333-ae72c1c97fcf_default" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_80a88b8d-6013-4b21-a333-ae72c1c97fcf" xlink:to="loc_srt_RangeMember_80a88b8d-6013-4b21-a333-ae72c1c97fcf_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_e8c617c2-b091-4948-94f8-e41087faaa36" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_80a88b8d-6013-4b21-a333-ae72c1c97fcf" xlink:to="loc_srt_RangeMember_e8c617c2-b091-4948-94f8-e41087faaa36" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_1cd4ffe1-e7e5-4d33-8f9f-4c221e7bbdf5" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_e8c617c2-b091-4948-94f8-e41087faaa36" xlink:to="loc_srt_MaximumMember_1cd4ffe1-e7e5-4d33-8f9f-4c221e7bbdf5" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/SharebasedawardsScheduleofWeightedAverageFairValuesandAssumptionsUsedDetails" xlink:type="simple" xlink:href="iclr-20220930.xsd#SharebasedawardsScheduleofWeightedAverageFairValuesandAssumptionsUsedDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/SharebasedawardsScheduleofWeightedAverageFairValuesandAssumptionsUsedDetails" xlink:type="extended" id="i961e1b83679a448286adff47131f31ab_SharebasedawardsScheduleofWeightedAverageFairValuesandAssumptionsUsedDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_efc05c41-935d-48ac-906d-e725befd04c8" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_9096bc8f-c26e-4408-b116-ed0e200d956e" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_efc05c41-935d-48ac-906d-e725befd04c8" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_9096bc8f-c26e-4408-b116-ed0e200d956e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsAndMethodologyAbstract_e26b636a-58f4-4b7b-9a2b-edd6d30e5719" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsAndMethodologyAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_efc05c41-935d-48ac-906d-e725befd04c8" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsAndMethodologyAbstract_e26b636a-58f4-4b7b-9a2b-edd6d30e5719" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_745b9318-5766-4027-941e-26d3c924d913" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsAndMethodologyAbstract_e26b636a-58f4-4b7b-9a2b-edd6d30e5719" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_745b9318-5766-4027-941e-26d3c924d913" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_48bbce39-baa5-45be-bba0-436fba3c8d7f" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsAndMethodologyAbstract_e26b636a-58f4-4b7b-9a2b-edd6d30e5719" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_48bbce39-baa5-45be-bba0-436fba3c8d7f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_481a6c47-caa6-4ce6-9d03-707f21499880" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsAndMethodologyAbstract_e26b636a-58f4-4b7b-9a2b-edd6d30e5719" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_481a6c47-caa6-4ce6-9d03-707f21499880" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_63e868f2-3a3c-4268-8728-d6ae28807c5c" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsAndMethodologyAbstract_e26b636a-58f4-4b7b-9a2b-edd6d30e5719" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_63e868f2-3a3c-4268-8728-d6ae28807c5c" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_35cd06d6-a821-4cc0-aab6-d4bb9a97b459" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_efc05c41-935d-48ac-906d-e725befd04c8" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_35cd06d6-a821-4cc0-aab6-d4bb9a97b459" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_5a835f49-d6a5-400c-a504-122e39a249de" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_35cd06d6-a821-4cc0-aab6-d4bb9a97b459" xlink:to="loc_us-gaap_AwardTypeAxis_5a835f49-d6a5-400c-a504-122e39a249de" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_5a835f49-d6a5-400c-a504-122e39a249de_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_5a835f49-d6a5-400c-a504-122e39a249de" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_5a835f49-d6a5-400c-a504-122e39a249de_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_b4435e5d-b2cb-43b6-8b87-f17f839ccb53" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_5a835f49-d6a5-400c-a504-122e39a249de" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_b4435e5d-b2cb-43b6-8b87-f17f839ccb53" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_69ecc0a4-99e6-4f6f-96e4-5b9f7c9c9de7" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_b4435e5d-b2cb-43b6-8b87-f17f839ccb53" xlink:to="loc_us-gaap_EmployeeStockOptionMember_69ecc0a4-99e6-4f6f-96e4-5b9f7c9c9de7" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/SharebasedawardsSummaryofRSUandPSUActivityDetails" xlink:type="simple" xlink:href="iclr-20220930.xsd#SharebasedawardsSummaryofRSUandPSUActivityDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/SharebasedawardsSummaryofRSUandPSUActivityDetails" xlink:type="extended" id="i118dde8337384cbf8b2a28e16597b8da_SharebasedawardsSummaryofRSUandPSUActivityDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_76616c18-d640-41d9-b6fa-992d484d50b1" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_d1e4039d-dbb6-4a34-9cab-c5a7ec69a9fc" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_76616c18-d640-41d9-b6fa-992d484d50b1" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_d1e4039d-dbb6-4a34-9cab-c5a7ec69a9fc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_121fa522-9fef-43e8-8493-c4538acecf3d" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_d1e4039d-dbb6-4a34-9cab-c5a7ec69a9fc" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_121fa522-9fef-43e8-8493-c4538acecf3d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_d062985b-16d7-4ac6-bcc9-2cfb9c0b5754" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_d1e4039d-dbb6-4a34-9cab-c5a7ec69a9fc" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_d062985b-16d7-4ac6-bcc9-2cfb9c0b5754" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_1aeddbdb-11c0-41d9-b24d-c465e8d155fb" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_d1e4039d-dbb6-4a34-9cab-c5a7ec69a9fc" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_1aeddbdb-11c0-41d9-b24d-c465e8d155fb" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_554cf3a6-2b19-49a1-9103-4f928e610129" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_d1e4039d-dbb6-4a34-9cab-c5a7ec69a9fc" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_554cf3a6-2b19-49a1-9103-4f928e610129" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_7ff67772-4fd5-43fd-9218-cde7d448b57c" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_71a94f28-c3a6-4e3e-ad8f-356695690652" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_76616c18-d640-41d9-b6fa-992d484d50b1" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_71a94f28-c3a6-4e3e-ad8f-356695690652" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_625e56a5-2975-4232-9ff7-d357b1978c96" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_71a94f28-c3a6-4e3e-ad8f-356695690652" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_625e56a5-2975-4232-9ff7-d357b1978c96" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_a2b0adf2-d3a0-40ca-8c6d-672643326c11" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_71a94f28-c3a6-4e3e-ad8f-356695690652" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_a2b0adf2-d3a0-40ca-8c6d-672643326c11" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_2b0c7cf6-e494-4422-b7dd-e967c26b51ac" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_71a94f28-c3a6-4e3e-ad8f-356695690652" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_2b0c7cf6-e494-4422-b7dd-e967c26b51ac" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_9f0747e8-dcbc-42ed-92b1-40c6a0eab7a4" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_71a94f28-c3a6-4e3e-ad8f-356695690652" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_9f0747e8-dcbc-42ed-92b1-40c6a0eab7a4" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_37afce0d-0d5d-4965-99f1-a27fa6589b9d" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_00524b66-8ff2-4a2d-ba55-30ddda09daac" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_76616c18-d640-41d9-b6fa-992d484d50b1" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_00524b66-8ff2-4a2d-ba55-30ddda09daac" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_b8785691-da09-4c2c-8493-d386bdd15fdd" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_00524b66-8ff2-4a2d-ba55-30ddda09daac" xlink:to="loc_us-gaap_AwardTypeAxis_b8785691-da09-4c2c-8493-d386bdd15fdd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_b8785691-da09-4c2c-8493-d386bdd15fdd_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_b8785691-da09-4c2c-8493-d386bdd15fdd" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_b8785691-da09-4c2c-8493-d386bdd15fdd_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_a5b1742a-4290-46de-b6c4-d10b07169c83" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_b8785691-da09-4c2c-8493-d386bdd15fdd" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_a5b1742a-4290-46de-b6c4-d10b07169c83" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_PerformanceShareUnitMember_afcfdc99-72c5-4948-bbb4-1eb3c40f129d" xlink:href="iclr-20220930.xsd#iclr_PerformanceShareUnitMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_a5b1742a-4290-46de-b6c4-d10b07169c83" xlink:to="loc_iclr_PerformanceShareUnitMember_afcfdc99-72c5-4948-bbb4-1eb3c40f129d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_d34b5985-c98b-4912-a73d-9dac1f14cbee" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_a5b1742a-4290-46de-b6c4-d10b07169c83" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_d34b5985-c98b-4912-a73d-9dac1f14cbee" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/SharebasedawardsScheduleofNoncashStockCompensationExpenseDetails" xlink:type="simple" xlink:href="iclr-20220930.xsd#SharebasedawardsScheduleofNoncashStockCompensationExpenseDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/SharebasedawardsScheduleofNoncashStockCompensationExpenseDetails" xlink:type="extended" id="i67491e7fc8c041919712fc50da12b82c_SharebasedawardsScheduleofNoncashStockCompensationExpenseDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_35a03374-7e9c-454a-baf1-dad3f35ee811" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensation_550dd3f2-dc19-42b7-93a7-fabe714c8df5" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_35a03374-7e9c-454a-baf1-dad3f35ee811" xlink:to="loc_us-gaap_ShareBasedCompensation_550dd3f2-dc19-42b7-93a7-fabe714c8df5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_d859b12c-2d40-49f4-95b4-d7693d9c4f0f" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_35a03374-7e9c-454a-baf1-dad3f35ee811" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_d859b12c-2d40-49f4-95b4-d7693d9c4f0f" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_cb656916-ac30-407f-9f72-caf74a5a2faf" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_IncomeStatementLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_d859b12c-2d40-49f4-95b4-d7693d9c4f0f" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_cb656916-ac30-407f-9f72-caf74a5a2faf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_cb656916-ac30-407f-9f72-caf74a5a2faf_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_cb656916-ac30-407f-9f72-caf74a5a2faf" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_cb656916-ac30-407f-9f72-caf74a5a2faf_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_a6398e84-c84c-4935-a820-67beef4f6c33" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_cb656916-ac30-407f-9f72-caf74a5a2faf" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_a6398e84-c84c-4935-a820-67beef4f6c33" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostOfSalesMember_b84e2821-3b95-4518-8417-0c9d92764055" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_CostOfSalesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_a6398e84-c84c-4935-a820-67beef4f6c33" xlink:to="loc_us-gaap_CostOfSalesMember_b84e2821-3b95-4518-8417-0c9d92764055" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SellingGeneralAndAdministrativeExpensesMember_148c4467-c0e4-42b0-a80a-235839f79ea9" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_SellingGeneralAndAdministrativeExpensesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_a6398e84-c84c-4935-a820-67beef4f6c33" xlink:to="loc_us-gaap_SellingGeneralAndAdministrativeExpensesMember_148c4467-c0e4-42b0-a80a-235839f79ea9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_TransactionAndIntegrationRelatedExpensesMember_1a42d08f-730e-454f-ae51-b75bb5f43c3b" xlink:href="iclr-20220930.xsd#iclr_TransactionAndIntegrationRelatedExpensesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_a6398e84-c84c-4935-a820-67beef4f6c33" xlink:to="loc_iclr_TransactionAndIntegrationRelatedExpensesMember_1a42d08f-730e-454f-ae51-b75bb5f43c3b" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/SharecapitalDetails" xlink:type="simple" xlink:href="iclr-20220930.xsd#SharecapitalDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/SharecapitalDetails" xlink:type="extended" id="i2c254d5aad4b4dfc894bf96662d46f10_SharecapitalDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityClassOfTreasuryStockLineItems_035d3c8a-a084-42b4-b447-de87537ef293" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_EquityClassOfTreasuryStockLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesAcquisitions_786810e3-08c3-4453-b905-4a2b92813d13" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StockIssuedDuringPeriodSharesAcquisitions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityClassOfTreasuryStockLineItems_035d3c8a-a084-42b4-b447-de87537ef293" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesAcquisitions_786810e3-08c3-4453-b905-4a2b92813d13" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_StockRepurchaseProgramAuthorizedPercentage_ee918559-06fa-4959-9293-1818333861ef" xlink:href="iclr-20220930.xsd#iclr_StockRepurchaseProgramAuthorizedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityClassOfTreasuryStockLineItems_035d3c8a-a084-42b4-b447-de87537ef293" xlink:to="loc_iclr_StockRepurchaseProgramAuthorizedPercentage_ee918559-06fa-4959-9293-1818333861ef" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodShares_f316b52d-9435-49b4-b19b-d3d39dff96d5" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StockRepurchasedAndRetiredDuringPeriodShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityClassOfTreasuryStockLineItems_035d3c8a-a084-42b4-b447-de87537ef293" xlink:to="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodShares_f316b52d-9435-49b4-b19b-d3d39dff96d5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodValue_96387383-566a-40fb-a1ef-17c4df42d93c" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StockRepurchasedAndRetiredDuringPeriodValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityClassOfTreasuryStockLineItems_035d3c8a-a084-42b4-b447-de87537ef293" xlink:to="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodValue_96387383-566a-40fb-a1ef-17c4df42d93c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfTreasuryStockTable_581fa853-7f9b-4659-acfa-eedb21b565cd" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ClassOfTreasuryStockTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EquityClassOfTreasuryStockLineItems_035d3c8a-a084-42b4-b447-de87537ef293" xlink:to="loc_us-gaap_ClassOfTreasuryStockTable_581fa853-7f9b-4659-acfa-eedb21b565cd" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareRepurchaseProgramAxis_cbc7390d-ac95-4def-91ce-6d781254357f" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareRepurchaseProgramAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ClassOfTreasuryStockTable_581fa853-7f9b-4659-acfa-eedb21b565cd" xlink:to="loc_us-gaap_ShareRepurchaseProgramAxis_cbc7390d-ac95-4def-91ce-6d781254357f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareRepurchaseProgramDomain_cbc7390d-ac95-4def-91ce-6d781254357f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareRepurchaseProgramDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ShareRepurchaseProgramAxis_cbc7390d-ac95-4def-91ce-6d781254357f" xlink:to="loc_us-gaap_ShareRepurchaseProgramDomain_cbc7390d-ac95-4def-91ce-6d781254357f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareRepurchaseProgramDomain_14a35e3b-ac77-4514-990f-0aab1e008a3e" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareRepurchaseProgramDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ShareRepurchaseProgramAxis_cbc7390d-ac95-4def-91ce-6d781254357f" xlink:to="loc_us-gaap_ShareRepurchaseProgramDomain_14a35e3b-ac77-4514-990f-0aab1e008a3e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BuybackProgramMember_c0d1688c-073d-4a78-8a4d-5e9731e8547c" xlink:href="iclr-20220930.xsd#iclr_BuybackProgramMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareRepurchaseProgramDomain_14a35e3b-ac77-4514-990f-0aab1e008a3e" xlink:to="loc_iclr_BuybackProgramMember_c0d1688c-073d-4a78-8a4d-5e9731e8547c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_df1c6159-508b-4347-aa8d-c5f86dc044e6" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ClassOfTreasuryStockTable_581fa853-7f9b-4659-acfa-eedb21b565cd" xlink:to="loc_srt_RangeAxis_df1c6159-508b-4347-aa8d-c5f86dc044e6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_df1c6159-508b-4347-aa8d-c5f86dc044e6_default" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_df1c6159-508b-4347-aa8d-c5f86dc044e6" xlink:to="loc_srt_RangeMember_df1c6159-508b-4347-aa8d-c5f86dc044e6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_dc9ca41f-2c3a-4fbd-8437-b0dc5a8f3a36" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_df1c6159-508b-4347-aa8d-c5f86dc044e6" xlink:to="loc_srt_RangeMember_dc9ca41f-2c3a-4fbd-8437-b0dc5a8f3a36" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_316b9eef-d977-4a99-a987-ae76231023fa" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_dc9ca41f-2c3a-4fbd-8437-b0dc5a8f3a36" xlink:to="loc_srt_MaximumMember_316b9eef-d977-4a99-a987-ae76231023fa" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_ea85c67d-061f-42d2-b60c-ab2cac512d61" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ClassOfTreasuryStockTable_581fa853-7f9b-4659-acfa-eedb21b565cd" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_ea85c67d-061f-42d2-b60c-ab2cac512d61" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_ea85c67d-061f-42d2-b60c-ab2cac512d61_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_ea85c67d-061f-42d2-b60c-ab2cac512d61" xlink:to="loc_us-gaap_EquityComponentDomain_ea85c67d-061f-42d2-b60c-ab2cac512d61_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_5b6ff71f-efd3-422d-acb2-a74008aede38" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_ea85c67d-061f-42d2-b60c-ab2cac512d61" xlink:to="loc_us-gaap_EquityComponentDomain_5b6ff71f-efd3-422d-acb2-a74008aede38" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_0cbce103-6309-4aef-8757-a06dbb1d5925" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_5b6ff71f-efd3-422d-acb2-a74008aede38" xlink:to="loc_us-gaap_CommonStockMember_0cbce103-6309-4aef-8757-a06dbb1d5925" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/BusinessSegmentandGeographicalInformationScheduleofDistributionofRevenuebyGeographicalAreaDetails" xlink:type="simple" xlink:href="iclr-20220930.xsd#BusinessSegmentandGeographicalInformationScheduleofDistributionofRevenuebyGeographicalAreaDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/BusinessSegmentandGeographicalInformationScheduleofDistributionofRevenuebyGeographicalAreaDetails" xlink:type="extended" id="i800d1a2bfd0e4a8fa341bdf4f4ef1ae8_BusinessSegmentandGeographicalInformationScheduleofDistributionofRevenuebyGeographicalAreaDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_3c7beb4b-4b16-4629-8df4-3321d0045f83" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_d0efe6c8-e013-420b-8d8e-20ffcf0a5436" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_3c7beb4b-4b16-4629-8df4-3321d0045f83" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_d0efe6c8-e013-420b-8d8e-20ffcf0a5436" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_4015ca76-b77b-4b2d-a4b5-344d8d763bb6" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_3c7beb4b-4b16-4629-8df4-3321d0045f83" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_4015ca76-b77b-4b2d-a4b5-344d8d763bb6" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementGeographicalAxis_6e0ec397-b2ca-48d5-9f45-4e04af2f535f" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_StatementGeographicalAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_4015ca76-b77b-4b2d-a4b5-344d8d763bb6" xlink:to="loc_srt_StatementGeographicalAxis_6e0ec397-b2ca-48d5-9f45-4e04af2f535f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_6e0ec397-b2ca-48d5-9f45-4e04af2f535f_default" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_6e0ec397-b2ca-48d5-9f45-4e04af2f535f" xlink:to="loc_srt_SegmentGeographicalDomain_6e0ec397-b2ca-48d5-9f45-4e04af2f535f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_cf6f93bd-6f87-4e81-a901-aa37093b4e6a" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_6e0ec397-b2ca-48d5-9f45-4e04af2f535f" xlink:to="loc_srt_SegmentGeographicalDomain_cf6f93bd-6f87-4e81-a901-aa37093b4e6a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_IE_2bf69afc-a141-42ab-9d1c-5302f64d938c" xlink:href="https://xbrl.sec.gov/country/2022/country-2022.xsd#country_IE"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_cf6f93bd-6f87-4e81-a901-aa37093b4e6a" xlink:to="loc_country_IE_2bf69afc-a141-42ab-9d1c-5302f64d938c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_RestOfEuropeMember_d3c90e9f-3ea7-4473-8e74-69c1728845c1" xlink:href="iclr-20220930.xsd#iclr_RestOfEuropeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_cf6f93bd-6f87-4e81-a901-aa37093b4e6a" xlink:to="loc_iclr_RestOfEuropeMember_d3c90e9f-3ea7-4473-8e74-69c1728845c1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_US_70a94401-e0b7-411e-8960-4faeaa826753" xlink:href="https://xbrl.sec.gov/country/2022/country-2022.xsd#country_US"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_cf6f93bd-6f87-4e81-a901-aa37093b4e6a" xlink:to="loc_country_US_70a94401-e0b7-411e-8960-4faeaa826753" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_RestOfWorldMember_9a64feea-a1b5-4ecc-a5c2-9918b79a36f5" xlink:href="iclr-20220930.xsd#iclr_RestOfWorldMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_cf6f93bd-6f87-4e81-a901-aa37093b4e6a" xlink:to="loc_iclr_RestOfWorldMember_9a64feea-a1b5-4ecc-a5c2-9918b79a36f5" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/BusinessSegmentandGeographicalInformationScheduleofDistributionofIncomefromOperationsbyGeographicalAreaDetails" xlink:type="simple" xlink:href="iclr-20220930.xsd#BusinessSegmentandGeographicalInformationScheduleofDistributionofIncomefromOperationsbyGeographicalAreaDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/BusinessSegmentandGeographicalInformationScheduleofDistributionofIncomefromOperationsbyGeographicalAreaDetails" xlink:type="extended" id="i1bfc45feac6e4a22b60351f96552e847_BusinessSegmentandGeographicalInformationScheduleofDistributionofIncomefromOperationsbyGeographicalAreaDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_e04adc6f-ca08-49a1-88e2-1cd095d22536" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingIncomeLoss_86ba55ad-caa9-48da-8bc7-703c0c5c980b" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_OperatingIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_e04adc6f-ca08-49a1-88e2-1cd095d22536" xlink:to="loc_us-gaap_OperatingIncomeLoss_86ba55ad-caa9-48da-8bc7-703c0c5c980b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_91c49912-b98b-4e71-a33f-0288c1c4770e" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_e04adc6f-ca08-49a1-88e2-1cd095d22536" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_91c49912-b98b-4e71-a33f-0288c1c4770e" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementGeographicalAxis_497c9437-d639-415e-b298-2e20c8b66262" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_StatementGeographicalAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_91c49912-b98b-4e71-a33f-0288c1c4770e" xlink:to="loc_srt_StatementGeographicalAxis_497c9437-d639-415e-b298-2e20c8b66262" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_497c9437-d639-415e-b298-2e20c8b66262_default" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_497c9437-d639-415e-b298-2e20c8b66262" xlink:to="loc_srt_SegmentGeographicalDomain_497c9437-d639-415e-b298-2e20c8b66262_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_2f635cf4-65b4-4681-b612-ea09b3a429db" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_497c9437-d639-415e-b298-2e20c8b66262" xlink:to="loc_srt_SegmentGeographicalDomain_2f635cf4-65b4-4681-b612-ea09b3a429db" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_IE_2f26f159-5c4a-4bad-a764-05d637acf1cf" xlink:href="https://xbrl.sec.gov/country/2022/country-2022.xsd#country_IE"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_2f635cf4-65b4-4681-b612-ea09b3a429db" xlink:to="loc_country_IE_2f26f159-5c4a-4bad-a764-05d637acf1cf" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_RestOfEuropeMember_69ace98d-825f-4e92-8378-5f2c62eb65f1" xlink:href="iclr-20220930.xsd#iclr_RestOfEuropeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_2f635cf4-65b4-4681-b612-ea09b3a429db" xlink:to="loc_iclr_RestOfEuropeMember_69ace98d-825f-4e92-8378-5f2c62eb65f1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_US_a6687ae0-32a1-41ab-b869-55d8d421c23f" xlink:href="https://xbrl.sec.gov/country/2022/country-2022.xsd#country_US"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_2f635cf4-65b4-4681-b612-ea09b3a429db" xlink:to="loc_country_US_a6687ae0-32a1-41ab-b869-55d8d421c23f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_RestOfWorldMember_9d4bd3b5-b957-41ee-aa6a-8b6bb76ed908" xlink:href="iclr-20220930.xsd#iclr_RestOfWorldMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_2f635cf4-65b4-4681-b612-ea09b3a429db" xlink:to="loc_iclr_RestOfWorldMember_9d4bd3b5-b957-41ee-aa6a-8b6bb76ed908" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/BusinessSegmentandGeographicalInformationScheduleofDistributionofDepreciationandAmortizationbyGeographicalAreaDetails" xlink:type="simple" xlink:href="iclr-20220930.xsd#BusinessSegmentandGeographicalInformationScheduleofDistributionofDepreciationandAmortizationbyGeographicalAreaDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/BusinessSegmentandGeographicalInformationScheduleofDistributionofDepreciationandAmortizationbyGeographicalAreaDetails" xlink:type="extended" id="ic122bdd1159744479d99fd01279b5ac2_BusinessSegmentandGeographicalInformationScheduleofDistributionofDepreciationandAmortizationbyGeographicalAreaDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_a3eb8b6c-d867-4310-bb68-71bc32bfbcbd" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NoncurrentAssets_14e9e66a-b3b9-429d-a02e-9532611f78f5" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_NoncurrentAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_a3eb8b6c-d867-4310-bb68-71bc32bfbcbd" xlink:to="loc_us-gaap_NoncurrentAssets_14e9e66a-b3b9-429d-a02e-9532611f78f5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_de02f6ac-1c4a-49d3-a6cb-3e18af3fb0a8" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_a3eb8b6c-d867-4310-bb68-71bc32bfbcbd" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_de02f6ac-1c4a-49d3-a6cb-3e18af3fb0a8" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementGeographicalAxis_bcb3d361-cc92-47f7-8d19-88f394a67aac" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_StatementGeographicalAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_de02f6ac-1c4a-49d3-a6cb-3e18af3fb0a8" xlink:to="loc_srt_StatementGeographicalAxis_bcb3d361-cc92-47f7-8d19-88f394a67aac" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_bcb3d361-cc92-47f7-8d19-88f394a67aac_default" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_bcb3d361-cc92-47f7-8d19-88f394a67aac" xlink:to="loc_srt_SegmentGeographicalDomain_bcb3d361-cc92-47f7-8d19-88f394a67aac_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_fd4ea2e9-3b9f-41ee-97ea-0f9f1814ad05" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_bcb3d361-cc92-47f7-8d19-88f394a67aac" xlink:to="loc_srt_SegmentGeographicalDomain_fd4ea2e9-3b9f-41ee-97ea-0f9f1814ad05" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_IE_46a68b83-2e53-469a-b252-f1b12701d5b5" xlink:href="https://xbrl.sec.gov/country/2022/country-2022.xsd#country_IE"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_fd4ea2e9-3b9f-41ee-97ea-0f9f1814ad05" xlink:to="loc_country_IE_46a68b83-2e53-469a-b252-f1b12701d5b5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_RestOfEuropeMember_9704017a-347b-4d05-b1b4-97639ecff7e9" xlink:href="iclr-20220930.xsd#iclr_RestOfEuropeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_fd4ea2e9-3b9f-41ee-97ea-0f9f1814ad05" xlink:to="loc_iclr_RestOfEuropeMember_9704017a-347b-4d05-b1b4-97639ecff7e9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_US_e1c5dd1c-bf26-4b44-b780-c972e70e5089" xlink:href="https://xbrl.sec.gov/country/2022/country-2022.xsd#country_US"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_fd4ea2e9-3b9f-41ee-97ea-0f9f1814ad05" xlink:to="loc_country_US_e1c5dd1c-bf26-4b44-b780-c972e70e5089" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_RestOfWorldMember_90181d91-edb7-40d1-a6e2-4608793c894a" xlink:href="iclr-20220930.xsd#iclr_RestOfWorldMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_fd4ea2e9-3b9f-41ee-97ea-0f9f1814ad05" xlink:to="loc_iclr_RestOfWorldMember_90181d91-edb7-40d1-a6e2-4608793c894a" xlink:type="arc" order="3"/>
  </link:definitionLink>
</link:linkbase>
