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

<!--XBRL Document Created with Wdesk from Workiva-->
<!--Copyright 2020 Workiva-->
<!--r:77157664-d885-402b-bd0c-341e64854ee1,g:5a455175-4fa2-47d9-938e-237d8cb2f8ee-->
<link:linkbase xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd">
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member"/>
  <link:roleRef roleURI="http://www.iconplc.com/role/Cover" xlink:type="simple" xlink:href="iclr-20200630.xsd#Cover"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/Cover" xlink:type="extended" id="i512acead12ac4c97b14ba81d96e3fd1b_Cover"/>
  <link:roleRef roleURI="http://www.iconplc.com/role/CondensedConsolidatedBalanceSheets" xlink:type="simple" xlink:href="iclr-20200630.xsd#CondensedConsolidatedBalanceSheets"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/CondensedConsolidatedBalanceSheets" xlink:type="extended" id="if1c6d50449bb41feac35c92fd874c82a_CondensedConsolidatedBalanceSheets"/>
  <link:roleRef roleURI="http://www.iconplc.com/role/CondensedConsolidatedBalanceSheetsParenthetical" xlink:type="simple" xlink:href="iclr-20200630.xsd#CondensedConsolidatedBalanceSheetsParenthetical"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/CondensedConsolidatedBalanceSheetsParenthetical" xlink:type="extended" id="if50978b738134a22ade0e3c200aa34ce_CondensedConsolidatedBalanceSheetsParenthetical"/>
  <link:roleRef roleURI="http://www.iconplc.com/role/CondensedConsolidatedStatementsofOperations" xlink:type="simple" xlink:href="iclr-20200630.xsd#CondensedConsolidatedStatementsofOperations"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/CondensedConsolidatedStatementsofOperations" xlink:type="extended" id="i4db7a4a04dc444c0b6c5438482ddad6d_CondensedConsolidatedStatementsofOperations"/>
  <link:roleRef roleURI="http://www.iconplc.com/role/CondensedConsolidatedStatementsofCashFlows" xlink:type="simple" xlink:href="iclr-20200630.xsd#CondensedConsolidatedStatementsofCashFlows"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/CondensedConsolidatedStatementsofCashFlows" xlink:type="extended" id="i79e7bf3408704d95b0205816c4a59a86_CondensedConsolidatedStatementsofCashFlows"/>
  <link:roleRef roleURI="http://www.iconplc.com/role/CondensedConsolidatedStatementsofShareholdersEquityandComprehensiveIncome" xlink:type="simple" xlink:href="iclr-20200630.xsd#CondensedConsolidatedStatementsofShareholdersEquityandComprehensiveIncome"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/CondensedConsolidatedStatementsofShareholdersEquityandComprehensiveIncome" xlink:type="extended" id="i5554784fff8c419dbabf2a8c0d9de44b_CondensedConsolidatedStatementsofShareholdersEquityandComprehensiveIncome">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_09531501-975f-4e24-a0be-3c44eaf01392" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_1093b954-cf52-4be3-b674-d3571d6f6ca6" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_09531501-975f-4e24-a0be-3c44eaf01392" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_1093b954-cf52-4be3-b674-d3571d6f6ca6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesIssued_03d27a5c-6ac5-4a97-bb14-221bbde3a5e4" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CommonStockSharesIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_1093b954-cf52-4be3-b674-d3571d6f6ca6" xlink:to="loc_us-gaap_CommonStockSharesIssued_03d27a5c-6ac5-4a97-bb14-221bbde3a5e4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_e140da2e-29ab-4bdf-8523-620f4390c0b6" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_1093b954-cf52-4be3-b674-d3571d6f6ca6" xlink:to="loc_us-gaap_StockholdersEquity_e140da2e-29ab-4bdf-8523-620f4390c0b6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterestAbstract_230ac916-939a-4244-b8b1-39d8240c94c8" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterestAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_1093b954-cf52-4be3-b674-d3571d6f6ca6" xlink:to="loc_us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterestAbstract_230ac916-939a-4244-b8b1-39d8240c94c8" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_6f98ba75-07eb-4e9b-9ebe-52f7c8145214" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterestAbstract_230ac916-939a-4244-b8b1-39d8240c94c8" xlink:to="loc_us-gaap_NetIncomeLoss_6f98ba75-07eb-4e9b-9ebe-52f7c8145214" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax_00102a6a-adc1-428f-afaf-73cc0b9fd7d1" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterestAbstract_230ac916-939a-4244-b8b1-39d8240c94c8" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax_00102a6a-adc1-428f-afaf-73cc0b9fd7d1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTaxLongTermFunding_f17b47b1-9060-431b-93eb-3d07090dd01e" xlink:href="iclr-20200630.xsd#iclr_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTaxLongTermFunding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterestAbstract_230ac916-939a-4244-b8b1-39d8240c94c8" xlink:to="loc_iclr_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTaxLongTermFunding_f17b47b1-9060-431b-93eb-3d07090dd01e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax_5a4e7353-3404-417e-9e4f-9a41e5843c20" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterestAbstract_230ac916-939a-4244-b8b1-39d8240c94c8" xlink:to="loc_us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax_5a4e7353-3404-417e-9e4f-9a41e5843c20" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossReclassificationAfterTax_c26c7f01-a7a8-4573-becd-2af6da3b77b9" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossReclassificationAfterTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterestAbstract_230ac916-939a-4244-b8b1-39d8240c94c8" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossReclassificationAfterTax_c26c7f01-a7a8-4573-becd-2af6da3b77b9" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossBeforeReclassificationAfterTax_bbc8cd4d-1dd5-41ee-a8a4-32aea20211a4" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossBeforeReclassificationAfterTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterestAbstract_230ac916-939a-4244-b8b1-39d8240c94c8" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossBeforeReclassificationAfterTax_bbc8cd4d-1dd5-41ee-a8a4-32aea20211a4" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ComprehensiveIncomeNetOfTax_8b69341a-d669-4ff1-a872-07e9b74b49a7" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ComprehensiveIncomeNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterestAbstract_230ac916-939a-4244-b8b1-39d8240c94c8" xlink:to="loc_us-gaap_ComprehensiveIncomeNetOfTax_8b69341a-d669-4ff1-a872-07e9b74b49a7" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_b3f42de6-5da5-416d-b3a7-8b04c1b85a46" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_1093b954-cf52-4be3-b674-d3571d6f6ca6" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_b3f42de6-5da5-416d-b3a7-8b04c1b85a46" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_88416d63-41cb-46eb-893a-445dd1a7d53d" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_1093b954-cf52-4be3-b674-d3571d6f6ca6" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_88416d63-41cb-46eb-893a-445dd1a7d53d" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation_72e6b185-aad5-49e2-bdf9-d016497a6a7c" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_1093b954-cf52-4be3-b674-d3571d6f6ca6" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation_72e6b185-aad5-49e2-bdf9-d016497a6a7c" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation_66220633-1639-49fb-b82d-5a4b8a4acc33" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_1093b954-cf52-4be3-b674-d3571d6f6ca6" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation_66220633-1639-49fb-b82d-5a4b8a4acc33" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_de074943-0ba1-472c-b57e-579addba699e" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_1093b954-cf52-4be3-b674-d3571d6f6ca6" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_de074943-0ba1-472c-b57e-579addba699e" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalStockIssuedIssuanceCosts_f665beb8-e13d-43ab-ab41-4256bd82034b" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalStockIssuedIssuanceCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_1093b954-cf52-4be3-b674-d3571d6f6ca6" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalStockIssuedIssuanceCosts_f665beb8-e13d-43ab-ab41-4256bd82034b" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodShares_c3746907-60a1-43ef-a3a1-265e7fea0f7a" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StockRepurchasedAndRetiredDuringPeriodShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_1093b954-cf52-4be3-b674-d3571d6f6ca6" xlink:to="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodShares_c3746907-60a1-43ef-a3a1-265e7fea0f7a" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodValue_0f65eb4e-0b39-43f9-937d-9219c17717ac" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StockRepurchasedAndRetiredDuringPeriodValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_1093b954-cf52-4be3-b674-d3571d6f6ca6" xlink:to="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodValue_0f65eb4e-0b39-43f9-937d-9219c17717ac" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_AdjustmentToRetainedEarningShareRepurchaseCosts_89c747da-f8b7-4fd3-96ef-9ca4a068b6cd" xlink:href="iclr-20200630.xsd#iclr_AdjustmentToRetainedEarningShareRepurchaseCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_1093b954-cf52-4be3-b674-d3571d6f6ca6" xlink:to="loc_iclr_AdjustmentToRetainedEarningShareRepurchaseCosts_89c747da-f8b7-4fd3-96ef-9ca4a068b6cd" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_RedeemableNoncontrollingInterestAccretionToRedemptionValue_1a1758f6-566b-4358-960c-b40cab9f7fcd" xlink:href="iclr-20200630.xsd#iclr_RedeemableNoncontrollingInterestAccretionToRedemptionValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_1093b954-cf52-4be3-b674-d3571d6f6ca6" xlink:to="loc_iclr_RedeemableNoncontrollingInterestAccretionToRedemptionValue_1a1758f6-566b-4358-960c-b40cab9f7fcd" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesIssued_646957d3-7374-4b19-9629-d07e8b4034fb" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CommonStockSharesIssued"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_05dddc00-45fb-4932-976e-a67906bf300c" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StockholdersEquity"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInTemporaryEquityRollForward_090bd385-6868-4ab7-9f4d-3e83a9b5a79c" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_IncreaseDecreaseInTemporaryEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_09531501-975f-4e24-a0be-3c44eaf01392" xlink:to="loc_us-gaap_IncreaseDecreaseInTemporaryEquityRollForward_090bd385-6868-4ab7-9f4d-3e83a9b5a79c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RedeemableNoncontrollingInterestEquityCarryingAmount_a98ee6ab-c90b-4aa3-a764-5f061dfe7576" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_RedeemableNoncontrollingInterestEquityCarryingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInTemporaryEquityRollForward_090bd385-6868-4ab7-9f4d-3e83a9b5a79c" xlink:to="loc_us-gaap_RedeemableNoncontrollingInterestEquityCarryingAmount_a98ee6ab-c90b-4aa3-a764-5f061dfe7576" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TemporaryEquityNetIncome_3ef67a2b-9ab1-4fa4-a9fb-89eaf46881b6" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_TemporaryEquityNetIncome"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInTemporaryEquityRollForward_090bd385-6868-4ab7-9f4d-3e83a9b5a79c" xlink:to="loc_us-gaap_TemporaryEquityNetIncome_3ef67a2b-9ab1-4fa4-a9fb-89eaf46881b6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_TemporaryEquityComprehensiveIncomeLoss_988166d3-f712-4a03-9a95-f66e9dcff115" xlink:href="iclr-20200630.xsd#iclr_TemporaryEquityComprehensiveIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInTemporaryEquityRollForward_090bd385-6868-4ab7-9f4d-3e83a9b5a79c" xlink:to="loc_iclr_TemporaryEquityComprehensiveIncomeLoss_988166d3-f712-4a03-9a95-f66e9dcff115" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TemporaryEquityAccretionToRedemptionValue_0f278ede-4df4-4149-8e6e-680b6efb247b" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_TemporaryEquityAccretionToRedemptionValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInTemporaryEquityRollForward_090bd385-6868-4ab7-9f4d-3e83a9b5a79c" xlink:to="loc_us-gaap_TemporaryEquityAccretionToRedemptionValue_0f278ede-4df4-4149-8e6e-680b6efb247b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_RedeemableNoncontrollingInterestDecreaseFromRedemptionsOrPurchaseOfInterests_91602b85-b347-4b88-a945-da91aa99f44f" xlink:href="iclr-20200630.xsd#iclr_RedeemableNoncontrollingInterestDecreaseFromRedemptionsOrPurchaseOfInterests"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInTemporaryEquityRollForward_090bd385-6868-4ab7-9f4d-3e83a9b5a79c" xlink:to="loc_iclr_RedeemableNoncontrollingInterestDecreaseFromRedemptionsOrPurchaseOfInterests_91602b85-b347-4b88-a945-da91aa99f44f" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RedeemableNoncontrollingInterestEquityCarryingAmount_cc411ca3-2495-46a9-aace-cf7fc489728b" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_RedeemableNoncontrollingInterestEquityCarryingAmount"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_bceb4e35-879d-490d-b9e6-68aaa97a5d47" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_09531501-975f-4e24-a0be-3c44eaf01392" xlink:to="loc_us-gaap_StatementTable_bceb4e35-879d-490d-b9e6-68aaa97a5d47" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_e870247b-1f94-4398-ae10-6550ee883a2d" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_bceb4e35-879d-490d-b9e6-68aaa97a5d47" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_e870247b-1f94-4398-ae10-6550ee883a2d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_e870247b-1f94-4398-ae10-6550ee883a2d_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_e870247b-1f94-4398-ae10-6550ee883a2d" xlink:to="loc_us-gaap_EquityComponentDomain_e870247b-1f94-4398-ae10-6550ee883a2d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_705d31a8-b01a-43c4-b39b-931eba747ec7" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_e870247b-1f94-4398-ae10-6550ee883a2d" xlink:to="loc_us-gaap_EquityComponentDomain_705d31a8-b01a-43c4-b39b-931eba747ec7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_fea3a527-450d-45b9-98f9-c7e15982a53a" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_705d31a8-b01a-43c4-b39b-931eba747ec7" xlink:to="loc_us-gaap_CommonStockMember_fea3a527-450d-45b9-98f9-c7e15982a53a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_d18bdef6-ac5e-4a1d-b822-088e8ea3841f" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AdditionalPaidInCapitalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_705d31a8-b01a-43c4-b39b-931eba747ec7" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_d18bdef6-ac5e-4a1d-b822-088e8ea3841f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_OtherUndenominatedCapitalMember_4676117c-4e00-4c02-bceb-4829eaa8705a" xlink:href="iclr-20200630.xsd#iclr_OtherUndenominatedCapitalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_705d31a8-b01a-43c4-b39b-931eba747ec7" xlink:to="loc_iclr_OtherUndenominatedCapitalMember_4676117c-4e00-4c02-bceb-4829eaa8705a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_266a7088-1807-473e-bb92-a6a73e8272c0" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_705d31a8-b01a-43c4-b39b-931eba747ec7" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_266a7088-1807-473e-bb92-a6a73e8272c0" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsMember_d8c2c377-0c1b-4026-b932-4d4540cbe7a6" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_RetainedEarningsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_705d31a8-b01a-43c4-b39b-931eba747ec7" xlink:to="loc_us-gaap_RetainedEarningsMember_d8c2c377-0c1b-4026-b932-4d4540cbe7a6" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_RedeemableNoncontrollingInterestMember_72f549aa-3d50-488f-9834-b2bb50f11e32" xlink:href="iclr-20200630.xsd#iclr_RedeemableNoncontrollingInterestMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_705d31a8-b01a-43c4-b39b-931eba747ec7" xlink:to="loc_iclr_RedeemableNoncontrollingInterestMember_72f549aa-3d50-488f-9834-b2bb50f11e32" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeInstrumentRiskAxis_b1f39f39-df80-4f5a-bc70-c450eaf64275" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DerivativeInstrumentRiskAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_bceb4e35-879d-490d-b9e6-68aaa97a5d47" xlink:to="loc_us-gaap_DerivativeInstrumentRiskAxis_b1f39f39-df80-4f5a-bc70-c450eaf64275" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_b1f39f39-df80-4f5a-bc70-c450eaf64275_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_b1f39f39-df80-4f5a-bc70-c450eaf64275" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_b1f39f39-df80-4f5a-bc70-c450eaf64275_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_4eb11f62-37ae-425b-8af5-55df7908dd24" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_b1f39f39-df80-4f5a-bc70-c450eaf64275" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_4eb11f62-37ae-425b-8af5-55df7908dd24" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestRateContractMember_00cc302c-09bf-4f60-83a8-ac5112bb9d7b" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_InterestRateContractMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_4eb11f62-37ae-425b-8af5-55df7908dd24" xlink:to="loc_us-gaap_InterestRateContractMember_00cc302c-09bf-4f60-83a8-ac5112bb9d7b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_CashFlowHedgeMember_3d56d6d6-0c82-45c7-b9d0-a5c1da3f986e" xlink:href="iclr-20200630.xsd#iclr_CashFlowHedgeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_4eb11f62-37ae-425b-8af5-55df7908dd24" xlink:to="loc_iclr_CashFlowHedgeMember_3d56d6d6-0c82-45c7-b9d0-a5c1da3f986e" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/Basisofpresentation" xlink:type="simple" xlink:href="iclr-20200630.xsd#Basisofpresentation"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/Basisofpresentation" xlink:type="extended" id="i2df2b6a3ca264176b0356925df7e7e55_Basisofpresentation"/>
  <link:roleRef roleURI="http://www.iconplc.com/role/Significantaccountingpolicies" xlink:type="simple" xlink:href="iclr-20200630.xsd#Significantaccountingpolicies"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/Significantaccountingpolicies" xlink:type="extended" id="id691ef2b7f8c49d1afe1d55e43683c73_Significantaccountingpolicies"/>
  <link:roleRef roleURI="http://www.iconplc.com/role/SignificantaccountingpoliciesPolicies" xlink:type="simple" xlink:href="iclr-20200630.xsd#SignificantaccountingpoliciesPolicies"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/SignificantaccountingpoliciesPolicies" xlink:type="extended" id="i08bc7760fd8047f7a125031c8e68c2a5_SignificantaccountingpoliciesPolicies"/>
  <link:roleRef roleURI="http://www.iconplc.com/role/SignificantaccountingpoliciesDetails" xlink:type="simple" xlink:href="iclr-20200630.xsd#SignificantaccountingpoliciesDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/SignificantaccountingpoliciesDetails" xlink:type="extended" id="i75f452babf2245b4accc7a59efd4e596_SignificantaccountingpoliciesDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_54b7b7bb-0f5e-47c0-8f57-6913623f6fc9" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationStepAcquisitionEquityInterestInAcquireeIncludingSubsequentAcquisitionPercentage_0b6a8ae5-cbe7-4b4c-893b-72ca6e664dec" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_BusinessCombinationStepAcquisitionEquityInterestInAcquireeIncludingSubsequentAcquisitionPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_54b7b7bb-0f5e-47c0-8f57-6913623f6fc9" xlink:to="loc_us-gaap_BusinessCombinationStepAcquisitionEquityInterestInAcquireeIncludingSubsequentAcquisitionPercentage_0b6a8ae5-cbe7-4b4c-893b-72ca6e664dec" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountingStandardsUpdateExtensibleList_ce7a756e-7797-423a-a1f9-871097bfb01a" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AccountingStandardsUpdateExtensibleList"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_54b7b7bb-0f5e-47c0-8f57-6913623f6fc9" xlink:to="loc_us-gaap_AccountingStandardsUpdateExtensibleList_ce7a756e-7797-423a-a1f9-871097bfb01a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseRightOfUseAsset_7480d4c8-e704-487d-ac9b-1efea6c8b686" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_OperatingLeaseRightOfUseAsset"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_54b7b7bb-0f5e-47c0-8f57-6913623f6fc9" xlink:to="loc_us-gaap_OperatingLeaseRightOfUseAsset_7480d4c8-e704-487d-ac9b-1efea6c8b686" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseLiability_d6eac812-783e-435c-b2e6-cb26859dedef" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_OperatingLeaseLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_54b7b7bb-0f5e-47c0-8f57-6913623f6fc9" xlink:to="loc_us-gaap_OperatingLeaseLiability_d6eac812-783e-435c-b2e6-cb26859dedef" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_b00eed36-5e54-495c-afc2-32d01d6f2062" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_54b7b7bb-0f5e-47c0-8f57-6913623f6fc9" xlink:to="loc_us-gaap_StockholdersEquity_b00eed36-5e54-495c-afc2-32d01d6f2062" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ChangeInAccountingPrincipleAccountingStandardsUpdateTransitionOptionElectedExtensibleList_9b0ad1e2-f00d-4190-845e-bd00aa841e9c" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ChangeInAccountingPrincipleAccountingStandardsUpdateTransitionOptionElectedExtensibleList"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_54b7b7bb-0f5e-47c0-8f57-6913623f6fc9" xlink:to="loc_us-gaap_ChangeInAccountingPrincipleAccountingStandardsUpdateTransitionOptionElectedExtensibleList_9b0ad1e2-f00d-4190-845e-bd00aa841e9c" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsForPreviousAcquisition_49a91519-0942-4171-b41d-a9d38e6813bf" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_PaymentsForPreviousAcquisition"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_54b7b7bb-0f5e-47c0-8f57-6913623f6fc9" xlink:to="loc_us-gaap_PaymentsForPreviousAcquisition_49a91519-0942-4171-b41d-a9d38e6813bf" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable_8f7f30b5-2b34-47da-b90e-81a70d12f15d" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_54b7b7bb-0f5e-47c0-8f57-6913623f6fc9" xlink:to="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable_8f7f30b5-2b34-47da-b90e-81a70d12f15d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_7bbe94b1-8819-4e98-a785-837fad7c98fc" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable_8f7f30b5-2b34-47da-b90e-81a70d12f15d" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_7bbe94b1-8819-4e98-a785-837fad7c98fc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_7bbe94b1-8819-4e98-a785-837fad7c98fc_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_7bbe94b1-8819-4e98-a785-837fad7c98fc" xlink:to="loc_us-gaap_EquityComponentDomain_7bbe94b1-8819-4e98-a785-837fad7c98fc_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_32775830-a4d7-4932-9bd5-4e972e4711fd" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_7bbe94b1-8819-4e98-a785-837fad7c98fc" xlink:to="loc_us-gaap_EquityComponentDomain_32775830-a4d7-4932-9bd5-4e972e4711fd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsMember_8f04e111-9f46-4b39-b6aa-cfce618002f7" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_RetainedEarningsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_32775830-a4d7-4932-9bd5-4e972e4711fd" xlink:to="loc_us-gaap_RetainedEarningsMember_8f04e111-9f46-4b39-b6aa-cfce618002f7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsForNewAccountingPronouncementsAxis_58f2a798-28c3-4f24-bbc5-4c1b2525cdeb" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AdjustmentsForNewAccountingPronouncementsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable_8f7f30b5-2b34-47da-b90e-81a70d12f15d" xlink:to="loc_us-gaap_AdjustmentsForNewAccountingPronouncementsAxis_58f2a798-28c3-4f24-bbc5-4c1b2525cdeb" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfAdoptionMember_58f2a798-28c3-4f24-bbc5-4c1b2525cdeb_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_TypeOfAdoptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AdjustmentsForNewAccountingPronouncementsAxis_58f2a798-28c3-4f24-bbc5-4c1b2525cdeb" xlink:to="loc_us-gaap_TypeOfAdoptionMember_58f2a798-28c3-4f24-bbc5-4c1b2525cdeb_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfAdoptionMember_cc9b8b67-3ea0-4b13-b9fb-d02629955162" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_TypeOfAdoptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AdjustmentsForNewAccountingPronouncementsAxis_58f2a798-28c3-4f24-bbc5-4c1b2525cdeb" xlink:to="loc_us-gaap_TypeOfAdoptionMember_cc9b8b67-3ea0-4b13-b9fb-d02629955162" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountingStandardsUpdate201602Member_0dc64401-ab1c-459b-aa9e-f19d813b19b2" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AccountingStandardsUpdate201602Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfAdoptionMember_cc9b8b67-3ea0-4b13-b9fb-d02629955162" xlink:to="loc_us-gaap_AccountingStandardsUpdate201602Member_0dc64401-ab1c-459b-aa9e-f19d813b19b2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_8b1b0b59-3548-4083-ab25-e4548b7af15b" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable_8f7f30b5-2b34-47da-b90e-81a70d12f15d" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_8b1b0b59-3548-4083-ab25-e4548b7af15b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_8b1b0b59-3548-4083-ab25-e4548b7af15b_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_8b1b0b59-3548-4083-ab25-e4548b7af15b" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_8b1b0b59-3548-4083-ab25-e4548b7af15b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_c31fea82-25be-4e32-a4e0-77298ae5cd3b" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_8b1b0b59-3548-4083-ab25-e4548b7af15b" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_c31fea82-25be-4e32-a4e0-77298ae5cd3b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_MeDiNovaResearchMember_f6f9f939-8278-4dd8-8974-30bf822c5587" xlink:href="iclr-20200630.xsd#iclr_MeDiNovaResearchMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_c31fea82-25be-4e32-a4e0-77298ae5cd3b" xlink:to="loc_iclr_MeDiNovaResearchMember_f6f9f939-8278-4dd8-8974-30bf822c5587" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeAxis_b2c69f56-8380-4555-ad8a-7d3538b79480" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_SubsequentEventTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable_8f7f30b5-2b34-47da-b90e-81a70d12f15d" xlink:to="loc_us-gaap_SubsequentEventTypeAxis_b2c69f56-8380-4555-ad8a-7d3538b79480" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_b2c69f56-8380-4555-ad8a-7d3538b79480_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_b2c69f56-8380-4555-ad8a-7d3538b79480" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_b2c69f56-8380-4555-ad8a-7d3538b79480_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_816b4e25-bbba-463a-8598-70aac2772df0" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_b2c69f56-8380-4555-ad8a-7d3538b79480" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_816b4e25-bbba-463a-8598-70aac2772df0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventMember_4038790b-a527-43a0-ac58-8c3301f06eaa" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_SubsequentEventMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventTypeDomain_816b4e25-bbba-463a-8598-70aac2772df0" xlink:to="loc_us-gaap_SubsequentEventMember_4038790b-a527-43a0-ac58-8c3301f06eaa" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/Revenue" xlink:type="simple" xlink:href="iclr-20200630.xsd#Revenue"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/Revenue" xlink:type="extended" id="i60ed0afaa925498c8fabd0a33b2cf88e_Revenue"/>
  <link:roleRef roleURI="http://www.iconplc.com/role/RevenueTables" xlink:type="simple" xlink:href="iclr-20200630.xsd#RevenueTables"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/RevenueTables" xlink:type="extended" id="i44e0f6ee15954ad89d8a219f02043e9f_RevenueTables"/>
  <link:roleRef roleURI="http://www.iconplc.com/role/RevenueDetails" xlink:type="simple" xlink:href="iclr-20200630.xsd#RevenueDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/RevenueDetails" xlink:type="extended" id="i652bc8c9aed44dec94b0b76897e5da76_RevenueDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueLineItems_475adc99-c71e-4f1c-8196-877de1dc6c1c" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DisaggregationOfRevenueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_fbb6ad9e-0086-4c17-9562-6b864803aae5" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_475adc99-c71e-4f1c-8196-877de1dc6c1c" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_fbb6ad9e-0086-4c17-9562-6b864803aae5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueTable_bd6d9d4e-2540-41bf-b1ae-e293887352b9" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DisaggregationOfRevenueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_475adc99-c71e-4f1c-8196-877de1dc6c1c" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_bd6d9d4e-2540-41bf-b1ae-e293887352b9" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MajorCustomersAxis_4a5088cb-b31d-4324-8f7c-107dc6b45fa7" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_MajorCustomersAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_bd6d9d4e-2540-41bf-b1ae-e293887352b9" xlink:to="loc_srt_MajorCustomersAxis_4a5088cb-b31d-4324-8f7c-107dc6b45fa7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_NameOfMajorCustomerDomain_4a5088cb-b31d-4324-8f7c-107dc6b45fa7_default" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_NameOfMajorCustomerDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_MajorCustomersAxis_4a5088cb-b31d-4324-8f7c-107dc6b45fa7" xlink:to="loc_srt_NameOfMajorCustomerDomain_4a5088cb-b31d-4324-8f7c-107dc6b45fa7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_NameOfMajorCustomerDomain_dc01dacd-7d65-488d-adc9-4b67a75a50ee" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_NameOfMajorCustomerDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_MajorCustomersAxis_4a5088cb-b31d-4324-8f7c-107dc6b45fa7" xlink:to="loc_srt_NameOfMajorCustomerDomain_dc01dacd-7d65-488d-adc9-4b67a75a50ee" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_TopClientMember_abbc7fbb-762f-4eec-b653-efcce45279ce" xlink:href="iclr-20200630.xsd#iclr_TopClientMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_dc01dacd-7d65-488d-adc9-4b67a75a50ee" xlink:to="loc_iclr_TopClientMember_abbc7fbb-762f-4eec-b653-efcce45279ce" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_Clients25Member_dc8bf784-826d-45bb-96f0-d73edb8e4257" xlink:href="iclr-20200630.xsd#iclr_Clients25Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_dc01dacd-7d65-488d-adc9-4b67a75a50ee" xlink:to="loc_iclr_Clients25Member_dc8bf784-826d-45bb-96f0-d73edb8e4257" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_Clients610Member_0908fc33-6579-44b4-8019-917a938df1d5" xlink:href="iclr-20200630.xsd#iclr_Clients610Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_dc01dacd-7d65-488d-adc9-4b67a75a50ee" xlink:to="loc_iclr_Clients610Member_0908fc33-6579-44b4-8019-917a938df1d5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_Clients1125Member_75d7bd16-2e24-4301-84c5-2bdade6389c5" xlink:href="iclr-20200630.xsd#iclr_Clients1125Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_dc01dacd-7d65-488d-adc9-4b67a75a50ee" xlink:to="loc_iclr_Clients1125Member_75d7bd16-2e24-4301-84c5-2bdade6389c5" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_OtherCustomersMember_6dca7db4-39ec-4ad4-8643-fa80f35cc2d9" xlink:href="iclr-20200630.xsd#iclr_OtherCustomersMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_dc01dacd-7d65-488d-adc9-4b67a75a50ee" xlink:to="loc_iclr_OtherCustomersMember_6dca7db4-39ec-4ad4-8643-fa80f35cc2d9" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/Accountsreceivableunbilledrevenuecontractassetsandunearnedrevenueorpaymentsonaccountcontractliabilities" xlink:type="simple" xlink:href="iclr-20200630.xsd#Accountsreceivableunbilledrevenuecontractassetsandunearnedrevenueorpaymentsonaccountcontractliabilities"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/Accountsreceivableunbilledrevenuecontractassetsandunearnedrevenueorpaymentsonaccountcontractliabilities" xlink:type="extended" id="ib7d72e0e1c804920ad2d473ed2082238_Accountsreceivableunbilledrevenuecontractassetsandunearnedrevenueorpaymentsonaccountcontractliabilities"/>
  <link:roleRef roleURI="http://www.iconplc.com/role/AccountsreceivableunbilledrevenuecontractassetsandunearnedrevenueorpaymentsonaccountcontractliabilitiesTables" xlink:type="simple" xlink:href="iclr-20200630.xsd#AccountsreceivableunbilledrevenuecontractassetsandunearnedrevenueorpaymentsonaccountcontractliabilitiesTables"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/AccountsreceivableunbilledrevenuecontractassetsandunearnedrevenueorpaymentsonaccountcontractliabilitiesTables" xlink:type="extended" id="i4196ab1cd05b45c4b1f0055226d07290_AccountsreceivableunbilledrevenuecontractassetsandunearnedrevenueorpaymentsonaccountcontractliabilitiesTables"/>
  <link:roleRef roleURI="http://www.iconplc.com/role/AccountsreceivableunbilledrevenuecontractassetsandunearnedrevenueorpaymentsonaccountcontractliabilitiesContractAssetsandLiabilitiesDetails" xlink:type="simple" xlink:href="iclr-20200630.xsd#AccountsreceivableunbilledrevenuecontractassetsandunearnedrevenueorpaymentsonaccountcontractliabilitiesContractAssetsandLiabilitiesDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/AccountsreceivableunbilledrevenuecontractassetsandunearnedrevenueorpaymentsonaccountcontractliabilitiesContractAssetsandLiabilitiesDetails" xlink:type="extended" id="ia2b0aa45fb7d42419dc750f22157c74e_AccountsreceivableunbilledrevenuecontractassetsandunearnedrevenueorpaymentsonaccountcontractliabilitiesContractAssetsandLiabilitiesDetails"/>
  <link:roleRef roleURI="http://www.iconplc.com/role/AccountsreceivableunbilledrevenuecontractassetsandunearnedrevenueorpaymentsonaccountcontractliabilitiesRevenueRemainingPerformanceObligationsDetails" xlink:type="simple" xlink:href="iclr-20200630.xsd#AccountsreceivableunbilledrevenuecontractassetsandunearnedrevenueorpaymentsonaccountcontractliabilitiesRevenueRemainingPerformanceObligationsDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/AccountsreceivableunbilledrevenuecontractassetsandunearnedrevenueorpaymentsonaccountcontractliabilitiesRevenueRemainingPerformanceObligationsDetails" xlink:type="extended" id="i2ac58d02a41547159eb450afc6abfbb5_AccountsreceivableunbilledrevenuecontractassetsandunearnedrevenueorpaymentsonaccountcontractliabilitiesRevenueRemainingPerformanceObligationsDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_29bd61d7-7e51-4526-871a-44fdb36be2a1" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationPercentage_a8f174ab-fb9c-4545-ae72-4a22fa51503e" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_RevenueRemainingPerformanceObligationPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_29bd61d7-7e51-4526-871a-44fdb36be2a1" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationPercentage_a8f174ab-fb9c-4545-ae72-4a22fa51503e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_c36b7569-14cd-40af-9274-f15cf1fb5d42" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_29bd61d7-7e51-4526-871a-44fdb36be2a1" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_c36b7569-14cd-40af-9274-f15cf1fb5d42" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_de622757-1268-403a-b88f-1b87c342b24a" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_29bd61d7-7e51-4526-871a-44fdb36be2a1" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_de622757-1268-403a-b88f-1b87c342b24a" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionStartDateAxis_196ce98e-40e3-49ae-885e-28aca0faf0f2" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionStartDateAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_de622757-1268-403a-b88f-1b87c342b24a" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionStartDateAxis_196ce98e-40e3-49ae-885e-28aca0faf0f2" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/AccountsreceivableunbilledrevenuecontractassetsandunearnedrevenueorpaymentsonaccountcontractliabilitiesRevenueRemainingPerformanceObligationsDetails_1" xlink:type="simple" xlink:href="iclr-20200630.xsd#AccountsreceivableunbilledrevenuecontractassetsandunearnedrevenueorpaymentsonaccountcontractliabilitiesRevenueRemainingPerformanceObligationsDetails_1"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/AccountsreceivableunbilledrevenuecontractassetsandunearnedrevenueorpaymentsonaccountcontractliabilitiesRevenueRemainingPerformanceObligationsDetails_1" xlink:type="extended" id="i1c52a590749543e5a6098dba59bc571f_AccountsreceivableunbilledrevenuecontractassetsandunearnedrevenueorpaymentsonaccountcontractliabilitiesRevenueRemainingPerformanceObligationsDetails_1">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_29bd61d7-7e51-4526-871a-44fdb36be2a1" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationPercentage_a8f174ab-fb9c-4545-ae72-4a22fa51503e" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_RevenueRemainingPerformanceObligationPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_29bd61d7-7e51-4526-871a-44fdb36be2a1" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationPercentage_a8f174ab-fb9c-4545-ae72-4a22fa51503e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_c36b7569-14cd-40af-9274-f15cf1fb5d42" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_29bd61d7-7e51-4526-871a-44fdb36be2a1" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_c36b7569-14cd-40af-9274-f15cf1fb5d42" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_de622757-1268-403a-b88f-1b87c342b24a" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_29bd61d7-7e51-4526-871a-44fdb36be2a1" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_de622757-1268-403a-b88f-1b87c342b24a" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/Goodwill" xlink:type="simple" xlink:href="iclr-20200630.xsd#Goodwill"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/Goodwill" xlink:type="extended" id="i2175c7fa47a8405d9e9613827044fc7d_Goodwill"/>
  <link:roleRef roleURI="http://www.iconplc.com/role/GoodwillTables" xlink:type="simple" xlink:href="iclr-20200630.xsd#GoodwillTables"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/GoodwillTables" xlink:type="extended" id="i3056aa779aa041d8b0cac7c4da8ca08c_GoodwillTables"/>
  <link:roleRef roleURI="http://www.iconplc.com/role/GoodwillDetails" xlink:type="simple" xlink:href="iclr-20200630.xsd#GoodwillDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/GoodwillDetails" xlink:type="extended" id="i627e9bc19ade423c865720ae2fdb4770_GoodwillDetails"/>
  <link:roleRef roleURI="http://www.iconplc.com/role/Businesscombinations" xlink:type="simple" xlink:href="iclr-20200630.xsd#Businesscombinations"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/Businesscombinations" xlink:type="extended" id="i79993efedce6412aa4debcaf819d2740_Businesscombinations"/>
  <link:roleRef roleURI="http://www.iconplc.com/role/BusinesscombinationsTables" xlink:type="simple" xlink:href="iclr-20200630.xsd#BusinesscombinationsTables"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/BusinesscombinationsTables" xlink:type="extended" id="ibe1389b856e64968ba23e5c4e1b52122_BusinesscombinationsTables"/>
  <link:roleRef roleURI="http://www.iconplc.com/role/BusinesscombinationsDetails" xlink:type="simple" xlink:href="iclr-20200630.xsd#BusinesscombinationsDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/BusinesscombinationsDetails" xlink:type="extended" id="iba8bbf326d59447e92b781c8599ecc2d_BusinesscombinationsDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_04a5c3aa-81ee-4114-8c1d-7f30e1cb470d" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_BusinessAcquisitionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionPercentageOfVotingInterestsAcquired_b0695df3-5a7f-4739-bd87-39223cfc27bc" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_BusinessAcquisitionPercentageOfVotingInterestsAcquired"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_04a5c3aa-81ee-4114-8c1d-7f30e1cb470d" xlink:to="loc_us-gaap_BusinessAcquisitionPercentageOfVotingInterestsAcquired_b0695df3-5a7f-4739-bd87-39223cfc27bc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationStepAcquisitionEquityInterestInAcquireeIncludingSubsequentAcquisitionPercentage_d146ee84-d699-44d5-b5f0-f9f867ab19b6" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_BusinessCombinationStepAcquisitionEquityInterestInAcquireeIncludingSubsequentAcquisitionPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_04a5c3aa-81ee-4114-8c1d-7f30e1cb470d" xlink:to="loc_us-gaap_BusinessCombinationStepAcquisitionEquityInterestInAcquireeIncludingSubsequentAcquisitionPercentage_d146ee84-d699-44d5-b5f0-f9f867ab19b6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents_b2e44a63-9c22-43f5-a17d-b9eb251061ba" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_04a5c3aa-81ee-4114-8c1d-7f30e1cb470d" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents_b2e44a63-9c22-43f5-a17d-b9eb251061ba" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment_16a9c1c5-90ee-43fc-8bb6-0b0257b8d5d7" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_04a5c3aa-81ee-4114-8c1d-7f30e1cb470d" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment_16a9c1c5-90ee-43fc-8bb6-0b0257b8d5d7" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOperatingLeaseRightOfUseAsset_7b697bc1-a27c-4a90-b99c-db7ee4ab17fe" xlink:href="iclr-20200630.xsd#iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOperatingLeaseRightOfUseAsset"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_04a5c3aa-81ee-4114-8c1d-7f30e1cb470d" xlink:to="loc_iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOperatingLeaseRightOfUseAsset_7b697bc1-a27c-4a90-b99c-db7ee4ab17fe" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_c0d433df-dd8d-447d-ab27-daf42225ea5f" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_Goodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_04a5c3aa-81ee-4114-8c1d-7f30e1cb470d" xlink:to="loc_us-gaap_Goodwill_c0d433df-dd8d-447d-ab27-daf42225ea5f" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles_9c570d28-040f-41a2-9290-4f141fae4e40" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_04a5c3aa-81ee-4114-8c1d-7f30e1cb470d" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles_9c570d28-040f-41a2-9290-4f141fae4e40" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsReceivables_bd477ff1-06b0-410d-91fb-3408b0dc2465" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsReceivables"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_04a5c3aa-81ee-4114-8c1d-7f30e1cb470d" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsReceivables_bd477ff1-06b0-410d-91fb-3408b0dc2465" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedUnbilledRevenue_8d1fc32d-1637-4dce-9aa3-bc1a68ffcc49" xlink:href="iclr-20200630.xsd#iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedUnbilledRevenue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_04a5c3aa-81ee-4114-8c1d-7f30e1cb470d" xlink:to="loc_iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedUnbilledRevenue_8d1fc32d-1637-4dce-9aa3-bc1a68ffcc49" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOtherReceivables_09141c8e-31a3-4a98-9906-f5c6b62809b1" xlink:href="iclr-20200630.xsd#iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOtherReceivables"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_04a5c3aa-81ee-4114-8c1d-7f30e1cb470d" xlink:to="loc_iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOtherReceivables_09141c8e-31a3-4a98-9906-f5c6b62809b1" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsPrepaidExpenseAndOtherAssets_8fe5640a-be21-4757-876a-37abc6f37e9e" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsPrepaidExpenseAndOtherAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_04a5c3aa-81ee-4114-8c1d-7f30e1cb470d" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsPrepaidExpenseAndOtherAssets_8fe5640a-be21-4757-876a-37abc6f37e9e" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxAssets_e432674d-8ea2-4bf9-9383-dbd32171524a" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_04a5c3aa-81ee-4114-8c1d-7f30e1cb470d" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxAssets_e432674d-8ea2-4bf9-9383-dbd32171524a" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesAccountsPayable_d9e5a5c2-f612-4696-8d2e-c49740c126c6" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesAccountsPayable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_04a5c3aa-81ee-4114-8c1d-7f30e1cb470d" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesAccountsPayable_d9e5a5c2-f612-4696-8d2e-c49740c126c6" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedContractWithCustomerLiabilityCurrent_6b46ef1d-c1a3-46d7-912b-a13152620f46" xlink:href="iclr-20200630.xsd#iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedContractWithCustomerLiabilityCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_04a5c3aa-81ee-4114-8c1d-7f30e1cb470d" xlink:to="loc_iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedContractWithCustomerLiabilityCurrent_6b46ef1d-c1a3-46d7-912b-a13152620f46" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesOther_6af75bb4-e0dd-4ec0-b225-4286f2fa0b4f" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesOther"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_04a5c3aa-81ee-4114-8c1d-7f30e1cb470d" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesOther_6af75bb4-e0dd-4ec0-b225-4286f2fa0b4f" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedCurrentOperatingLeaseLiability_85c63802-2bbf-4966-bec9-a570a41d83f2" xlink:href="iclr-20200630.xsd#iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedCurrentOperatingLeaseLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_04a5c3aa-81ee-4114-8c1d-7f30e1cb470d" xlink:to="loc_iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedCurrentOperatingLeaseLiability_85c63802-2bbf-4966-bec9-a570a41d83f2" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedNoncurrentOperatingLeaseLiability_4669114d-d681-4aa9-8f65-4327f75df4cd" xlink:href="iclr-20200630.xsd#iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedNoncurrentOperatingLeaseLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_04a5c3aa-81ee-4114-8c1d-7f30e1cb470d" xlink:to="loc_iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedNoncurrentOperatingLeaseLiability_4669114d-d681-4aa9-8f65-4327f75df4cd" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilitiesOther_3e639174-501d-42cb-a9d5-5bb11be89295" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilitiesOther"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_04a5c3aa-81ee-4114-8c1d-7f30e1cb470d" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilitiesOther_3e639174-501d-42cb-a9d5-5bb11be89295" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxLiabilities_ee406ba1-ae74-4efe-9b1b-5d7b1c2a987c" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_04a5c3aa-81ee-4114-8c1d-7f30e1cb470d" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxLiabilities_ee406ba1-ae74-4efe-9b1b-5d7b1c2a987c" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet_9759d27c-b6ae-4727-8e63-26885b82d6dd" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_04a5c3aa-81ee-4114-8c1d-7f30e1cb470d" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet_9759d27c-b6ae-4727-8e63-26885b82d6dd" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquireBusinessesGross_3f153ba6-aa99-4e1a-8f1d-969393a19a2f" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_PaymentsToAcquireBusinessesGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_04a5c3aa-81ee-4114-8c1d-7f30e1cb470d" xlink:to="loc_us-gaap_PaymentsToAcquireBusinessesGross_3f153ba6-aa99-4e1a-8f1d-969393a19a2f" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BusinessCombinationConsiderationTransferredWorkingCapitalAdjustments_5607dd56-553f-4a50-b024-9b078b8a7de8" xlink:href="iclr-20200630.xsd#iclr_BusinessCombinationConsiderationTransferredWorkingCapitalAdjustments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_04a5c3aa-81ee-4114-8c1d-7f30e1cb470d" xlink:to="loc_iclr_BusinessCombinationConsiderationTransferredWorkingCapitalAdjustments_5607dd56-553f-4a50-b024-9b078b8a7de8" xlink:type="arc" order="21"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BusinessCombinationAcquisitionOfLessThan100PercentRedeemableNoncontrollingInterestFairValue_a636ff8a-4046-48e9-82f3-e8e563169a19" xlink:href="iclr-20200630.xsd#iclr_BusinessCombinationAcquisitionOfLessThan100PercentRedeemableNoncontrollingInterestFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_04a5c3aa-81ee-4114-8c1d-7f30e1cb470d" xlink:to="loc_iclr_BusinessCombinationAcquisitionOfLessThan100PercentRedeemableNoncontrollingInterestFairValue_a636ff8a-4046-48e9-82f3-e8e563169a19" xlink:type="arc" order="22"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationContingentConsiderationLiability_a4c7e247-7cd2-4a2b-b852-81fd82e66999" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_BusinessCombinationContingentConsiderationLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_04a5c3aa-81ee-4114-8c1d-7f30e1cb470d" xlink:to="loc_us-gaap_BusinessCombinationContingentConsiderationLiability_a4c7e247-7cd2-4a2b-b852-81fd82e66999" xlink:type="arc" order="23"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationConsiderationTransferred1_7d49ecce-99ce-49a1-a63c-e21928c3c8c1" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_BusinessCombinationConsiderationTransferred1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_04a5c3aa-81ee-4114-8c1d-7f30e1cb470d" xlink:to="loc_us-gaap_BusinessCombinationConsiderationTransferred1_7d49ecce-99ce-49a1-a63c-e21928c3c8c1" xlink:type="arc" order="24"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionPurchasePriceAllocationGoodwillExpectedTaxDeductibleAmount_66f155ab-332e-45b4-a44a-63362baf71a8" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_BusinessAcquisitionPurchasePriceAllocationGoodwillExpectedTaxDeductibleAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_04a5c3aa-81ee-4114-8c1d-7f30e1cb470d" xlink:to="loc_us-gaap_BusinessAcquisitionPurchasePriceAllocationGoodwillExpectedTaxDeductibleAmount_66f155ab-332e-45b4-a44a-63362baf71a8" xlink:type="arc" order="25"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationContingentConsiderationArrangementsRangeOfOutcomesValueHigh_e1748d64-3191-404b-a3f8-1159c9436aac" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_BusinessCombinationContingentConsiderationArrangementsRangeOfOutcomesValueHigh"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_04a5c3aa-81ee-4114-8c1d-7f30e1cb470d" xlink:to="loc_us-gaap_BusinessCombinationContingentConsiderationArrangementsRangeOfOutcomesValueHigh_e1748d64-3191-404b-a3f8-1159c9436aac" xlink:type="arc" order="26"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BusinessCombinationFairValueIncreaseDecreaseOtherLiabilities_e88d6602-1b5e-458a-9030-7083fa26b29e" xlink:href="iclr-20200630.xsd#iclr_BusinessCombinationFairValueIncreaseDecreaseOtherLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_04a5c3aa-81ee-4114-8c1d-7f30e1cb470d" xlink:to="loc_iclr_BusinessCombinationFairValueIncreaseDecreaseOtherLiabilities_e88d6602-1b5e-458a-9030-7083fa26b29e" xlink:type="arc" order="27"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentOperatingLeaseRightOfUseAsset_090e4106-b4f9-48dc-9f88-fc0721b848b0" xlink:href="iclr-20200630.xsd#iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentOperatingLeaseRightOfUseAsset"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_04a5c3aa-81ee-4114-8c1d-7f30e1cb470d" xlink:to="loc_iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentOperatingLeaseRightOfUseAsset_090e4106-b4f9-48dc-9f88-fc0721b848b0" xlink:type="arc" order="28"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentOperatingLeaseLiabilityCurrent_2ffd0efe-0857-426b-a2f4-617fb667a536" xlink:href="iclr-20200630.xsd#iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentOperatingLeaseLiabilityCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_04a5c3aa-81ee-4114-8c1d-7f30e1cb470d" xlink:to="loc_iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentOperatingLeaseLiabilityCurrent_2ffd0efe-0857-426b-a2f4-617fb667a536" xlink:type="arc" order="29"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BusinessCombinationsFairValueIncreaseDecreaseNonCurrentLeaseLiabilities_23bc4680-6657-4ea7-a7ff-dcee6ed7a333" xlink:href="iclr-20200630.xsd#iclr_BusinessCombinationsFairValueIncreaseDecreaseNonCurrentLeaseLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_04a5c3aa-81ee-4114-8c1d-7f30e1cb470d" xlink:to="loc_iclr_BusinessCombinationsFairValueIncreaseDecreaseNonCurrentLeaseLiabilities_23bc4680-6657-4ea7-a7ff-dcee6ed7a333" xlink:type="arc" order="30"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentDeferredTaxLiabilities_db3f2e67-3380-4346-81f8-eedbeccc3f55" xlink:href="iclr-20200630.xsd#iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentDeferredTaxLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_04a5c3aa-81ee-4114-8c1d-7f30e1cb470d" xlink:to="loc_iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentDeferredTaxLiabilities_db3f2e67-3380-4346-81f8-eedbeccc3f55" xlink:type="arc" order="31"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsForPreviousAcquisition_219ece2c-cd2b-4e87-82d0-1f2e47af518e" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_PaymentsForPreviousAcquisition"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_04a5c3aa-81ee-4114-8c1d-7f30e1cb470d" xlink:to="loc_us-gaap_PaymentsForPreviousAcquisition_219ece2c-cd2b-4e87-82d0-1f2e47af518e" xlink:type="arc" order="32"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_3433ca9c-ac9d-4ecd-85b7-f8112b89c216" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_04a5c3aa-81ee-4114-8c1d-7f30e1cb470d" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_3433ca9c-ac9d-4ecd-85b7-f8112b89c216" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_c4112dad-037d-463d-ba62-f0b634bd1c02" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_3433ca9c-ac9d-4ecd-85b7-f8112b89c216" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_c4112dad-037d-463d-ba62-f0b634bd1c02" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_c4112dad-037d-463d-ba62-f0b634bd1c02_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_c4112dad-037d-463d-ba62-f0b634bd1c02" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_c4112dad-037d-463d-ba62-f0b634bd1c02_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_7f67a4aa-1673-41e4-b847-ad721c1d4436" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_c4112dad-037d-463d-ba62-f0b634bd1c02" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_7f67a4aa-1673-41e4-b847-ad721c1d4436" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerRelationshipsMember_695f4965-2b07-4458-a598-7fd66edff142" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CustomerRelationshipsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_7f67a4aa-1673-41e4-b847-ad721c1d4436" xlink:to="loc_us-gaap_CustomerRelationshipsMember_695f4965-2b07-4458-a598-7fd66edff142" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OrderOrProductionBacklogMember_1e820ec8-09b2-4a8a-8767-1d1847b49b3c" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_OrderOrProductionBacklogMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_7f67a4aa-1673-41e4-b847-ad721c1d4436" xlink:to="loc_us-gaap_OrderOrProductionBacklogMember_1e820ec8-09b2-4a8a-8767-1d1847b49b3c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DatabasesMember_a90b847d-d084-43c8-a506-c39dd810c417" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DatabasesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_7f67a4aa-1673-41e4-b847-ad721c1d4436" xlink:to="loc_us-gaap_DatabasesMember_a90b847d-d084-43c8-a506-c39dd810c417" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_bb0afb64-d90f-428e-8a32-9ea78097c338" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_3433ca9c-ac9d-4ecd-85b7-f8112b89c216" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_bb0afb64-d90f-428e-8a32-9ea78097c338" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_bb0afb64-d90f-428e-8a32-9ea78097c338_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_bb0afb64-d90f-428e-8a32-9ea78097c338" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_bb0afb64-d90f-428e-8a32-9ea78097c338_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_975a7dc7-eb01-4505-8c5c-8a67ee9b8b3d" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_bb0afb64-d90f-428e-8a32-9ea78097c338" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_975a7dc7-eb01-4505-8c5c-8a67ee9b8b3d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_MedpassGroupLimitedMember_93d7b789-20b8-4f1f-a7a5-6ff8f366eb87" xlink:href="iclr-20200630.xsd#iclr_MedpassGroupLimitedMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_975a7dc7-eb01-4505-8c5c-8a67ee9b8b3d" xlink:to="loc_iclr_MedpassGroupLimitedMember_93d7b789-20b8-4f1f-a7a5-6ff8f366eb87" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_CRNHoldingsLLCSymphonyMember_c9a31b12-c17f-4a47-be41-42d6b3409281" xlink:href="iclr-20200630.xsd#iclr_CRNHoldingsLLCSymphonyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_975a7dc7-eb01-4505-8c5c-8a67ee9b8b3d" xlink:to="loc_iclr_CRNHoldingsLLCSymphonyMember_c9a31b12-c17f-4a47-be41-42d6b3409281" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_MeDiNovaResearchMember_1f30e762-5bc6-467f-8cff-16556222d154" xlink:href="iclr-20200630.xsd#iclr_MeDiNovaResearchMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_975a7dc7-eb01-4505-8c5c-8a67ee9b8b3d" xlink:to="loc_iclr_MeDiNovaResearchMember_1f30e762-5bc6-467f-8cff-16556222d154" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_MolecularMDCorpMember_60771e39-00ad-46b6-9c92-d9a25653cec8" xlink:href="iclr-20200630.xsd#iclr_MolecularMDCorpMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_975a7dc7-eb01-4505-8c5c-8a67ee9b8b3d" xlink:to="loc_iclr_MolecularMDCorpMember_60771e39-00ad-46b6-9c92-d9a25653cec8" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeAxis_6e49c901-7116-4495-aa52-c1366f48b76b" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_SubsequentEventTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_3433ca9c-ac9d-4ecd-85b7-f8112b89c216" xlink:to="loc_us-gaap_SubsequentEventTypeAxis_6e49c901-7116-4495-aa52-c1366f48b76b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_6e49c901-7116-4495-aa52-c1366f48b76b_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_6e49c901-7116-4495-aa52-c1366f48b76b" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_6e49c901-7116-4495-aa52-c1366f48b76b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_b65e33e1-ccbb-46f6-81a7-4676481ed646" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_6e49c901-7116-4495-aa52-c1366f48b76b" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_b65e33e1-ccbb-46f6-81a7-4676481ed646" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventMember_32ae494c-863b-4b89-aaee-ba32b05583c8" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_SubsequentEventMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventTypeDomain_b65e33e1-ccbb-46f6-81a7-4676481ed646" xlink:to="loc_us-gaap_SubsequentEventMember_32ae494c-863b-4b89-aaee-ba32b05583c8" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/Restructuring" xlink:type="simple" xlink:href="iclr-20200630.xsd#Restructuring"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/Restructuring" xlink:type="extended" id="i4ddde75be98746599e22ab927eb0e0a7_Restructuring"/>
  <link:roleRef roleURI="http://www.iconplc.com/role/RestructuringTables" xlink:type="simple" xlink:href="iclr-20200630.xsd#RestructuringTables"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/RestructuringTables" xlink:type="extended" id="i42e9c6e0a826403ca8b1bd26c07000b0_RestructuringTables"/>
  <link:roleRef roleURI="http://www.iconplc.com/role/RestructuringNarrativeDetails" xlink:type="simple" xlink:href="iclr-20200630.xsd#RestructuringNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/RestructuringNarrativeDetails" xlink:type="extended" id="if321f19fe085404e8b452888db73f9dc_RestructuringNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCostAndReserveLineItems_d8ba17fb-61b4-4547-bf46-df6e5332ba62" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_RestructuringCostAndReserveLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCharges_27e01e50-d079-40e2-8415-8e1655abd8de" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_RestructuringCharges"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_d8ba17fb-61b4-4547-bf46-df6e5332ba62" xlink:to="loc_us-gaap_RestructuringCharges_27e01e50-d079-40e2-8415-8e1655abd8de" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseImpairmentLoss_0cb7f2d9-2caa-4a8a-9607-68c87e4ee80f" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_OperatingLeaseImpairmentLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_d8ba17fb-61b4-4547-bf46-df6e5332ba62" xlink:to="loc_us-gaap_OperatingLeaseImpairmentLoss_0cb7f2d9-2caa-4a8a-9607-68c87e4ee80f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringReserveCurrent_312be7f7-2afc-4d0d-8504-8107ec33eeff" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_RestructuringReserveCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_d8ba17fb-61b4-4547-bf46-df6e5332ba62" xlink:to="loc_us-gaap_RestructuringReserveCurrent_312be7f7-2afc-4d0d-8504-8107ec33eeff" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringReserveNoncurrent_7f66c44e-f0fe-4beb-b7e6-04f9f54334b0" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_RestructuringReserveNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_d8ba17fb-61b4-4547-bf46-df6e5332ba62" xlink:to="loc_us-gaap_RestructuringReserveNoncurrent_7f66c44e-f0fe-4beb-b7e6-04f9f54334b0" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_6af9b80d-0e5d-45e8-bae8-396eca312bdd" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ScheduleOfRestructuringAndRelatedCostsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_d8ba17fb-61b4-4547-bf46-df6e5332ba62" xlink:to="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_6af9b80d-0e5d-45e8-bae8-396eca312bdd" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringPlanAxis_c5bfe8e8-9deb-4e8d-909d-eff88a686b2a" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_RestructuringPlanAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_6af9b80d-0e5d-45e8-bae8-396eca312bdd" xlink:to="loc_us-gaap_RestructuringPlanAxis_c5bfe8e8-9deb-4e8d-909d-eff88a686b2a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringPlanDomain_c5bfe8e8-9deb-4e8d-909d-eff88a686b2a_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_RestructuringPlanDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RestructuringPlanAxis_c5bfe8e8-9deb-4e8d-909d-eff88a686b2a" xlink:to="loc_us-gaap_RestructuringPlanDomain_c5bfe8e8-9deb-4e8d-909d-eff88a686b2a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringPlanDomain_1e40fecf-32ca-4887-8b7c-e2f0d7de8f2f" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_RestructuringPlanDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RestructuringPlanAxis_c5bfe8e8-9deb-4e8d-909d-eff88a686b2a" xlink:to="loc_us-gaap_RestructuringPlanDomain_1e40fecf-32ca-4887-8b7c-e2f0d7de8f2f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_ResourceRationalization2020Member_f753cc21-e515-4ccf-b5b3-e43ac01c8bed" xlink:href="iclr-20200630.xsd#iclr_ResourceRationalization2020Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringPlanDomain_1e40fecf-32ca-4887-8b7c-e2f0d7de8f2f" xlink:to="loc_iclr_ResourceRationalization2020Member_f753cc21-e515-4ccf-b5b3-e43ac01c8bed" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_ResourceRationalizations2018Member_7df4e19b-50cb-4cf1-9c35-34339c967f54" xlink:href="iclr-20200630.xsd#iclr_ResourceRationalizations2018Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringPlanDomain_1e40fecf-32ca-4887-8b7c-e2f0d7de8f2f" xlink:to="loc_iclr_ResourceRationalizations2018Member_7df4e19b-50cb-4cf1-9c35-34339c967f54" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationAxis_9693082b-1aa8-4ca4-9886-e6a35b566613" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_BalanceSheetLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_6af9b80d-0e5d-45e8-bae8-396eca312bdd" xlink:to="loc_us-gaap_BalanceSheetLocationAxis_9693082b-1aa8-4ca4-9886-e6a35b566613" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_9693082b-1aa8-4ca4-9886-e6a35b566613_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_BalanceSheetLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_9693082b-1aa8-4ca4-9886-e6a35b566613" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_9693082b-1aa8-4ca4-9886-e6a35b566613_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_540597a1-953b-4207-8a35-26060a132e5c" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_BalanceSheetLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_9693082b-1aa8-4ca4-9886-e6a35b566613" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_540597a1-953b-4207-8a35-26060a132e5c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherCurrentLiabilitiesMember_ccc29b5a-398c-4994-b085-c47fbe77bf71" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_OtherCurrentLiabilitiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_540597a1-953b-4207-8a35-26060a132e5c" xlink:to="loc_us-gaap_OtherCurrentLiabilitiesMember_ccc29b5a-398c-4994-b085-c47fbe77bf71" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherNoncurrentLiabilitiesMember_fca48fe1-f53b-40dc-a441-7f69d2351b7e" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_OtherNoncurrentLiabilitiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_540597a1-953b-4207-8a35-26060a132e5c" xlink:to="loc_us-gaap_OtherNoncurrentLiabilitiesMember_fca48fe1-f53b-40dc-a441-7f69d2351b7e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCostAndReserveAxis_ce3f70fe-eb21-456c-a6e6-5ded12b0a096" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_RestructuringCostAndReserveAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_6af9b80d-0e5d-45e8-bae8-396eca312bdd" xlink:to="loc_us-gaap_RestructuringCostAndReserveAxis_ce3f70fe-eb21-456c-a6e6-5ded12b0a096" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfRestructuringDomain_ce3f70fe-eb21-456c-a6e6-5ded12b0a096_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_TypeOfRestructuringDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RestructuringCostAndReserveAxis_ce3f70fe-eb21-456c-a6e6-5ded12b0a096" xlink:to="loc_us-gaap_TypeOfRestructuringDomain_ce3f70fe-eb21-456c-a6e6-5ded12b0a096_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfRestructuringDomain_2c824c12-e285-481c-ab38-0ce477f11ed2" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_TypeOfRestructuringDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RestructuringCostAndReserveAxis_ce3f70fe-eb21-456c-a6e6-5ded12b0a096" xlink:to="loc_us-gaap_TypeOfRestructuringDomain_2c824c12-e285-481c-ab38-0ce477f11ed2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_WorkforceReductionMember_6e19eb8b-da4f-406a-8acf-0094f65a085d" xlink:href="iclr-20200630.xsd#iclr_WorkforceReductionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_2c824c12-e285-481c-ab38-0ce477f11ed2" xlink:to="loc_iclr_WorkforceReductionMember_6e19eb8b-da4f-406a-8acf-0094f65a085d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_OnerousLeaseMember_4c16e1a7-5524-448a-a8ee-c8149e2afdda" xlink:href="iclr-20200630.xsd#iclr_OnerousLeaseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_2c824c12-e285-481c-ab38-0ce477f11ed2" xlink:to="loc_iclr_OnerousLeaseMember_4c16e1a7-5524-448a-a8ee-c8149e2afdda" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherRestructuringMember_45b8216d-eb5a-4306-b026-ec50ee17dfc7" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_OtherRestructuringMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_2c824c12-e285-481c-ab38-0ce477f11ed2" xlink:to="loc_us-gaap_OtherRestructuringMember_45b8216d-eb5a-4306-b026-ec50ee17dfc7" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/RestructuringRollforwardActivityDetails" xlink:type="simple" xlink:href="iclr-20200630.xsd#RestructuringRollforwardActivityDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/RestructuringRollforwardActivityDetails" xlink:type="extended" id="i4575edf99e2d4c02a84ec38ac903e55f_RestructuringRollforwardActivityDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCostAndReserveLineItems_6cc1e5d9-a986-471f-853c-1d2edff81c0b" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_RestructuringCostAndReserveLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringReserveRollForward_8cf73175-33da-4527-a62d-ebef2e649b94" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_RestructuringReserveRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_6cc1e5d9-a986-471f-853c-1d2edff81c0b" xlink:to="loc_us-gaap_RestructuringReserveRollForward_8cf73175-33da-4527-a62d-ebef2e649b94" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCharges_bad2fb26-c4f6-4f26-a10c-363cc572a21f" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_RestructuringCharges"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringReserveRollForward_8cf73175-33da-4527-a62d-ebef2e649b94" xlink:to="loc_us-gaap_RestructuringCharges_bad2fb26-c4f6-4f26-a10c-363cc572a21f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringReserve_9704ff69-bf47-4e7d-9ca3-6f981edfbe89" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_RestructuringReserve"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringReserveRollForward_8cf73175-33da-4527-a62d-ebef2e649b94" xlink:to="loc_us-gaap_RestructuringReserve_9704ff69-bf47-4e7d-9ca3-6f981edfbe89" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_RestructuringReserveUtilizations_588f7da1-aef2-4009-b01e-7c808fad8936" xlink:href="iclr-20200630.xsd#iclr_RestructuringReserveUtilizations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringReserveRollForward_8cf73175-33da-4527-a62d-ebef2e649b94" xlink:to="loc_iclr_RestructuringReserveUtilizations_588f7da1-aef2-4009-b01e-7c808fad8936" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringReserve_9f6457ec-b781-4f09-9e41-6b4bd539920f" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_RestructuringReserve"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_167df1e0-b754-4e61-b93c-5887ca90fd15" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ScheduleOfRestructuringAndRelatedCostsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_6cc1e5d9-a986-471f-853c-1d2edff81c0b" xlink:to="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_167df1e0-b754-4e61-b93c-5887ca90fd15" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringPlanAxis_80f7e38e-8fba-47cd-939f-0c58838977c8" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_RestructuringPlanAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_167df1e0-b754-4e61-b93c-5887ca90fd15" xlink:to="loc_us-gaap_RestructuringPlanAxis_80f7e38e-8fba-47cd-939f-0c58838977c8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringPlanDomain_80f7e38e-8fba-47cd-939f-0c58838977c8_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_RestructuringPlanDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RestructuringPlanAxis_80f7e38e-8fba-47cd-939f-0c58838977c8" xlink:to="loc_us-gaap_RestructuringPlanDomain_80f7e38e-8fba-47cd-939f-0c58838977c8_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringPlanDomain_53e3fd87-1e3e-4ee2-a87a-e3bae21a967a" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_RestructuringPlanDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RestructuringPlanAxis_80f7e38e-8fba-47cd-939f-0c58838977c8" xlink:to="loc_us-gaap_RestructuringPlanDomain_53e3fd87-1e3e-4ee2-a87a-e3bae21a967a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_ResourceRationalization2020Member_8ba60363-d820-408a-8166-09d4f25f77a8" xlink:href="iclr-20200630.xsd#iclr_ResourceRationalization2020Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringPlanDomain_53e3fd87-1e3e-4ee2-a87a-e3bae21a967a" xlink:to="loc_iclr_ResourceRationalization2020Member_8ba60363-d820-408a-8166-09d4f25f77a8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_ResourceRationalizations2018Member_cdcd34d3-360d-406f-a5ac-a90381ae6102" xlink:href="iclr-20200630.xsd#iclr_ResourceRationalizations2018Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringPlanDomain_53e3fd87-1e3e-4ee2-a87a-e3bae21a967a" xlink:to="loc_iclr_ResourceRationalizations2018Member_cdcd34d3-360d-406f-a5ac-a90381ae6102" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCostAndReserveAxis_60e2c3eb-ba19-423a-9034-6feb62f56beb" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_RestructuringCostAndReserveAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_167df1e0-b754-4e61-b93c-5887ca90fd15" xlink:to="loc_us-gaap_RestructuringCostAndReserveAxis_60e2c3eb-ba19-423a-9034-6feb62f56beb" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfRestructuringDomain_60e2c3eb-ba19-423a-9034-6feb62f56beb_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_TypeOfRestructuringDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RestructuringCostAndReserveAxis_60e2c3eb-ba19-423a-9034-6feb62f56beb" xlink:to="loc_us-gaap_TypeOfRestructuringDomain_60e2c3eb-ba19-423a-9034-6feb62f56beb_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfRestructuringDomain_b0bf7c67-7769-4139-bedf-7ac2a9078685" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_TypeOfRestructuringDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RestructuringCostAndReserveAxis_60e2c3eb-ba19-423a-9034-6feb62f56beb" xlink:to="loc_us-gaap_TypeOfRestructuringDomain_b0bf7c67-7769-4139-bedf-7ac2a9078685" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_WorkforceReductionMember_f0bc0829-33aa-4815-b292-ef360707ba93" xlink:href="iclr-20200630.xsd#iclr_WorkforceReductionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_b0bf7c67-7769-4139-bedf-7ac2a9078685" xlink:to="loc_iclr_WorkforceReductionMember_f0bc0829-33aa-4815-b292-ef360707ba93" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_OnerousLeaseMember_c5b258f2-c131-4c18-911f-b377b5b3d4fc" xlink:href="iclr-20200630.xsd#iclr_OnerousLeaseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_b0bf7c67-7769-4139-bedf-7ac2a9078685" xlink:to="loc_iclr_OnerousLeaseMember_c5b258f2-c131-4c18-911f-b377b5b3d4fc" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/Operatingleases" xlink:type="simple" xlink:href="iclr-20200630.xsd#Operatingleases"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/Operatingleases" xlink:type="extended" id="id7057833e94447e39edf44eba42528a5_Operatingleases"/>
  <link:roleRef roleURI="http://www.iconplc.com/role/OperatingleasesTables" xlink:type="simple" xlink:href="iclr-20200630.xsd#OperatingleasesTables"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/OperatingleasesTables" xlink:type="extended" id="ic091aa1cbd48496db2fc2971e5c51b6c_OperatingleasesTables"/>
  <link:roleRef roleURI="http://www.iconplc.com/role/OperatingleasesLeaseCostsDetails" xlink:type="simple" xlink:href="iclr-20200630.xsd#OperatingleasesLeaseCostsDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/OperatingleasesLeaseCostsDetails" xlink:type="extended" id="i314e7854c5424dddb62032ce8f59ac9b_OperatingleasesLeaseCostsDetails"/>
  <link:roleRef roleURI="http://www.iconplc.com/role/OperatingleasesNarrativeDetails" xlink:type="simple" xlink:href="iclr-20200630.xsd#OperatingleasesNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/OperatingleasesNarrativeDetails" xlink:type="extended" id="if8967552d2b34d15a8f1fc9c12872634_OperatingleasesNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeLeaseDescriptionLineItems_faa52eb2-8df3-4be5-a6e0-8d9f09882403" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_LesseeLeaseDescriptionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LeaseCost_bd91c9b0-1e9c-437c-bb62-895dc67b4727" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_LeaseCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_faa52eb2-8df3-4be5-a6e0-8d9f09882403" xlink:to="loc_us-gaap_LeaseCost_bd91c9b0-1e9c-437c-bb62-895dc67b4727" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RightOfUseAssetObtainedInExchangeForOperatingLeaseLiability_94190204-cda6-485a-9d80-8b4ebee8bf8c" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_RightOfUseAssetObtainedInExchangeForOperatingLeaseLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_faa52eb2-8df3-4be5-a6e0-8d9f09882403" xlink:to="loc_us-gaap_RightOfUseAssetObtainedInExchangeForOperatingLeaseLiability_94190204-cda6-485a-9d80-8b4ebee8bf8c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseImpairmentLoss_7566306f-128c-4280-8d72-f2983c91125f" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_OperatingLeaseImpairmentLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_faa52eb2-8df3-4be5-a6e0-8d9f09882403" xlink:to="loc_us-gaap_OperatingLeaseImpairmentLoss_7566306f-128c-4280-8d72-f2983c91125f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseWeightedAverageRemainingLeaseTerm1_c14bce91-d05f-47ff-9ee7-2d323d672c2e" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_OperatingLeaseWeightedAverageRemainingLeaseTerm1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_faa52eb2-8df3-4be5-a6e0-8d9f09882403" xlink:to="loc_us-gaap_OperatingLeaseWeightedAverageRemainingLeaseTerm1_c14bce91-d05f-47ff-9ee7-2d323d672c2e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseWeightedAverageDiscountRatePercent_bacdbf9d-ddf6-4243-bce8-ded92acb79a0" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_OperatingLeaseWeightedAverageDiscountRatePercent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_faa52eb2-8df3-4be5-a6e0-8d9f09882403" xlink:to="loc_us-gaap_OperatingLeaseWeightedAverageDiscountRatePercent_bacdbf9d-ddf6-4243-bce8-ded92acb79a0" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseLiabilityCurrent_9963cb62-56b9-4c1f-b1cc-46476a849a7c" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_OperatingLeaseLiabilityCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_faa52eb2-8df3-4be5-a6e0-8d9f09882403" xlink:to="loc_us-gaap_OperatingLeaseLiabilityCurrent_9963cb62-56b9-4c1f-b1cc-46476a849a7c" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeLeaseDescriptionTable_158ff123-f5b1-4f7c-876d-6821aa0cbe31" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_LesseeLeaseDescriptionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_faa52eb2-8df3-4be5-a6e0-8d9f09882403" xlink:to="loc_us-gaap_LesseeLeaseDescriptionTable_158ff123-f5b1-4f7c-876d-6821aa0cbe31" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_021eab05-c217-4963-b41c-54c1177c6c89" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_IncomeStatementLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LesseeLeaseDescriptionTable_158ff123-f5b1-4f7c-876d-6821aa0cbe31" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_021eab05-c217-4963-b41c-54c1177c6c89" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_021eab05-c217-4963-b41c-54c1177c6c89_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_021eab05-c217-4963-b41c-54c1177c6c89" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_021eab05-c217-4963-b41c-54c1177c6c89_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_1e3969d7-abe0-4de8-9564-e34458118e43" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_021eab05-c217-4963-b41c-54c1177c6c89" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_1e3969d7-abe0-4de8-9564-e34458118e43" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SellingGeneralAndAdministrativeExpensesMember_14380032-ed2a-4e36-ae04-3695ceef2590" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_SellingGeneralAndAdministrativeExpensesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_1e3969d7-abe0-4de8-9564-e34458118e43" xlink:to="loc_us-gaap_SellingGeneralAndAdministrativeExpensesMember_14380032-ed2a-4e36-ae04-3695ceef2590" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostOfGoodsAndServicesSold_af8b2322-6bb2-445b-b5a8-89ca5c04cf87" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CostOfGoodsAndServicesSold"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_1e3969d7-abe0-4de8-9564-e34458118e43" xlink:to="loc_us-gaap_CostOfGoodsAndServicesSold_af8b2322-6bb2-445b-b5a8-89ca5c04cf87" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationAxis_1061f7c4-c669-45ce-a3c1-1a66aae906b2" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_BalanceSheetLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LesseeLeaseDescriptionTable_158ff123-f5b1-4f7c-876d-6821aa0cbe31" xlink:to="loc_us-gaap_BalanceSheetLocationAxis_1061f7c4-c669-45ce-a3c1-1a66aae906b2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_1061f7c4-c669-45ce-a3c1-1a66aae906b2_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_BalanceSheetLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_1061f7c4-c669-45ce-a3c1-1a66aae906b2" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_1061f7c4-c669-45ce-a3c1-1a66aae906b2_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_7d9ab20f-8a0c-4c51-b36e-c107d666eee4" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_BalanceSheetLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_1061f7c4-c669-45ce-a3c1-1a66aae906b2" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_7d9ab20f-8a0c-4c51-b36e-c107d666eee4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherCurrentLiabilitiesMember_8be94bf3-cc97-460d-ab77-a4def10c7a85" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_OtherCurrentLiabilitiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_7d9ab20f-8a0c-4c51-b36e-c107d666eee4" xlink:to="loc_us-gaap_OtherCurrentLiabilitiesMember_8be94bf3-cc97-460d-ab77-a4def10c7a85" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/OperatingleasesOperatingLeaseMaturityDetails" xlink:type="simple" xlink:href="iclr-20200630.xsd#OperatingleasesOperatingLeaseMaturityDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/OperatingleasesOperatingLeaseMaturityDetails" xlink:type="extended" id="i1c41db99cfb84d2d9c809260b1a85647_OperatingleasesOperatingLeaseMaturityDetails"/>
  <link:roleRef roleURI="http://www.iconplc.com/role/Incometaxes" xlink:type="simple" xlink:href="iclr-20200630.xsd#Incometaxes"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/Incometaxes" xlink:type="extended" id="i08dafda8fe284afeb4251f27c59b7413_Incometaxes"/>
  <link:roleRef roleURI="http://www.iconplc.com/role/IncometaxesTables" xlink:type="simple" xlink:href="iclr-20200630.xsd#IncometaxesTables"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/IncometaxesTables" xlink:type="extended" id="ibc0a7c1ae49c4f52a42e78908c20c3b9_IncometaxesTables"/>
  <link:roleRef roleURI="http://www.iconplc.com/role/IncometaxesIncomeTaxesRecognizedDuringPeriodDetails" xlink:type="simple" xlink:href="iclr-20200630.xsd#IncometaxesIncomeTaxesRecognizedDuringPeriodDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/IncometaxesIncomeTaxesRecognizedDuringPeriodDetails" xlink:type="extended" id="i39a45b29090c4c35a342ccfc48088f98_IncometaxesIncomeTaxesRecognizedDuringPeriodDetails"/>
  <link:roleRef roleURI="http://www.iconplc.com/role/IncometaxesNarrativeDetails" xlink:type="simple" xlink:href="iclr-20200630.xsd#IncometaxesNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/IncometaxesNarrativeDetails" xlink:type="extended" id="i1b22de405a2f49929c6cf4a574d8579d_IncometaxesNarrativeDetails"/>
  <link:roleRef roleURI="http://www.iconplc.com/role/Netincomeperordinaryshare" xlink:type="simple" xlink:href="iclr-20200630.xsd#Netincomeperordinaryshare"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/Netincomeperordinaryshare" xlink:type="extended" id="ib9e66bb2d70746908bc0ea5ef2172b53_Netincomeperordinaryshare"/>
  <link:roleRef roleURI="http://www.iconplc.com/role/NetincomeperordinaryshareTables" xlink:type="simple" xlink:href="iclr-20200630.xsd#NetincomeperordinaryshareTables"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/NetincomeperordinaryshareTables" xlink:type="extended" id="ie972793384a24a93a400a794cf62756b_NetincomeperordinaryshareTables"/>
  <link:roleRef roleURI="http://www.iconplc.com/role/NetincomeperordinaryshareReconciliationofNumberofSharesUsedinComputationofBasicandDilutedNetIncomePerOrdinaryShareDetails" xlink:type="simple" xlink:href="iclr-20200630.xsd#NetincomeperordinaryshareReconciliationofNumberofSharesUsedinComputationofBasicandDilutedNetIncomePerOrdinaryShareDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/NetincomeperordinaryshareReconciliationofNumberofSharesUsedinComputationofBasicandDilutedNetIncomePerOrdinaryShareDetails" xlink:type="extended" id="id31d8e98683b4a568031d3354cf2ad3b_NetincomeperordinaryshareReconciliationofNumberofSharesUsedinComputationofBasicandDilutedNetIncomePerOrdinaryShareDetails"/>
  <link:roleRef roleURI="http://www.iconplc.com/role/NetincomeperordinaryshareReconciliationofNetIncomeAttributabletotheGroupandNetIncomeAttributableTotheGroupIncludingNCIRedemptionAmountDetails" xlink:type="simple" xlink:href="iclr-20200630.xsd#NetincomeperordinaryshareReconciliationofNetIncomeAttributabletotheGroupandNetIncomeAttributableTotheGroupIncludingNCIRedemptionAmountDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/NetincomeperordinaryshareReconciliationofNetIncomeAttributabletotheGroupandNetIncomeAttributableTotheGroupIncludingNCIRedemptionAmountDetails" xlink:type="extended" id="id9356c09e65f4b0a9aa2671c1d7961ba_NetincomeperordinaryshareReconciliationofNetIncomeAttributabletotheGroupandNetIncomeAttributableTotheGroupIncludingNCIRedemptionAmountDetails"/>
  <link:roleRef roleURI="http://www.iconplc.com/role/Sharebasedawards" xlink:type="simple" xlink:href="iclr-20200630.xsd#Sharebasedawards"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/Sharebasedawards" xlink:type="extended" id="i41b71b7eff8a4e9cbf730143ec0517de_Sharebasedawards"/>
  <link:roleRef roleURI="http://www.iconplc.com/role/SharebasedawardsTables" xlink:type="simple" xlink:href="iclr-20200630.xsd#SharebasedawardsTables"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/SharebasedawardsTables" xlink:type="extended" id="if9780639df58432a815367c1925a706e_SharebasedawardsTables"/>
  <link:roleRef roleURI="http://www.iconplc.com/role/SharebasedawardsNarrativeDetails" xlink:type="simple" xlink:href="iclr-20200630.xsd#SharebasedawardsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/SharebasedawardsNarrativeDetails" xlink:type="extended" id="i98a3c65b822e4d0fbee7f677b71d6b2b_SharebasedawardsNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureLineItems_0e1212b2-1948-44e8-8a7e-4e23fd7cdaea" xlink:href="iclr-20200630.xsd#iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized_eb6eb608-14fd-4223-8cd1-f26d88ded1aa" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureLineItems_0e1212b2-1948-44e8-8a7e-4e23fd7cdaea" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized_eb6eb608-14fd-4223-8cd1-f26d88ded1aa" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_LimitOfSharesIssuedOrToBeIssuedPursuantToOptionsGranted_1cf09a3c-2220-48f9-a503-72d0c07c7cb1" xlink:href="iclr-20200630.xsd#iclr_LimitOfSharesIssuedOrToBeIssuedPursuantToOptionsGranted"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureLineItems_0e1212b2-1948-44e8-8a7e-4e23fd7cdaea" xlink:to="loc_iclr_LimitOfSharesIssuedOrToBeIssuedPursuantToOptionsGranted_1cf09a3c-2220-48f9-a503-72d0c07c7cb1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardMaximumNumberOfSharesPerEmployee_579872b6-e250-45d4-9c38-0a165e1771f4" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardMaximumNumberOfSharesPerEmployee"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureLineItems_0e1212b2-1948-44e8-8a7e-4e23fd7cdaea" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardMaximumNumberOfSharesPerEmployee_579872b6-e250-45d4-9c38-0a165e1771f4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardPurchasePriceOfCommonStockPercent_82c0a787-d619-4ca0-a7c5-5328668d59ee" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardPurchasePriceOfCommonStockPercent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureLineItems_0e1212b2-1948-44e8-8a7e-4e23fd7cdaea" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardPurchasePriceOfCommonStockPercent_82c0a787-d619-4ca0-a7c5-5328668d59ee" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardPercentageOfOutstandingStockMaximum_6a73d7b5-1ce4-4298-aec2-19e0a026f0ed" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardPercentageOfOutstandingStockMaximum"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureLineItems_0e1212b2-1948-44e8-8a7e-4e23fd7cdaea" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardPercentageOfOutstandingStockMaximum_6a73d7b5-1ce4-4298-aec2-19e0a026f0ed" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_30989043-dbf7-43f7-8052-a519f0ccd737" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureLineItems_0e1212b2-1948-44e8-8a7e-4e23fd7cdaea" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_30989043-dbf7-43f7-8052-a519f0ccd737" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod_f3db25aa-edf8-4b72-bc5a-1fc53783df8c" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureLineItems_0e1212b2-1948-44e8-8a7e-4e23fd7cdaea" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod_f3db25aa-edf8-4b72-bc5a-1fc53783df8c" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2_14d7a08f-a69e-416e-b2ac-a9691dd9fd0c" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureLineItems_0e1212b2-1948-44e8-8a7e-4e23fd7cdaea" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2_14d7a08f-a69e-416e-b2ac-a9691dd9fd0c" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingOptionPrice_3e1f4dd5-33ce-4b0d-936a-2f2830bca076" xlink:href="iclr-20200630.xsd#iclr_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingOptionPrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureLineItems_0e1212b2-1948-44e8-8a7e-4e23fd7cdaea" xlink:to="loc_iclr_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingOptionPrice_3e1f4dd5-33ce-4b0d-936a-2f2830bca076" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageOptionPrice_59e7d9d4-e544-4bc4-a9e0-4285ff8a17da" xlink:href="iclr-20200630.xsd#iclr_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageOptionPrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureLineItems_0e1212b2-1948-44e8-8a7e-4e23fd7cdaea" xlink:to="loc_iclr_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageOptionPrice_59e7d9d4-e544-4bc4-a9e0-4285ff8a17da" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber_bf043bdd-f2d6-4524-89b7-f897a3560eb5" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureLineItems_0e1212b2-1948-44e8-8a7e-4e23fd7cdaea" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber_bf043bdd-f2d6-4524-89b7-f897a3560eb5" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1_72b374ea-2baf-423b-a21e-6e5ced436423" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureLineItems_0e1212b2-1948-44e8-8a7e-4e23fd7cdaea" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1_72b374ea-2baf-423b-a21e-6e5ced436423" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice_9c9ae0a2-852a-4fa6-8ac2-1fb3f1706f42" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureLineItems_0e1212b2-1948-44e8-8a7e-4e23fd7cdaea" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice_9c9ae0a2-852a-4fa6-8ac2-1fb3f1706f42" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1_87c23ce0-354a-4d7a-8317-803f983de98f" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureLineItems_0e1212b2-1948-44e8-8a7e-4e23fd7cdaea" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1_87c23ce0-354a-4d7a-8317-803f983de98f" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue_09799aec-16a6-4e26-af02-0c5e192db426" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureLineItems_0e1212b2-1948-44e8-8a7e-4e23fd7cdaea" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue_09799aec-16a6-4e26-af02-0c5e192db426" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized_66c24d6b-c3fa-437a-8e02-5ce67a7e32ea" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureLineItems_0e1212b2-1948-44e8-8a7e-4e23fd7cdaea" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized_66c24d6b-c3fa-437a-8e02-5ce67a7e32ea" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue_79ae196d-053c-4139-8f34-d45f72716188" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureLineItems_0e1212b2-1948-44e8-8a7e-4e23fd7cdaea" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue_79ae196d-053c-4139-8f34-d45f72716188" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_ef1b5cd8-a433-43af-ae96-1ed8e3b6929c" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureLineItems_0e1212b2-1948-44e8-8a7e-4e23fd7cdaea" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_ef1b5cd8-a433-43af-ae96-1ed8e3b6929c" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_5d9ae74c-b41f-4f4e-b727-2d30a66ac48c" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureLineItems_0e1212b2-1948-44e8-8a7e-4e23fd7cdaea" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_5d9ae74c-b41f-4f4e-b727-2d30a66ac48c" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_35a3de45-47c1-4325-8d00-427fb73a762f" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureLineItems_0e1212b2-1948-44e8-8a7e-4e23fd7cdaea" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_35a3de45-47c1-4325-8d00-427fb73a762f" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureTable_93539369-ba80-49a8-9272-d9b41008f5a3" xlink:href="iclr-20200630.xsd#iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureLineItems_0e1212b2-1948-44e8-8a7e-4e23fd7cdaea" xlink:to="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureTable_93539369-ba80-49a8-9272-d9b41008f5a3" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualAxis_64c9010d-d677-4a77-9e9d-3ae17ee1e200" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_TitleOfIndividualAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureTable_93539369-ba80-49a8-9272-d9b41008f5a3" xlink:to="loc_srt_TitleOfIndividualAxis_64c9010d-d677-4a77-9e9d-3ae17ee1e200" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_64c9010d-d677-4a77-9e9d-3ae17ee1e200_default" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_TitleOfIndividualWithRelationshipToEntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_TitleOfIndividualAxis_64c9010d-d677-4a77-9e9d-3ae17ee1e200" xlink:to="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_64c9010d-d677-4a77-9e9d-3ae17ee1e200_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_716bb397-bdbf-4641-806e-27bb42ca3553" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_TitleOfIndividualWithRelationshipToEntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_TitleOfIndividualAxis_64c9010d-d677-4a77-9e9d-3ae17ee1e200" xlink:to="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_716bb397-bdbf-4641-806e-27bb42ca3553" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_NonexecutiveDirectorMember_901e313d-c9d5-4ee5-bf75-50009c1c1b0a" xlink:href="iclr-20200630.xsd#iclr_NonexecutiveDirectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_716bb397-bdbf-4641-806e-27bb42ca3553" xlink:to="loc_iclr_NonexecutiveDirectorMember_901e313d-c9d5-4ee5-bf75-50009c1c1b0a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardDateAxis_3b3de119-b574-4614-8f26-1a849a9f3b91" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AwardDateAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureTable_93539369-ba80-49a8-9272-d9b41008f5a3" xlink:to="loc_us-gaap_AwardDateAxis_3b3de119-b574-4614-8f26-1a849a9f3b91" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardDateDomain_3b3de119-b574-4614-8f26-1a849a9f3b91_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AwardDateDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardDateAxis_3b3de119-b574-4614-8f26-1a849a9f3b91" xlink:to="loc_us-gaap_AwardDateDomain_3b3de119-b574-4614-8f26-1a849a9f3b91_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardDateDomain_5eafdd6e-80d2-4a6f-8697-88bb4a9109be" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AwardDateDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardDateAxis_3b3de119-b574-4614-8f26-1a849a9f3b91" xlink:to="loc_us-gaap_AwardDateDomain_5eafdd6e-80d2-4a6f-8697-88bb4a9109be" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_AwardDateAllYearsExcluding2018Member_c64846ab-bf69-4d89-aebc-94a52ce5ee92" xlink:href="iclr-20200630.xsd#iclr_AwardDateAllYearsExcluding2018Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AwardDateDomain_5eafdd6e-80d2-4a6f-8697-88bb4a9109be" xlink:to="loc_iclr_AwardDateAllYearsExcluding2018Member_c64846ab-bf69-4d89-aebc-94a52ce5ee92" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_AwardDate2018Member_2bcaf9b4-3f15-4d66-b131-184f5b3f2b74" xlink:href="iclr-20200630.xsd#iclr_AwardDate2018Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AwardDateDomain_5eafdd6e-80d2-4a6f-8697-88bb4a9109be" xlink:to="loc_iclr_AwardDate2018Member_2bcaf9b4-3f15-4d66-b131-184f5b3f2b74" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_54855a80-6f31-41d0-b5f3-4824917ef781" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureTable_93539369-ba80-49a8-9272-d9b41008f5a3" xlink:to="loc_us-gaap_AwardTypeAxis_54855a80-6f31-41d0-b5f3-4824917ef781" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_54855a80-6f31-41d0-b5f3-4824917ef781_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_54855a80-6f31-41d0-b5f3-4824917ef781" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_54855a80-6f31-41d0-b5f3-4824917ef781_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_d3e13216-6f9e-4d03-bcd1-3154f869a388" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_54855a80-6f31-41d0-b5f3-4824917ef781" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_d3e13216-6f9e-4d03-bcd1-3154f869a388" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_ConsultantsStockPlan2008PlanMember_0d5a9d06-c41f-4fbd-85e6-0ef9df0884d6" xlink:href="iclr-20200630.xsd#iclr_ConsultantsStockPlan2008PlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_d3e13216-6f9e-4d03-bcd1-3154f869a388" xlink:to="loc_iclr_ConsultantsStockPlan2008PlanMember_0d5a9d06-c41f-4fbd-85e6-0ef9df0884d6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_EmployeeStockPlanTwentyZeroEightPlanMember_b13bb736-c8df-4e6f-b91d-e670f8bedb41" xlink:href="iclr-20200630.xsd#iclr_EmployeeStockPlanTwentyZeroEightPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_d3e13216-6f9e-4d03-bcd1-3154f869a388" xlink:to="loc_iclr_EmployeeStockPlanTwentyZeroEightPlanMember_b13bb736-c8df-4e6f-b91d-e670f8bedb41" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_OptionPlans2008Member_293747bc-fae2-46a5-a75f-726969cd6310" xlink:href="iclr-20200630.xsd#iclr_OptionPlans2008Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_d3e13216-6f9e-4d03-bcd1-3154f869a388" xlink:to="loc_iclr_OptionPlans2008Member_293747bc-fae2-46a5-a75f-726969cd6310" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_EmployeeStockPlanTwentyZeroThreePlanMember_2df5b356-e3ea-4d08-b82e-271af05af56d" xlink:href="iclr-20200630.xsd#iclr_EmployeeStockPlanTwentyZeroThreePlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_d3e13216-6f9e-4d03-bcd1-3154f869a388" xlink:to="loc_iclr_EmployeeStockPlanTwentyZeroThreePlanMember_2df5b356-e3ea-4d08-b82e-271af05af56d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_ae17b019-5e33-43d9-a29c-9d610d170553" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_d3e13216-6f9e-4d03-bcd1-3154f869a388" xlink:to="loc_us-gaap_EmployeeStockOptionMember_ae17b019-5e33-43d9-a29c-9d610d170553" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_RestrictedStockUnitsTwentyThirteenMember_01f6f11a-6516-4b67-8661-cc2528c899cb" xlink:href="iclr-20200630.xsd#iclr_RestrictedStockUnitsTwentyThirteenMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_d3e13216-6f9e-4d03-bcd1-3154f869a388" xlink:to="loc_iclr_RestrictedStockUnitsTwentyThirteenMember_01f6f11a-6516-4b67-8661-cc2528c899cb" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_ConsultantsRestrictedStockUnitsTwentyNineteenMember_05e75a54-5968-4bc3-8784-7db0ca547ab7" xlink:href="iclr-20200630.xsd#iclr_ConsultantsRestrictedStockUnitsTwentyNineteenMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_d3e13216-6f9e-4d03-bcd1-3154f869a388" xlink:to="loc_iclr_ConsultantsRestrictedStockUnitsTwentyNineteenMember_05e75a54-5968-4bc3-8784-7db0ca547ab7" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_PerformanceShareUnitMember_10f1ea91-71d6-4d2d-864d-c7223dba3078" xlink:href="iclr-20200630.xsd#iclr_PerformanceShareUnitMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_d3e13216-6f9e-4d03-bcd1-3154f869a388" xlink:to="loc_iclr_PerformanceShareUnitMember_10f1ea91-71d6-4d2d-864d-c7223dba3078" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_170c975d-784c-4205-b3b3-ea49351798cc" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_d3e13216-6f9e-4d03-bcd1-3154f869a388" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_170c975d-784c-4205-b3b3-ea49351798cc" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_PerformanceBasedGrantsMember_8a7e8be7-f4d3-4a72-be46-4d3c538e9b76" xlink:href="iclr-20200630.xsd#iclr_PerformanceBasedGrantsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_d3e13216-6f9e-4d03-bcd1-3154f869a388" xlink:to="loc_iclr_PerformanceBasedGrantsMember_8a7e8be7-f4d3-4a72-be46-4d3c538e9b76" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_a72ab4db-4a50-4768-9e3f-a6874548d3e1" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureTable_93539369-ba80-49a8-9272-d9b41008f5a3" xlink:to="loc_srt_RangeAxis_a72ab4db-4a50-4768-9e3f-a6874548d3e1" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_a72ab4db-4a50-4768-9e3f-a6874548d3e1_default" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_a72ab4db-4a50-4768-9e3f-a6874548d3e1" xlink:to="loc_srt_RangeMember_a72ab4db-4a50-4768-9e3f-a6874548d3e1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_ecee4ebe-5728-4f96-a2a1-18c2a5a6b340" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_a72ab4db-4a50-4768-9e3f-a6874548d3e1" xlink:to="loc_srt_RangeMember_ecee4ebe-5728-4f96-a2a1-18c2a5a6b340" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_6a7e6117-527d-4af8-934e-3b3df34f4d3a" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_ecee4ebe-5728-4f96-a2a1-18c2a5a6b340" xlink:to="loc_srt_MinimumMember_6a7e6117-527d-4af8-934e-3b3df34f4d3a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_2cfd6ce2-4660-4da0-a0ce-3dc18d240a6f" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_ecee4ebe-5728-4f96-a2a1-18c2a5a6b340" xlink:to="loc_srt_MaximumMember_2cfd6ce2-4660-4da0-a0ce-3dc18d240a6f" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/SharebasedawardsSummaryofStockOptionActivityDetails" xlink:type="simple" xlink:href="iclr-20200630.xsd#SharebasedawardsSummaryofStockOptionActivityDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/SharebasedawardsSummaryofStockOptionActivityDetails" xlink:type="extended" id="i50c84720d40e4bcba4d066720127fbd0_SharebasedawardsSummaryofStockOptionActivityDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_d96f86ea-efd9-4916-83ef-679b47665a73" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_0be7ade0-36f7-4d7c-9cf1-e1046b8b3ed8" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_d96f86ea-efd9-4916-83ef-679b47665a73" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_0be7ade0-36f7-4d7c-9cf1-e1046b8b3ed8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_7683117b-9fe5-487a-b01d-af917fb61928" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_0be7ade0-36f7-4d7c-9cf1-e1046b8b3ed8" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_7683117b-9fe5-487a-b01d-af917fb61928" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross_d7783ba4-1ed9-4336-a0ea-c79f4f4fa2e5" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_0be7ade0-36f7-4d7c-9cf1-e1046b8b3ed8" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross_d7783ba4-1ed9-4336-a0ea-c79f4f4fa2e5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_23b0e84b-204c-4271-82e1-fc19cd6a5602" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_0be7ade0-36f7-4d7c-9cf1-e1046b8b3ed8" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_23b0e84b-204c-4271-82e1-fc19cd6a5602" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriod_6edd8405-d81f-4505-bb78-30878aba4277" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_0be7ade0-36f7-4d7c-9cf1-e1046b8b3ed8" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriod_6edd8405-d81f-4505-bb78-30878aba4277" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_2ddd3c38-1859-44cb-8bf4-70259d490596" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber_0c91eb47-54d4-46d0-8104-944bd3bb4a4c" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_d96f86ea-efd9-4916-83ef-679b47665a73" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber_0c91eb47-54d4-46d0-8104-944bd3bb4a4c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_8ae31168-aac9-4d64-8e7a-6087dcabaed0" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_d96f86ea-efd9-4916-83ef-679b47665a73" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_8ae31168-aac9-4d64-8e7a-6087dcabaed0" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice_927a3110-25cb-471e-9b91-1a61bff3e3c9" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_8ae31168-aac9-4d64-8e7a-6087dcabaed0" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice_927a3110-25cb-471e-9b91-1a61bff3e3c9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice_d7454230-b5a1-459d-aedf-28de8a621cb3" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_8ae31168-aac9-4d64-8e7a-6087dcabaed0" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice_d7454230-b5a1-459d-aedf-28de8a621cb3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice_7ec12ede-d538-4be6-af7d-f2a9890d873a" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_8ae31168-aac9-4d64-8e7a-6087dcabaed0" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice_7ec12ede-d538-4be6-af7d-f2a9890d873a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriodWeightedAverageExercisePrice_cdb711d3-e373-4719-bd05-9962af9ef8ca" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriodWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_8ae31168-aac9-4d64-8e7a-6087dcabaed0" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriodWeightedAverageExercisePrice_cdb711d3-e373-4719-bd05-9962af9ef8ca" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice_367f6227-e7a0-40ac-a06f-bb22734d5d81" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice_7bcfb168-7a07-4b26-bbe3-adc0bc14b1d9" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_d96f86ea-efd9-4916-83ef-679b47665a73" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice_7bcfb168-7a07-4b26-bbe3-adc0bc14b1d9" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsWeightedAverageGrantDateFairValueAbstract_56d703c0-4d0d-4a5d-a9fb-b94084817c65" xlink:href="iclr-20200630.xsd#iclr_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsWeightedAverageGrantDateFairValueAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_d96f86ea-efd9-4916-83ef-679b47665a73" xlink:to="loc_iclr_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsWeightedAverageGrantDateFairValueAbstract_56d703c0-4d0d-4a5d-a9fb-b94084817c65" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsWeightedAverageGrantDateFairValue_1ce1bac0-2e91-475f-af49-3e0b0ebe2ed9" xlink:href="iclr-20200630.xsd#iclr_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iclr_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsWeightedAverageGrantDateFairValueAbstract_56d703c0-4d0d-4a5d-a9fb-b94084817c65" xlink:to="loc_iclr_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsWeightedAverageGrantDateFairValue_1ce1bac0-2e91-475f-af49-3e0b0ebe2ed9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_00b251b5-e54e-4932-bf93-1dd81facf328" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iclr_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsWeightedAverageGrantDateFairValueAbstract_56d703c0-4d0d-4a5d-a9fb-b94084817c65" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_00b251b5-e54e-4932-bf93-1dd81facf328" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisedInPeriodWeightedAverageGrantDateFairValue_f4af87fe-8e0b-4270-906f-b28561039ea6" xlink:href="iclr-20200630.xsd#iclr_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisedInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iclr_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsWeightedAverageGrantDateFairValueAbstract_56d703c0-4d0d-4a5d-a9fb-b94084817c65" xlink:to="loc_iclr_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisedInPeriodWeightedAverageGrantDateFairValue_f4af87fe-8e0b-4270-906f-b28561039ea6" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsForfeituresAndExpirationsInPeriodWeightedAverageGrantDateFairValue_275bc1e0-6afc-42c2-b76a-398eb25cb5e1" xlink:href="iclr-20200630.xsd#iclr_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsForfeituresAndExpirationsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iclr_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsWeightedAverageGrantDateFairValueAbstract_56d703c0-4d0d-4a5d-a9fb-b94084817c65" xlink:to="loc_iclr_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsForfeituresAndExpirationsInPeriodWeightedAverageGrantDateFairValue_275bc1e0-6afc-42c2-b76a-398eb25cb5e1" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsWeightedAverageGrantDateFairValue_55ed2b24-5199-4335-b70e-913f145c07fa" xlink:href="iclr-20200630.xsd#iclr_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsWeightedAverageGrantDateFairValue"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageGrantDateFairValue_3c3fa659-f1b2-4587-8831-7e31db4919de" xlink:href="iclr-20200630.xsd#iclr_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_d96f86ea-efd9-4916-83ef-679b47665a73" xlink:to="loc_iclr_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageGrantDateFairValue_3c3fa659-f1b2-4587-8831-7e31db4919de" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsWeightedAverageRemainingContractualTermAbstract_e84985b4-f36c-42ee-9db9-51b1bf60a5e1" xlink:href="iclr-20200630.xsd#iclr_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsWeightedAverageRemainingContractualTermAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_d96f86ea-efd9-4916-83ef-679b47665a73" xlink:to="loc_iclr_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsWeightedAverageRemainingContractualTermAbstract_e84985b4-f36c-42ee-9db9-51b1bf60a5e1" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2_5b1987e8-7fac-40d8-aeff-dea7ee2f1a67" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iclr_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsWeightedAverageRemainingContractualTermAbstract_e84985b4-f36c-42ee-9db9-51b1bf60a5e1" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2_5b1987e8-7fac-40d8-aeff-dea7ee2f1a67" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1_b1d9fbf5-a87f-4fae-9b4c-d97159bba001" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iclr_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsWeightedAverageRemainingContractualTermAbstract_e84985b4-f36c-42ee-9db9-51b1bf60a5e1" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1_b1d9fbf5-a87f-4fae-9b4c-d97159bba001" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_4fefdbdd-cd9d-477d-911b-ba51d225e3bf" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_d96f86ea-efd9-4916-83ef-679b47665a73" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_4fefdbdd-cd9d-477d-911b-ba51d225e3bf" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_2b67780b-70e7-4f18-a93f-c12f0a5ee205" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_4fefdbdd-cd9d-477d-911b-ba51d225e3bf" xlink:to="loc_us-gaap_AwardTypeAxis_2b67780b-70e7-4f18-a93f-c12f0a5ee205" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_2b67780b-70e7-4f18-a93f-c12f0a5ee205_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_2b67780b-70e7-4f18-a93f-c12f0a5ee205" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_2b67780b-70e7-4f18-a93f-c12f0a5ee205_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_65a39510-0ea7-449c-9097-d10276590194" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_2b67780b-70e7-4f18-a93f-c12f0a5ee205" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_65a39510-0ea7-449c-9097-d10276590194" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_b8ed69e3-3293-45cd-b8a3-ccd7a17a3808" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_65a39510-0ea7-449c-9097-d10276590194" xlink:to="loc_us-gaap_EmployeeStockOptionMember_b8ed69e3-3293-45cd-b8a3-ccd7a17a3808" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/SharebasedawardsSummaryofMovementinNonVestedShareOptionsDetails" xlink:type="simple" xlink:href="iclr-20200630.xsd#SharebasedawardsSummaryofMovementinNonVestedShareOptionsDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/SharebasedawardsSummaryofMovementinNonVestedShareOptionsDetails" xlink:type="extended" id="ic4384bcffbd646f0a0576ac587a9d31a_SharebasedawardsSummaryofMovementinNonVestedShareOptionsDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_f2422fd8-b048-456e-b6d8-9fd5b84b71e2" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedNumberOfSharesRollForward_1f35364a-c542-4f26-b3d2-652c1629eed7" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedNumberOfSharesRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_f2422fd8-b048-456e-b6d8-9fd5b84b71e2" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedNumberOfSharesRollForward_1f35364a-c542-4f26-b3d2-652c1629eed7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedNumberOfShares_e12f80b9-e792-4579-af1e-c1b7430273b6" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedNumberOfShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedNumberOfSharesRollForward_1f35364a-c542-4f26-b3d2-652c1629eed7" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedNumberOfShares_e12f80b9-e792-4579-af1e-c1b7430273b6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross_63c1e9e8-4f21-4f51-8928-3f015cab5b71" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedNumberOfSharesRollForward_1f35364a-c542-4f26-b3d2-652c1629eed7" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross_63c1e9e8-4f21-4f51-8928-3f015cab5b71" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedNumberOfShares_2e6fd1bb-6023-451b-a79f-9d273ad67145" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedNumberOfShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedNumberOfSharesRollForward_1f35364a-c542-4f26-b3d2-652c1629eed7" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedNumberOfShares_2e6fd1bb-6023-451b-a79f-9d273ad67145" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedOptionsForfeitedNumberOfShares_19207b89-f23e-48c7-b4e9-67037d836b3b" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedOptionsForfeitedNumberOfShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedNumberOfSharesRollForward_1f35364a-c542-4f26-b3d2-652c1629eed7" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedOptionsForfeitedNumberOfShares_19207b89-f23e-48c7-b4e9-67037d836b3b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedNumberOfShares_a018b586-d4d1-4539-bab1-4a17fc5aab35" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedNumberOfShares"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonvestedOptionsOutstandingExercisePriceAbstract_d479131c-c82e-4c3f-971a-da9068c36fb5" xlink:href="iclr-20200630.xsd#iclr_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonvestedOptionsOutstandingExercisePriceAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_f2422fd8-b048-456e-b6d8-9fd5b84b71e2" xlink:to="loc_iclr_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonvestedOptionsOutstandingExercisePriceAbstract_d479131c-c82e-4c3f-971a-da9068c36fb5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonvestedOptionsOutstandingWeightedAverageExercisePrice_91f97ff2-ca59-4b70-ba3f-24649f84155a" xlink:href="iclr-20200630.xsd#iclr_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonvestedOptionsOutstandingWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iclr_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonvestedOptionsOutstandingExercisePriceAbstract_d479131c-c82e-4c3f-971a-da9068c36fb5" xlink:to="loc_iclr_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonvestedOptionsOutstandingWeightedAverageExercisePrice_91f97ff2-ca59-4b70-ba3f-24649f84155a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonvestedOptionsGrantedInPeriodWeightedAverageExercisePrice_b3a88d85-838b-4a7e-abd4-574e22a0fa72" xlink:href="iclr-20200630.xsd#iclr_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonvestedOptionsGrantedInPeriodWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iclr_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonvestedOptionsOutstandingExercisePriceAbstract_d479131c-c82e-4c3f-971a-da9068c36fb5" xlink:to="loc_iclr_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonvestedOptionsGrantedInPeriodWeightedAverageExercisePrice_b3a88d85-838b-4a7e-abd4-574e22a0fa72" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonvestedOptionsVestedInPeriodWeightedAverageExercisePrice_f397692e-97d3-4419-9cf5-89ad8b81abce" xlink:href="iclr-20200630.xsd#iclr_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonvestedOptionsVestedInPeriodWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iclr_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonvestedOptionsOutstandingExercisePriceAbstract_d479131c-c82e-4c3f-971a-da9068c36fb5" xlink:to="loc_iclr_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonvestedOptionsVestedInPeriodWeightedAverageExercisePrice_f397692e-97d3-4419-9cf5-89ad8b81abce" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonvestedOptionsForfeitedInPeriodWeightedAverageExercisePrice_f4ffa4e7-8911-4142-b20e-42c75cd3f4cb" xlink:href="iclr-20200630.xsd#iclr_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonvestedOptionsForfeitedInPeriodWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iclr_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonvestedOptionsOutstandingExercisePriceAbstract_d479131c-c82e-4c3f-971a-da9068c36fb5" xlink:to="loc_iclr_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonvestedOptionsForfeitedInPeriodWeightedAverageExercisePrice_f4ffa4e7-8911-4142-b20e-42c75cd3f4cb" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonvestedOptionsOutstandingWeightedAverageExercisePrice_e2ce97f3-9e4d-4861-9228-06b77cf2d99b" xlink:href="iclr-20200630.xsd#iclr_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonvestedOptionsOutstandingWeightedAverageExercisePrice"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedWeightedAverageGrantDateFairValueAbstract_0dedee49-255b-4818-8478-35bd85c852ee" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedWeightedAverageGrantDateFairValueAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_f2422fd8-b048-456e-b6d8-9fd5b84b71e2" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedWeightedAverageGrantDateFairValueAbstract_0dedee49-255b-4818-8478-35bd85c852ee" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedWeightedAverageGrantDateFairValue_c9f082e4-a7a4-49bd-8c5d-e619605178db" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedWeightedAverageGrantDateFairValueAbstract_0dedee49-255b-4818-8478-35bd85c852ee" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedWeightedAverageGrantDateFairValue_c9f082e4-a7a4-49bd-8c5d-e619605178db" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_709f7e1e-8a38-4dcb-9cc0-7fe0ed0be9f7" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedWeightedAverageGrantDateFairValueAbstract_0dedee49-255b-4818-8478-35bd85c852ee" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_709f7e1e-8a38-4dcb-9cc0-7fe0ed0be9f7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedWeightedAverageGrantDateFairValue_de64e544-9c06-4a0c-b95c-d33e0d8ed3c1" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedWeightedAverageGrantDateFairValueAbstract_0dedee49-255b-4818-8478-35bd85c852ee" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedWeightedAverageGrantDateFairValue_de64e544-9c06-4a0c-b95c-d33e0d8ed3c1" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedOptionsForfeitedWeightedAverageGrantDateFairValue_63b2b8b5-2b19-4cd8-b753-6d78437a297e" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedOptionsForfeitedWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedWeightedAverageGrantDateFairValueAbstract_0dedee49-255b-4818-8478-35bd85c852ee" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedOptionsForfeitedWeightedAverageGrantDateFairValue_63b2b8b5-2b19-4cd8-b753-6d78437a297e" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedWeightedAverageGrantDateFairValue_8abe397f-57e4-49b2-a270-19c4b9beb27c" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedWeightedAverageGrantDateFairValue"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_f85d487d-ee1c-4dca-9fbd-5f40cedceae1" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_f2422fd8-b048-456e-b6d8-9fd5b84b71e2" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_f85d487d-ee1c-4dca-9fbd-5f40cedceae1" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_ff1c5681-644c-4e7a-b0f1-4ad7ff4418a1" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_f85d487d-ee1c-4dca-9fbd-5f40cedceae1" xlink:to="loc_us-gaap_AwardTypeAxis_ff1c5681-644c-4e7a-b0f1-4ad7ff4418a1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_ff1c5681-644c-4e7a-b0f1-4ad7ff4418a1_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_ff1c5681-644c-4e7a-b0f1-4ad7ff4418a1" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_ff1c5681-644c-4e7a-b0f1-4ad7ff4418a1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_7b28e50b-bdc2-43bf-b053-bfdfa0a75b9d" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_ff1c5681-644c-4e7a-b0f1-4ad7ff4418a1" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_7b28e50b-bdc2-43bf-b053-bfdfa0a75b9d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_2d66d83d-2c4f-4fcf-bdb5-d05c754d00e5" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_7b28e50b-bdc2-43bf-b053-bfdfa0a75b9d" xlink:to="loc_us-gaap_EmployeeStockOptionMember_2d66d83d-2c4f-4fcf-bdb5-d05c754d00e5" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/SharebasedawardsScheduleofWeightedAverageFairValuesandAssumptionsUsedDetails" xlink:type="simple" xlink:href="iclr-20200630.xsd#SharebasedawardsScheduleofWeightedAverageFairValuesandAssumptionsUsedDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/SharebasedawardsScheduleofWeightedAverageFairValuesandAssumptionsUsedDetails" xlink:type="extended" id="i757ebd1e833142e4b7975eeed16980f0_SharebasedawardsScheduleofWeightedAverageFairValuesandAssumptionsUsedDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_eea8923e-13f3-4f02-8a3c-82fd63f98f70" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_638999df-03e6-4e33-b631-7fe510e02576" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_eea8923e-13f3-4f02-8a3c-82fd63f98f70" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_638999df-03e6-4e33-b631-7fe510e02576" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsAndMethodologyAbstract_57a50266-dd63-4d4c-9cd6-b26bf7335edd" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsAndMethodologyAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_eea8923e-13f3-4f02-8a3c-82fd63f98f70" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsAndMethodologyAbstract_57a50266-dd63-4d4c-9cd6-b26bf7335edd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_dd08fbeb-4dab-4c61-80a9-23d5e2f182a4" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsAndMethodologyAbstract_57a50266-dd63-4d4c-9cd6-b26bf7335edd" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_dd08fbeb-4dab-4c61-80a9-23d5e2f182a4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_8c9e62ed-b238-4e09-ad03-bc33394354bf" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsAndMethodologyAbstract_57a50266-dd63-4d4c-9cd6-b26bf7335edd" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_8c9e62ed-b238-4e09-ad03-bc33394354bf" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_18cff667-3ee0-415a-bea1-830ef74d032d" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsAndMethodologyAbstract_57a50266-dd63-4d4c-9cd6-b26bf7335edd" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_18cff667-3ee0-415a-bea1-830ef74d032d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_874a6122-756f-43b3-b92a-2940a71b56f9" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsAndMethodologyAbstract_57a50266-dd63-4d4c-9cd6-b26bf7335edd" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_874a6122-756f-43b3-b92a-2940a71b56f9" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_4857e0e8-4bdc-4454-a0d4-9d97afd3d30b" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_eea8923e-13f3-4f02-8a3c-82fd63f98f70" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_4857e0e8-4bdc-4454-a0d4-9d97afd3d30b" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_241ef9f8-32fc-48b3-b452-508e50d37ef9" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_4857e0e8-4bdc-4454-a0d4-9d97afd3d30b" xlink:to="loc_us-gaap_AwardTypeAxis_241ef9f8-32fc-48b3-b452-508e50d37ef9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_241ef9f8-32fc-48b3-b452-508e50d37ef9_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_241ef9f8-32fc-48b3-b452-508e50d37ef9" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_241ef9f8-32fc-48b3-b452-508e50d37ef9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_d9b1a690-396f-4401-92d3-b0a9147c5baf" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_241ef9f8-32fc-48b3-b452-508e50d37ef9" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_d9b1a690-396f-4401-92d3-b0a9147c5baf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_89f45132-7d25-4880-880f-471deb2076fe" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_d9b1a690-396f-4401-92d3-b0a9147c5baf" xlink:to="loc_us-gaap_EmployeeStockOptionMember_89f45132-7d25-4880-880f-471deb2076fe" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/SharebasedawardsSummaryofRSUandPSUActivityDetails" xlink:type="simple" xlink:href="iclr-20200630.xsd#SharebasedawardsSummaryofRSUandPSUActivityDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/SharebasedawardsSummaryofRSUandPSUActivityDetails" xlink:type="extended" id="i74440a2218b24f39941d0dd9d70495aa_SharebasedawardsSummaryofRSUandPSUActivityDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_3b5abf21-ef81-490c-ad75-73096ff9f59a" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_0b5f77df-132e-4c09-b6db-ba1fc1b22498" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_3b5abf21-ef81-490c-ad75-73096ff9f59a" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_0b5f77df-132e-4c09-b6db-ba1fc1b22498" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_3991a567-efe9-416f-bdbd-1e0c73e9eac7" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_0b5f77df-132e-4c09-b6db-ba1fc1b22498" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_3991a567-efe9-416f-bdbd-1e0c73e9eac7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_fde1305e-1b4d-4ce9-acc2-48596bf16204" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_0b5f77df-132e-4c09-b6db-ba1fc1b22498" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_fde1305e-1b4d-4ce9-acc2-48596bf16204" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_26024dc9-4cbc-48b2-9308-74e1fd8457df" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_0b5f77df-132e-4c09-b6db-ba1fc1b22498" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_26024dc9-4cbc-48b2-9308-74e1fd8457df" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_517151fe-edbd-4a29-a1ad-6f482d261f61" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_0b5f77df-132e-4c09-b6db-ba1fc1b22498" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_517151fe-edbd-4a29-a1ad-6f482d261f61" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_c3489235-e8d8-4aec-aee7-6425b4cd10e6" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_3b2ed537-480b-45e6-a7e6-2e3335c95c77" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_3b5abf21-ef81-490c-ad75-73096ff9f59a" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_3b2ed537-480b-45e6-a7e6-2e3335c95c77" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_28e008ab-6f55-4f9f-96f5-df8d1ea96dea" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_3b2ed537-480b-45e6-a7e6-2e3335c95c77" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_28e008ab-6f55-4f9f-96f5-df8d1ea96dea" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_9e0df06a-832e-4d92-9a84-5441331ebd33" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_3b2ed537-480b-45e6-a7e6-2e3335c95c77" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_9e0df06a-832e-4d92-9a84-5441331ebd33" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_920d3fe9-4ed9-4f9c-9295-1870534ce1bb" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_3b2ed537-480b-45e6-a7e6-2e3335c95c77" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_920d3fe9-4ed9-4f9c-9295-1870534ce1bb" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_c01e5d52-3348-44b0-af05-edc0e1c6a2b6" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_3b2ed537-480b-45e6-a7e6-2e3335c95c77" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_c01e5d52-3348-44b0-af05-edc0e1c6a2b6" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_52e315d5-2341-4d40-8917-1eaf94df5d4b" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAdditionalDisclosuresAbstract_7358298a-da5f-41c5-91de-2de212d59cec" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAdditionalDisclosuresAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_3b5abf21-ef81-490c-ad75-73096ff9f59a" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAdditionalDisclosuresAbstract_7358298a-da5f-41c5-91de-2de212d59cec" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsOutstandingWeightedAverageRemainingContractualTerms_5ffbb352-ecd1-4320-be4f-9433d9457f18" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsOutstandingWeightedAverageRemainingContractualTerms"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAdditionalDisclosuresAbstract_7358298a-da5f-41c5-91de-2de212d59cec" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsOutstandingWeightedAverageRemainingContractualTerms_5ffbb352-ecd1-4320-be4f-9433d9457f18" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_917b8365-2890-4b6c-a8ab-210cff802836" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_3b5abf21-ef81-490c-ad75-73096ff9f59a" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_917b8365-2890-4b6c-a8ab-210cff802836" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_ec2f7581-014c-45ce-9f20-110efa438c15" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_917b8365-2890-4b6c-a8ab-210cff802836" xlink:to="loc_us-gaap_AwardTypeAxis_ec2f7581-014c-45ce-9f20-110efa438c15" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_ec2f7581-014c-45ce-9f20-110efa438c15_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_ec2f7581-014c-45ce-9f20-110efa438c15" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_ec2f7581-014c-45ce-9f20-110efa438c15_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_74809043-31e1-4210-b173-78eefc81efdc" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_ec2f7581-014c-45ce-9f20-110efa438c15" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_74809043-31e1-4210-b173-78eefc81efdc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_PerformanceShareUnitMember_8158ff5a-07f8-4203-86cc-83dd345c0cf3" xlink:href="iclr-20200630.xsd#iclr_PerformanceShareUnitMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_74809043-31e1-4210-b173-78eefc81efdc" xlink:to="loc_iclr_PerformanceShareUnitMember_8158ff5a-07f8-4203-86cc-83dd345c0cf3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_577d721a-f567-4d54-8a03-d7f7620f6ad4" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_74809043-31e1-4210-b173-78eefc81efdc" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_577d721a-f567-4d54-8a03-d7f7620f6ad4" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/SharebasedawardsScheduleofNoncashStockCompensationExpenseDetails" xlink:type="simple" xlink:href="iclr-20200630.xsd#SharebasedawardsScheduleofNoncashStockCompensationExpenseDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/SharebasedawardsScheduleofNoncashStockCompensationExpenseDetails" xlink:type="extended" id="i57d0abfd473444008c1813bb2e6dc31b_SharebasedawardsScheduleofNoncashStockCompensationExpenseDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_18a98699-c6ae-446e-860d-45263da5a6ca" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensation_ca162c86-5301-4c9d-8754-51fb23c579e3" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_18a98699-c6ae-446e-860d-45263da5a6ca" xlink:to="loc_us-gaap_ShareBasedCompensation_ca162c86-5301-4c9d-8754-51fb23c579e3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_72115bcc-d27d-4039-8a84-3814bb92ec45" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_18a98699-c6ae-446e-860d-45263da5a6ca" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_72115bcc-d27d-4039-8a84-3814bb92ec45" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_fbcc9964-6490-4e0e-adab-f3d23e9af235" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_IncomeStatementLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_72115bcc-d27d-4039-8a84-3814bb92ec45" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_fbcc9964-6490-4e0e-adab-f3d23e9af235" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_fbcc9964-6490-4e0e-adab-f3d23e9af235_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_fbcc9964-6490-4e0e-adab-f3d23e9af235" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_fbcc9964-6490-4e0e-adab-f3d23e9af235_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_b68a8d66-f49b-4cd4-acc4-3738ea644693" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_fbcc9964-6490-4e0e-adab-f3d23e9af235" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_b68a8d66-f49b-4cd4-acc4-3738ea644693" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostOfSalesMember_e5521d04-5acf-4025-aa32-a0a9f08c552f" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CostOfSalesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_b68a8d66-f49b-4cd4-acc4-3738ea644693" xlink:to="loc_us-gaap_CostOfSalesMember_e5521d04-5acf-4025-aa32-a0a9f08c552f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SellingGeneralAndAdministrativeExpensesMember_a470b1ae-63f2-4637-97b0-347b8d2cee59" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_SellingGeneralAndAdministrativeExpensesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_b68a8d66-f49b-4cd4-acc4-3738ea644693" xlink:to="loc_us-gaap_SellingGeneralAndAdministrativeExpensesMember_a470b1ae-63f2-4637-97b0-347b8d2cee59" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/Sharecapital" xlink:type="simple" xlink:href="iclr-20200630.xsd#Sharecapital"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/Sharecapital" xlink:type="extended" id="ief3d8d03513c4042bd808557cb59a579_Sharecapital"/>
  <link:roleRef roleURI="http://www.iconplc.com/role/SharecapitalDetails" xlink:type="simple" xlink:href="iclr-20200630.xsd#SharecapitalDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/SharecapitalDetails" xlink:type="extended" id="i636b41c9081c450e8f5827ef11e65245_SharecapitalDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityClassOfTreasuryStockLineItems_8b20304b-1731-4287-bfee-9d085c6211f2" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_EquityClassOfTreasuryStockLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_StockRepurchaseProgramAuthorizedPercentage_1acc1f9b-5e9f-4e92-b0f3-6a1ca6664ecb" xlink:href="iclr-20200630.xsd#iclr_StockRepurchaseProgramAuthorizedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityClassOfTreasuryStockLineItems_8b20304b-1731-4287-bfee-9d085c6211f2" xlink:to="loc_iclr_StockRepurchaseProgramAuthorizedPercentage_1acc1f9b-5e9f-4e92-b0f3-6a1ca6664ecb" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchaseProgramNumberOfSharesAuthorizedToBeRepurchased_79763808-80a2-4686-84e4-ee7eebfbf0de" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StockRepurchaseProgramNumberOfSharesAuthorizedToBeRepurchased"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityClassOfTreasuryStockLineItems_8b20304b-1731-4287-bfee-9d085c6211f2" xlink:to="loc_us-gaap_StockRepurchaseProgramNumberOfSharesAuthorizedToBeRepurchased_79763808-80a2-4686-84e4-ee7eebfbf0de" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodValue_6e537ba7-46ab-4668-bc41-4078baf3528a" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StockRepurchasedAndRetiredDuringPeriodValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityClassOfTreasuryStockLineItems_8b20304b-1731-4287-bfee-9d085c6211f2" xlink:to="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodValue_6e537ba7-46ab-4668-bc41-4078baf3528a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodShares_30e804ab-447d-41d3-8cda-32491cd775bd" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StockRepurchasedAndRetiredDuringPeriodShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityClassOfTreasuryStockLineItems_8b20304b-1731-4287-bfee-9d085c6211f2" xlink:to="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodShares_30e804ab-447d-41d3-8cda-32491cd775bd" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfTreasuryStockTable_1cd1c3c5-65e4-4108-9b36-6fb9155a7895" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ClassOfTreasuryStockTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EquityClassOfTreasuryStockLineItems_8b20304b-1731-4287-bfee-9d085c6211f2" xlink:to="loc_us-gaap_ClassOfTreasuryStockTable_1cd1c3c5-65e4-4108-9b36-6fb9155a7895" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareRepurchaseProgramAxis_d73cf6ef-cabc-49cc-bbd9-039b3eb0c8af" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareRepurchaseProgramAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ClassOfTreasuryStockTable_1cd1c3c5-65e4-4108-9b36-6fb9155a7895" xlink:to="loc_us-gaap_ShareRepurchaseProgramAxis_d73cf6ef-cabc-49cc-bbd9-039b3eb0c8af" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareRepurchaseProgramDomain_d73cf6ef-cabc-49cc-bbd9-039b3eb0c8af_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareRepurchaseProgramDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ShareRepurchaseProgramAxis_d73cf6ef-cabc-49cc-bbd9-039b3eb0c8af" xlink:to="loc_us-gaap_ShareRepurchaseProgramDomain_d73cf6ef-cabc-49cc-bbd9-039b3eb0c8af_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareRepurchaseProgramDomain_a2c2b58c-6752-409b-8b67-a9f8fa56d1be" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareRepurchaseProgramDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ShareRepurchaseProgramAxis_d73cf6ef-cabc-49cc-bbd9-039b3eb0c8af" xlink:to="loc_us-gaap_ShareRepurchaseProgramDomain_a2c2b58c-6752-409b-8b67-a9f8fa56d1be" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BuybackProgramMember_9b33667c-c2d9-4b3d-b986-b7779d8eb459" xlink:href="iclr-20200630.xsd#iclr_BuybackProgramMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareRepurchaseProgramDomain_a2c2b58c-6752-409b-8b67-a9f8fa56d1be" xlink:to="loc_iclr_BuybackProgramMember_9b33667c-c2d9-4b3d-b986-b7779d8eb459" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_960cd81a-6f19-4c18-ae98-edd90523e82e" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ClassOfTreasuryStockTable_1cd1c3c5-65e4-4108-9b36-6fb9155a7895" xlink:to="loc_srt_RangeAxis_960cd81a-6f19-4c18-ae98-edd90523e82e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_960cd81a-6f19-4c18-ae98-edd90523e82e_default" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_960cd81a-6f19-4c18-ae98-edd90523e82e" xlink:to="loc_srt_RangeMember_960cd81a-6f19-4c18-ae98-edd90523e82e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_e6d6c2e0-c678-4824-88f9-b8a324186dd4" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_960cd81a-6f19-4c18-ae98-edd90523e82e" xlink:to="loc_srt_RangeMember_e6d6c2e0-c678-4824-88f9-b8a324186dd4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_10865b70-7e34-401a-9c59-eeb0e1ffd6e1" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_e6d6c2e0-c678-4824-88f9-b8a324186dd4" xlink:to="loc_srt_MaximumMember_10865b70-7e34-401a-9c59-eeb0e1ffd6e1" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/Businesssegmentinformation" xlink:type="simple" xlink:href="iclr-20200630.xsd#Businesssegmentinformation"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/Businesssegmentinformation" xlink:type="extended" id="i1ec1adae00ac468ebcca567c966ca4de_Businesssegmentinformation"/>
  <link:roleRef roleURI="http://www.iconplc.com/role/BusinesssegmentinformationTables" xlink:type="simple" xlink:href="iclr-20200630.xsd#BusinesssegmentinformationTables"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/BusinesssegmentinformationTables" xlink:type="extended" id="ic4947b33a2b74cb3918e013ed179f060_BusinesssegmentinformationTables"/>
  <link:roleRef roleURI="http://www.iconplc.com/role/BusinesssegmentinformationNarrativeDetails" xlink:type="simple" xlink:href="iclr-20200630.xsd#BusinesssegmentinformationNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/BusinesssegmentinformationNarrativeDetails" xlink:type="extended" id="ie41182a6c44140058f47658f8cfb063a_BusinesssegmentinformationNarrativeDetails"/>
  <link:roleRef roleURI="http://www.iconplc.com/role/BusinesssegmentinformationDistributionofRevenuebyGeographicalAreaDetails" xlink:type="simple" xlink:href="iclr-20200630.xsd#BusinesssegmentinformationDistributionofRevenuebyGeographicalAreaDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/BusinesssegmentinformationDistributionofRevenuebyGeographicalAreaDetails" xlink:type="extended" id="i4a0e20a23fcb47869d0e4b69162697dd_BusinesssegmentinformationDistributionofRevenuebyGeographicalAreaDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_040e7922-6fee-4b30-8598-75729bb841fb" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_60006e4a-5541-4bf7-802b-3aeb5ee13524" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_040e7922-6fee-4b30-8598-75729bb841fb" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_60006e4a-5541-4bf7-802b-3aeb5ee13524" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_62137281-a066-4977-9b92-27702753798c" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_040e7922-6fee-4b30-8598-75729bb841fb" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_62137281-a066-4977-9b92-27702753798c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementGeographicalAxis_77d1dfa0-d50f-4279-98bd-9c10b51c033c" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_StatementGeographicalAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_62137281-a066-4977-9b92-27702753798c" xlink:to="loc_srt_StatementGeographicalAxis_77d1dfa0-d50f-4279-98bd-9c10b51c033c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_77d1dfa0-d50f-4279-98bd-9c10b51c033c_default" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_77d1dfa0-d50f-4279-98bd-9c10b51c033c" xlink:to="loc_srt_SegmentGeographicalDomain_77d1dfa0-d50f-4279-98bd-9c10b51c033c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_bd493c95-796e-43bf-8940-2bcb312ccbab" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_77d1dfa0-d50f-4279-98bd-9c10b51c033c" xlink:to="loc_srt_SegmentGeographicalDomain_bd493c95-796e-43bf-8940-2bcb312ccbab" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_IE_8ef7ef37-847a-48be-a989-347818d3cd34" xlink:href="https://xbrl.sec.gov/country/2020/country-2020-01-31.xsd#country_IE"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_bd493c95-796e-43bf-8940-2bcb312ccbab" xlink:to="loc_country_IE_8ef7ef37-847a-48be-a989-347818d3cd34" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_RestOfEuropeMember_f107ddfb-be6d-4628-a32b-aeea36a961bb" xlink:href="iclr-20200630.xsd#iclr_RestOfEuropeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_bd493c95-796e-43bf-8940-2bcb312ccbab" xlink:to="loc_iclr_RestOfEuropeMember_f107ddfb-be6d-4628-a32b-aeea36a961bb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_US_8009b0a7-97c9-4a55-9795-8fdf8c1e819b" xlink:href="https://xbrl.sec.gov/country/2020/country-2020-01-31.xsd#country_US"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_bd493c95-796e-43bf-8940-2bcb312ccbab" xlink:to="loc_country_US_8009b0a7-97c9-4a55-9795-8fdf8c1e819b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_RestOfWorldMember_e4db786c-8d3a-4ca1-891d-6ebf85f7db55" xlink:href="iclr-20200630.xsd#iclr_RestOfWorldMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_bd493c95-796e-43bf-8940-2bcb312ccbab" xlink:to="loc_iclr_RestOfWorldMember_e4db786c-8d3a-4ca1-891d-6ebf85f7db55" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/BusinesssegmentinformationDistributionofIncomefromOperationsIncludingRestructuringbyGeographicalAreaDetails" xlink:type="simple" xlink:href="iclr-20200630.xsd#BusinesssegmentinformationDistributionofIncomefromOperationsIncludingRestructuringbyGeographicalAreaDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/BusinesssegmentinformationDistributionofIncomefromOperationsIncludingRestructuringbyGeographicalAreaDetails" xlink:type="extended" id="i4084839ebe97426ca7a3b4eba48bad8c_BusinesssegmentinformationDistributionofIncomefromOperationsIncludingRestructuringbyGeographicalAreaDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_61c8b5b6-650f-4903-a158-d580a9d7f7ac" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingIncomeLoss_6a62ecd0-fe31-4260-a738-bf66668bd9b3" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_OperatingIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_61c8b5b6-650f-4903-a158-d580a9d7f7ac" xlink:to="loc_us-gaap_OperatingIncomeLoss_6a62ecd0-fe31-4260-a738-bf66668bd9b3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_13d50ca5-b1f5-44b8-b6f3-1a19c9f92f87" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_61c8b5b6-650f-4903-a158-d580a9d7f7ac" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_13d50ca5-b1f5-44b8-b6f3-1a19c9f92f87" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementGeographicalAxis_e854047a-f202-43c8-8d6b-f58630d6dd97" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_StatementGeographicalAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_13d50ca5-b1f5-44b8-b6f3-1a19c9f92f87" xlink:to="loc_srt_StatementGeographicalAxis_e854047a-f202-43c8-8d6b-f58630d6dd97" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_e854047a-f202-43c8-8d6b-f58630d6dd97_default" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_e854047a-f202-43c8-8d6b-f58630d6dd97" xlink:to="loc_srt_SegmentGeographicalDomain_e854047a-f202-43c8-8d6b-f58630d6dd97_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_d5593de2-03b3-43a0-b95f-bc21089b6ce5" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_e854047a-f202-43c8-8d6b-f58630d6dd97" xlink:to="loc_srt_SegmentGeographicalDomain_d5593de2-03b3-43a0-b95f-bc21089b6ce5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_IE_6ce1ef63-6a9c-4b06-bcd1-ea94bdc7cf99" xlink:href="https://xbrl.sec.gov/country/2020/country-2020-01-31.xsd#country_IE"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_d5593de2-03b3-43a0-b95f-bc21089b6ce5" xlink:to="loc_country_IE_6ce1ef63-6a9c-4b06-bcd1-ea94bdc7cf99" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_RestOfEuropeMember_cdac30d6-a8dd-4de2-9e57-523c6bf0de45" xlink:href="iclr-20200630.xsd#iclr_RestOfEuropeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_d5593de2-03b3-43a0-b95f-bc21089b6ce5" xlink:to="loc_iclr_RestOfEuropeMember_cdac30d6-a8dd-4de2-9e57-523c6bf0de45" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_US_61b41012-ed5f-4d1d-84ab-86b7e36db1d1" xlink:href="https://xbrl.sec.gov/country/2020/country-2020-01-31.xsd#country_US"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_d5593de2-03b3-43a0-b95f-bc21089b6ce5" xlink:to="loc_country_US_61b41012-ed5f-4d1d-84ab-86b7e36db1d1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_RestOfWorldMember_2929cb30-3403-4578-a10d-ebdb4c946a26" xlink:href="iclr-20200630.xsd#iclr_RestOfWorldMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_d5593de2-03b3-43a0-b95f-bc21089b6ce5" xlink:to="loc_iclr_RestOfWorldMember_2929cb30-3403-4578-a10d-ebdb4c946a26" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/BusinesssegmentinformationDistributionofIncomefromOperationsExcludingRestructuringbyGeographicalAreaDetails" xlink:type="simple" xlink:href="iclr-20200630.xsd#BusinesssegmentinformationDistributionofIncomefromOperationsExcludingRestructuringbyGeographicalAreaDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/BusinesssegmentinformationDistributionofIncomefromOperationsExcludingRestructuringbyGeographicalAreaDetails" xlink:type="extended" id="iad95fc6ef72a4230ac6c5eb7ef971933_BusinesssegmentinformationDistributionofIncomefromOperationsExcludingRestructuringbyGeographicalAreaDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_264242c4-3a98-46b6-9989-24d548a0926a" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_OperatingIncomeLossExcludingRestructuring_47a07b47-3fe4-4bbb-9022-6c17479c223d" xlink:href="iclr-20200630.xsd#iclr_OperatingIncomeLossExcludingRestructuring"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_264242c4-3a98-46b6-9989-24d548a0926a" xlink:to="loc_iclr_OperatingIncomeLossExcludingRestructuring_47a07b47-3fe4-4bbb-9022-6c17479c223d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_7ffc8605-22d4-4809-b648-b2d43038dff8" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_264242c4-3a98-46b6-9989-24d548a0926a" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_7ffc8605-22d4-4809-b648-b2d43038dff8" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementGeographicalAxis_1aec3c64-c40e-42a6-830b-d6519e748bcd" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_StatementGeographicalAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_7ffc8605-22d4-4809-b648-b2d43038dff8" xlink:to="loc_srt_StatementGeographicalAxis_1aec3c64-c40e-42a6-830b-d6519e748bcd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_1aec3c64-c40e-42a6-830b-d6519e748bcd_default" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_1aec3c64-c40e-42a6-830b-d6519e748bcd" xlink:to="loc_srt_SegmentGeographicalDomain_1aec3c64-c40e-42a6-830b-d6519e748bcd_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_f788a45f-2eb4-4945-8de3-13c7d3b54ec6" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_1aec3c64-c40e-42a6-830b-d6519e748bcd" xlink:to="loc_srt_SegmentGeographicalDomain_f788a45f-2eb4-4945-8de3-13c7d3b54ec6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_IE_1747759c-7e02-4584-a582-e8fece416f51" xlink:href="https://xbrl.sec.gov/country/2020/country-2020-01-31.xsd#country_IE"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_f788a45f-2eb4-4945-8de3-13c7d3b54ec6" xlink:to="loc_country_IE_1747759c-7e02-4584-a582-e8fece416f51" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_RestOfEuropeMember_9623b828-a725-452d-a078-dfb009f1449d" xlink:href="iclr-20200630.xsd#iclr_RestOfEuropeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_f788a45f-2eb4-4945-8de3-13c7d3b54ec6" xlink:to="loc_iclr_RestOfEuropeMember_9623b828-a725-452d-a078-dfb009f1449d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_US_5dd57c0e-b015-47ab-8fd2-1bfb31292f1f" xlink:href="https://xbrl.sec.gov/country/2020/country-2020-01-31.xsd#country_US"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_f788a45f-2eb4-4945-8de3-13c7d3b54ec6" xlink:to="loc_country_US_5dd57c0e-b015-47ab-8fd2-1bfb31292f1f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_RestOfWorldMember_2ddacc45-936b-429a-aab6-a7090d412081" xlink:href="iclr-20200630.xsd#iclr_RestOfWorldMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_f788a45f-2eb4-4945-8de3-13c7d3b54ec6" xlink:to="loc_iclr_RestOfWorldMember_2ddacc45-936b-429a-aab6-a7090d412081" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/BusinesssegmentinformationDistributionofLonglivedAssetsNetbyGeographicalAreaDetails" xlink:type="simple" xlink:href="iclr-20200630.xsd#BusinesssegmentinformationDistributionofLonglivedAssetsNetbyGeographicalAreaDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/BusinesssegmentinformationDistributionofLonglivedAssetsNetbyGeographicalAreaDetails" xlink:type="extended" id="iefb4921a56bc410f94de36e258417ef2_BusinesssegmentinformationDistributionofLonglivedAssetsNetbyGeographicalAreaDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_7c61681b-00c7-4ff4-8121-b5b62717a94c" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NoncurrentAssets_4d8b8511-6edd-4fb8-8a53-5b31da321445" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_NoncurrentAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_7c61681b-00c7-4ff4-8121-b5b62717a94c" xlink:to="loc_us-gaap_NoncurrentAssets_4d8b8511-6edd-4fb8-8a53-5b31da321445" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_96761091-3aa8-495a-be51-702c74b0eaa8" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_7c61681b-00c7-4ff4-8121-b5b62717a94c" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_96761091-3aa8-495a-be51-702c74b0eaa8" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementGeographicalAxis_95cd5904-243d-447e-8063-875176b992e4" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_StatementGeographicalAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_96761091-3aa8-495a-be51-702c74b0eaa8" xlink:to="loc_srt_StatementGeographicalAxis_95cd5904-243d-447e-8063-875176b992e4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_95cd5904-243d-447e-8063-875176b992e4_default" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_95cd5904-243d-447e-8063-875176b992e4" xlink:to="loc_srt_SegmentGeographicalDomain_95cd5904-243d-447e-8063-875176b992e4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_7f8eeb0b-ba8a-41c6-8b72-c72427c1ae02" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_95cd5904-243d-447e-8063-875176b992e4" xlink:to="loc_srt_SegmentGeographicalDomain_7f8eeb0b-ba8a-41c6-8b72-c72427c1ae02" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_IE_c6fb7a22-91f3-4352-9323-54713d4d4565" xlink:href="https://xbrl.sec.gov/country/2020/country-2020-01-31.xsd#country_IE"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_7f8eeb0b-ba8a-41c6-8b72-c72427c1ae02" xlink:to="loc_country_IE_c6fb7a22-91f3-4352-9323-54713d4d4565" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_RestOfEuropeMember_ed0d6273-8b92-4cdf-9937-9d01451ee21c" xlink:href="iclr-20200630.xsd#iclr_RestOfEuropeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_7f8eeb0b-ba8a-41c6-8b72-c72427c1ae02" xlink:to="loc_iclr_RestOfEuropeMember_ed0d6273-8b92-4cdf-9937-9d01451ee21c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_US_4cc1f32c-ccf8-4d42-bf06-8bf3b238123b" xlink:href="https://xbrl.sec.gov/country/2020/country-2020-01-31.xsd#country_US"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_7f8eeb0b-ba8a-41c6-8b72-c72427c1ae02" xlink:to="loc_country_US_4cc1f32c-ccf8-4d42-bf06-8bf3b238123b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_RestOfWorldMember_d1b892ce-80af-4441-b5f9-dde1b94f2aa0" xlink:href="iclr-20200630.xsd#iclr_RestOfWorldMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_7f8eeb0b-ba8a-41c6-8b72-c72427c1ae02" xlink:to="loc_iclr_RestOfWorldMember_d1b892ce-80af-4441-b5f9-dde1b94f2aa0" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/BusinesssegmentinformationDistributionofDepreciationandAmortizationbyGeographicalAreaDetails" xlink:type="simple" xlink:href="iclr-20200630.xsd#BusinesssegmentinformationDistributionofDepreciationandAmortizationbyGeographicalAreaDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/BusinesssegmentinformationDistributionofDepreciationandAmortizationbyGeographicalAreaDetails" xlink:type="extended" id="ib721025ffafc472690988c86ecabe503_BusinesssegmentinformationDistributionofDepreciationandAmortizationbyGeographicalAreaDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_c1bc39b6-840c-496b-80b9-68abd75d3749" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_DepreciationAmortizationAndOperatingLeaseRightofUseAssetAmortizationExpense_2bec2dbe-cccf-4e36-b382-882bf5e6c24f" xlink:href="iclr-20200630.xsd#iclr_DepreciationAmortizationAndOperatingLeaseRightofUseAssetAmortizationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_c1bc39b6-840c-496b-80b9-68abd75d3749" xlink:to="loc_iclr_DepreciationAmortizationAndOperatingLeaseRightofUseAssetAmortizationExpense_2bec2dbe-cccf-4e36-b382-882bf5e6c24f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_88f9adbb-f9b5-4b37-bb6a-2433d2dce7da" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_c1bc39b6-840c-496b-80b9-68abd75d3749" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_88f9adbb-f9b5-4b37-bb6a-2433d2dce7da" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementGeographicalAxis_f2abda94-feab-4087-bb2f-ac7c96e01194" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_StatementGeographicalAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_88f9adbb-f9b5-4b37-bb6a-2433d2dce7da" xlink:to="loc_srt_StatementGeographicalAxis_f2abda94-feab-4087-bb2f-ac7c96e01194" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_f2abda94-feab-4087-bb2f-ac7c96e01194_default" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_f2abda94-feab-4087-bb2f-ac7c96e01194" xlink:to="loc_srt_SegmentGeographicalDomain_f2abda94-feab-4087-bb2f-ac7c96e01194_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_e9bcaa5a-ec48-4359-9b53-d0550ef87899" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_f2abda94-feab-4087-bb2f-ac7c96e01194" xlink:to="loc_srt_SegmentGeographicalDomain_e9bcaa5a-ec48-4359-9b53-d0550ef87899" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_IE_b04c4632-b90a-4b7c-bda3-9944148fdfb6" xlink:href="https://xbrl.sec.gov/country/2020/country-2020-01-31.xsd#country_IE"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_e9bcaa5a-ec48-4359-9b53-d0550ef87899" xlink:to="loc_country_IE_b04c4632-b90a-4b7c-bda3-9944148fdfb6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_RestOfEuropeMember_473418d3-251c-435b-9138-7bf84dc05413" xlink:href="iclr-20200630.xsd#iclr_RestOfEuropeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_e9bcaa5a-ec48-4359-9b53-d0550ef87899" xlink:to="loc_iclr_RestOfEuropeMember_473418d3-251c-435b-9138-7bf84dc05413" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_US_f747b543-733b-4a17-b349-acfcef35ee2d" xlink:href="https://xbrl.sec.gov/country/2020/country-2020-01-31.xsd#country_US"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_e9bcaa5a-ec48-4359-9b53-d0550ef87899" xlink:to="loc_country_US_f747b543-733b-4a17-b349-acfcef35ee2d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_RestOfWorldMember_0673051a-b02c-491f-a3fc-2db238770c8e" xlink:href="iclr-20200630.xsd#iclr_RestOfWorldMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_e9bcaa5a-ec48-4359-9b53-d0550ef87899" xlink:to="loc_iclr_RestOfWorldMember_0673051a-b02c-491f-a3fc-2db238770c8e" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/BusinesssegmentinformationDistributionofTotalAssetsbyGeographicalAreaDetails" xlink:type="simple" xlink:href="iclr-20200630.xsd#BusinesssegmentinformationDistributionofTotalAssetsbyGeographicalAreaDetails"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/BusinesssegmentinformationDistributionofTotalAssetsbyGeographicalAreaDetails" xlink:type="extended" id="i789336b2595647eeacdf11bde3f0eaf2_BusinesssegmentinformationDistributionofTotalAssetsbyGeographicalAreaDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_40409899-a12d-41d2-ada4-56553f274a31" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Assets_7283ed63-592b-4e6e-b9dc-d0e54cc5645b" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_Assets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_40409899-a12d-41d2-ada4-56553f274a31" xlink:to="loc_us-gaap_Assets_7283ed63-592b-4e6e-b9dc-d0e54cc5645b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_737f8e68-92c0-4241-b712-4771177ac5df" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_40409899-a12d-41d2-ada4-56553f274a31" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_737f8e68-92c0-4241-b712-4771177ac5df" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementGeographicalAxis_553c8e44-514e-4aec-ad80-2e03082b9228" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_StatementGeographicalAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_737f8e68-92c0-4241-b712-4771177ac5df" xlink:to="loc_srt_StatementGeographicalAxis_553c8e44-514e-4aec-ad80-2e03082b9228" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_553c8e44-514e-4aec-ad80-2e03082b9228_default" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_553c8e44-514e-4aec-ad80-2e03082b9228" xlink:to="loc_srt_SegmentGeographicalDomain_553c8e44-514e-4aec-ad80-2e03082b9228_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_a93356c9-a9bc-4b44-9dbd-a7ef59173591" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_553c8e44-514e-4aec-ad80-2e03082b9228" xlink:to="loc_srt_SegmentGeographicalDomain_a93356c9-a9bc-4b44-9dbd-a7ef59173591" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_IE_6c7e2091-1d17-4667-886c-92f0d20a0a2d" xlink:href="https://xbrl.sec.gov/country/2020/country-2020-01-31.xsd#country_IE"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_a93356c9-a9bc-4b44-9dbd-a7ef59173591" xlink:to="loc_country_IE_6c7e2091-1d17-4667-886c-92f0d20a0a2d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_RestOfEuropeMember_1c9234ff-d1f1-4ffc-997e-4f3223c229c5" xlink:href="iclr-20200630.xsd#iclr_RestOfEuropeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_a93356c9-a9bc-4b44-9dbd-a7ef59173591" xlink:to="loc_iclr_RestOfEuropeMember_1c9234ff-d1f1-4ffc-997e-4f3223c229c5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_US_ebbb1689-db03-4eeb-9460-d8f3b386fdb0" xlink:href="https://xbrl.sec.gov/country/2020/country-2020-01-31.xsd#country_US"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_a93356c9-a9bc-4b44-9dbd-a7ef59173591" xlink:to="loc_country_US_ebbb1689-db03-4eeb-9460-d8f3b386fdb0" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_RestOfWorldMember_bdfe59b6-c5c2-414e-b77a-32772ddfc6f5" xlink:href="iclr-20200630.xsd#iclr_RestOfWorldMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_a93356c9-a9bc-4b44-9dbd-a7ef59173591" xlink:to="loc_iclr_RestOfWorldMember_bdfe59b6-c5c2-414e-b77a-32772ddfc6f5" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/Impactofchangeinaccountingpolicies" xlink:type="simple" xlink:href="iclr-20200630.xsd#Impactofchangeinaccountingpolicies"/>
  <link:definitionLink xlink:role="http://www.iconplc.com/role/Impactofchangeinaccountingpolicies" xlink:type="extended" id="i7ef037897ab94c6aa184eae3377ce7b9_Impactofchangeinaccountingpolicies"/>
</link:linkbase>
