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

<!--XBRL Document Created with the Workiva Platform-->
<!--Copyright 2026 Workiva-->
<!--r:019f28d8-38bb-7759-8a91-62e341cce7c4,g:fa1a9941-46c6-49a7-b8c6-1af1c7c56bd9-->
<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.archcapgroup.com/role/DocumentandEntityInformation" xlink:type="simple" xlink:href="acgl-20260630.xsd#DocumentandEntityInformation"/>
  <link:definitionLink xlink:role="http://www.archcapgroup.com/role/DocumentandEntityInformation" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityListingsLineItems_019f28d8-3bfe-7458-a682-49c33deac3bd" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityListingsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityListingsTable_019f28d8-3bfe-7e72-b319-94d682e63aff" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityListingsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_dei_EntityListingsLineItems_019f28d8-3bfe-7458-a682-49c33deac3bd" xlink:to="loc_dei_EntityListingsTable_019f28d8-3bfe-7e72-b319-94d682e63aff" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_019f28d8-3bfe-7a59-8d99-1e33100d2b22" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_dei_EntityListingsTable_019f28d8-3bfe-7e72-b319-94d682e63aff" xlink:to="loc_us-gaap_StatementClassOfStockAxis_019f28d8-3bfe-7a59-8d99-1e33100d2b22" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019f28d8-3bfe-7a59-8d99-1e33100d2b22_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019f28d8-3bfe-7a59-8d99-1e33100d2b22" xlink:to="loc_us-gaap_ClassOfStockDomain_019f28d8-3bfe-7a59-8d99-1e33100d2b22_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019f28d8-3bfe-754e-8c79-1923f83b0e72" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019f28d8-3bfe-7a59-8d99-1e33100d2b22" xlink:to="loc_us-gaap_ClassOfStockDomain_019f28d8-3bfe-754e-8c79-1923f83b0e72" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_019f28d8-3bfe-7727-abdd-93d8033dab70" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_019f28d8-3bfe-754e-8c79-1923f83b0e72" xlink:to="loc_us-gaap_CommonStockMember_019f28d8-3bfe-7727-abdd-93d8033dab70" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_SeriesFDepositaryShareEquivalentMember_019f28d8-3bfe-7fa9-81ee-4c2b5896e5b0" xlink:href="acgl-20260630.xsd#acgl_SeriesFDepositaryShareEquivalentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_019f28d8-3bfe-754e-8c79-1923f83b0e72" xlink:to="loc_acgl_SeriesFDepositaryShareEquivalentMember_019f28d8-3bfe-7fa9-81ee-4c2b5896e5b0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_SeriesGDepositaryShareEquivalentMember_019f28d8-3bfe-786e-95f9-46d07b361747" xlink:href="acgl-20260630.xsd#acgl_SeriesGDepositaryShareEquivalentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_019f28d8-3bfe-754e-8c79-1923f83b0e72" xlink:to="loc_acgl_SeriesGDepositaryShareEquivalentMember_019f28d8-3bfe-786e-95f9-46d07b361747" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_Security12bTitle_019f28d8-3bfe-7684-ae38-5a95cecafa41" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_Security12bTitle"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityListingsLineItems_019f28d8-3bfe-7458-a682-49c33deac3bd" xlink:to="loc_dei_Security12bTitle_019f28d8-3bfe-7684-ae38-5a95cecafa41" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_TradingSymbol_019f28d8-3bfe-7250-b0be-552bad717801" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_TradingSymbol"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityListingsLineItems_019f28d8-3bfe-7458-a682-49c33deac3bd" xlink:to="loc_dei_TradingSymbol_019f28d8-3bfe-7250-b0be-552bad717801" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_SecurityExchangeName_019f28d8-3bfe-7b02-8d3a-f1250e6fd127" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_SecurityExchangeName"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityListingsLineItems_019f28d8-3bfe-7458-a682-49c33deac3bd" xlink:to="loc_dei_SecurityExchangeName_019f28d8-3bfe-7b02-8d3a-f1250e6fd127" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.archcapgroup.com/role/UnauditedConsolidatedBalanceSheets" xlink:type="simple" xlink:href="acgl-20260630.xsd#UnauditedConsolidatedBalanceSheets"/>
  <link:definitionLink xlink:role="http://www.archcapgroup.com/role/UnauditedConsolidatedBalanceSheets" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_019f28d8-3bfe-7023-b3a7-d20528514262" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_019f28d8-3bfe-7167-8617-a848f4fbddfb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_019f28d8-3bfe-7023-b3a7-d20528514262" xlink:to="loc_us-gaap_StatementTable_019f28d8-3bfe-7167-8617-a848f4fbddfb" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_019f28d8-3bfe-74c5-a4d1-5afb891f60b5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019f28d8-3bfe-7167-8617-a848f4fbddfb" xlink:to="loc_us-gaap_FinancialInstrumentAxis_019f28d8-3bfe-74c5-a4d1-5afb891f60b5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3bfe-74c5-a4d1-5afb891f60b5_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019f28d8-3bfe-74c5-a4d1-5afb891f60b5" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3bfe-74c5-a4d1-5afb891f60b5_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3bfe-75b1-9aa2-1e67c7e80803" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019f28d8-3bfe-74c5-a4d1-5afb891f60b5" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3bfe-75b1-9aa2-1e67c7e80803" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FixedMaturitiesMember_019f28d8-3bfe-704e-b5ee-854cc1df5962" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FixedMaturitiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3bfe-75b1-9aa2-1e67c7e80803" xlink:to="loc_us-gaap_FixedMaturitiesMember_019f28d8-3bfe-704e-b5ee-854cc1df5962" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortTermInvestmentsMember_019f28d8-3bfe-7fff-88c5-4394e17dcc4a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShortTermInvestmentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3bfe-75b1-9aa2-1e67c7e80803" xlink:to="loc_us-gaap_ShortTermInvestmentsMember_019f28d8-3bfe-7fff-88c5-4394e17dcc4a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsAbstract_019f28d8-3bfe-7007-acb9-4ad655d36d10" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AssetsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019f28d8-3bfe-7023-b3a7-d20528514262" xlink:to="loc_us-gaap_AssetsAbstract_019f28d8-3bfe-7007-acb9-4ad655d36d10" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentsAbstract_019f28d8-3bfe-7211-9f19-bac45ecaa11d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InvestmentsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019f28d8-3bfe-7007-acb9-4ad655d36d10" xlink:to="loc_us-gaap_InvestmentsAbstract_019f28d8-3bfe-7211-9f19-bac45ecaa11d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_019f28d8-3bfe-7908-8944-0ba609085091" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleSecuritiesDebtSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentsAbstract_019f28d8-3bfe-7211-9f19-bac45ecaa11d" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_019f28d8-3bfe-7908-8944-0ba609085091" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesFvNiCurrentAndNoncurrent_019f28d8-3bfe-74f9-af7a-b2cfbc287825" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquitySecuritiesFvNiCurrentAndNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentsAbstract_019f28d8-3bfe-7211-9f19-bac45ecaa11d" xlink:to="loc_us-gaap_EquitySecuritiesFvNiCurrentAndNoncurrent_019f28d8-3bfe-74f9-af7a-b2cfbc287825" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherInvestments_019f28d8-3bfe-7324-98d2-48ada94e613a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherInvestments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentsAbstract_019f28d8-3bfe-7211-9f19-bac45ecaa11d" xlink:to="loc_us-gaap_OtherInvestments_019f28d8-3bfe-7324-98d2-48ada94e613a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityMethodInvestments_019f28d8-3bfe-7066-85be-412b2446dc28" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityMethodInvestments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentsAbstract_019f28d8-3bfe-7211-9f19-bac45ecaa11d" xlink:to="loc_us-gaap_EquityMethodInvestments_019f28d8-3bfe-7066-85be-412b2446dc28" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Investments_019f28d8-3bfe-7e3e-8044-15a4e56230c2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_Investments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentsAbstract_019f28d8-3bfe-7211-9f19-bac45ecaa11d" xlink:to="loc_us-gaap_Investments_019f28d8-3bfe-7e3e-8044-15a4e56230c2" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_019f28d8-3bfe-7c9f-b9ee-d92db6f3d52b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashAndCashEquivalentsAtCarryingValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019f28d8-3bfe-7007-acb9-4ad655d36d10" xlink:to="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_019f28d8-3bfe-7c9f-b9ee-d92db6f3d52b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccruedInvestmentIncomeReceivable_019f28d8-3bfe-7af1-8ed9-cdd7e2e67af4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccruedInvestmentIncomeReceivable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019f28d8-3bfe-7007-acb9-4ad655d36d10" xlink:to="loc_us-gaap_AccruedInvestmentIncomeReceivable_019f28d8-3bfe-7af1-8ed9-cdd7e2e67af4" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_InvestmentInOperatingAffiliateEquityMethod_019f28d8-3bfe-7bca-a8a5-9f15ccdd612b" xlink:href="acgl-20260630.xsd#acgl_InvestmentInOperatingAffiliateEquityMethod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019f28d8-3bfe-7007-acb9-4ad655d36d10" xlink:to="loc_acgl_InvestmentInOperatingAffiliateEquityMethod_019f28d8-3bfe-7bca-a8a5-9f15ccdd612b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PremiumsReceivableAtCarryingValue_019f28d8-3bfe-7e3a-a36e-8d51c6f8288e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PremiumsReceivableAtCarryingValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019f28d8-3bfe-7007-acb9-4ad655d36d10" xlink:to="loc_us-gaap_PremiumsReceivableAtCarryingValue_019f28d8-3bfe-7e3a-a36e-8d51c6f8288e" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReinsuranceRecoverablesOnPaidAndUnpaidLosses_019f28d8-3bfe-7408-a00b-f2d82bdacec6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ReinsuranceRecoverablesOnPaidAndUnpaidLosses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019f28d8-3bfe-7007-acb9-4ad655d36d10" xlink:to="loc_us-gaap_ReinsuranceRecoverablesOnPaidAndUnpaidLosses_019f28d8-3bfe-7408-a00b-f2d82bdacec6" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_ContractholderDeductiblesReceivable_019f28d8-3bfe-7c45-b8b3-b01aa19d2693" xlink:href="acgl-20260630.xsd#acgl_ContractholderDeductiblesReceivable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019f28d8-3bfe-7007-acb9-4ad655d36d10" xlink:to="loc_acgl_ContractholderDeductiblesReceivable_019f28d8-3bfe-7c45-b8b3-b01aa19d2693" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PrepaidReinsurancePremiums_019f28d8-3bfe-725c-a42b-693ea771deac" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PrepaidReinsurancePremiums"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019f28d8-3bfe-7007-acb9-4ad655d36d10" xlink:to="loc_us-gaap_PrepaidReinsurancePremiums_019f28d8-3bfe-725c-a42b-693ea771deac" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredPolicyAcquisitionCosts_019f28d8-3bfe-79d0-a346-802675207d7d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DeferredPolicyAcquisitionCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019f28d8-3bfe-7007-acb9-4ad655d36d10" xlink:to="loc_us-gaap_DeferredPolicyAcquisitionCosts_019f28d8-3bfe-79d0-a346-802675207d7d" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReceivablesFromBrokersDealersAndClearingOrganizations_019f28d8-3bfe-76a2-8e17-fa7bddbf5074" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ReceivablesFromBrokersDealersAndClearingOrganizations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019f28d8-3bfe-7007-acb9-4ad655d36d10" xlink:to="loc_us-gaap_ReceivablesFromBrokersDealersAndClearingOrganizations_019f28d8-3bfe-76a2-8e17-fa7bddbf5074" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IntangibleAssetsNetIncludingGoodwill_019f28d8-3bfe-71c6-b062-f10decbf36a2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IntangibleAssetsNetIncludingGoodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019f28d8-3bfe-7007-acb9-4ad655d36d10" xlink:to="loc_us-gaap_IntangibleAssetsNetIncludingGoodwill_019f28d8-3bfe-71c6-b062-f10decbf36a2" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherAssets_019f28d8-3bfe-7525-99a6-d8bbebbbfc35" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019f28d8-3bfe-7007-acb9-4ad655d36d10" xlink:to="loc_us-gaap_OtherAssets_019f28d8-3bfe-7525-99a6-d8bbebbbfc35" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Assets_019f28d8-3bfe-711c-8040-82ffeee1328c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_Assets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019f28d8-3bfe-7007-acb9-4ad655d36d10" xlink:to="loc_us-gaap_Assets_019f28d8-3bfe-711c-8040-82ffeee1328c" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesAbstract_019f28d8-3bfe-7559-ab44-b5967dbd4188" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LiabilitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019f28d8-3bfe-7023-b3a7-d20528514262" xlink:to="loc_us-gaap_LiabilitiesAbstract_019f28d8-3bfe-7559-ab44-b5967dbd4188" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense_019f28d8-3bfe-7405-803c-37e4ed5245f5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAbstract_019f28d8-3bfe-7559-ab44-b5967dbd4188" xlink:to="loc_us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense_019f28d8-3bfe-7405-803c-37e4ed5245f5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnearnedPremiums_019f28d8-3bfe-758f-a534-a85120613ed6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_UnearnedPremiums"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAbstract_019f28d8-3bfe-7559-ab44-b5967dbd4188" xlink:to="loc_us-gaap_UnearnedPremiums_019f28d8-3bfe-758f-a534-a85120613ed6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReinsurancePayable_019f28d8-3bfe-7114-9cc6-394111b1bcda" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ReinsurancePayable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAbstract_019f28d8-3bfe-7559-ab44-b5967dbd4188" xlink:to="loc_us-gaap_ReinsurancePayable_019f28d8-3bfe-7114-9cc6-394111b1bcda" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_ContractHolderPayables_019f28d8-3bfe-70b3-ab03-bc6b4fc501aa" xlink:href="acgl-20260630.xsd#acgl_ContractHolderPayables"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAbstract_019f28d8-3bfe-7559-ab44-b5967dbd4188" xlink:to="loc_acgl_ContractHolderPayables_019f28d8-3bfe-70b3-ab03-bc6b4fc501aa" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_CollateralHeldForInsuredObligations_019f28d8-3bfe-775b-b8c9-ce7ea4803615" xlink:href="acgl-20260630.xsd#acgl_CollateralHeldForInsuredObligations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAbstract_019f28d8-3bfe-7559-ab44-b5967dbd4188" xlink:to="loc_acgl_CollateralHeldForInsuredObligations_019f28d8-3bfe-775b-b8c9-ce7ea4803615" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SeniorLongTermNotes_019f28d8-3bfe-7023-9441-a2ba85a88526" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SeniorLongTermNotes"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAbstract_019f28d8-3bfe-7559-ab44-b5967dbd4188" xlink:to="loc_us-gaap_SeniorLongTermNotes_019f28d8-3bfe-7023-9441-a2ba85a88526" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_PayablesToBrokerDealersAndClearingOrganizations_019f28d8-3bfe-7404-91ff-d53c845a2e79" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_PayablesToBrokerDealersAndClearingOrganizations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAbstract_019f28d8-3bfe-7559-ab44-b5967dbd4188" xlink:to="loc_srt_PayablesToBrokerDealersAndClearingOrganizations_019f28d8-3bfe-7404-91ff-d53c845a2e79" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherLiabilities_019f28d8-3bfe-7310-ab8c-45da371bbf70" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAbstract_019f28d8-3bfe-7559-ab44-b5967dbd4188" xlink:to="loc_us-gaap_OtherLiabilities_019f28d8-3bfe-7310-ab8c-45da371bbf70" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Liabilities_019f28d8-3bfe-7c1d-8ef3-99be794b058a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_Liabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAbstract_019f28d8-3bfe-7559-ab44-b5967dbd4188" xlink:to="loc_us-gaap_Liabilities_019f28d8-3bfe-7c1d-8ef3-99be794b058a" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommitmentsAndContingencies_019f28d8-3bfe-7c60-a344-e3912c83b6f4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommitmentsAndContingencies"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019f28d8-3bfe-7023-b3a7-d20528514262" xlink:to="loc_us-gaap_CommitmentsAndContingencies_019f28d8-3bfe-7c60-a344-e3912c83b6f4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquityAbstract_019f28d8-3bfe-7532-89de-f96bfad363e6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquityAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019f28d8-3bfe-7023-b3a7-d20528514262" xlink:to="loc_us-gaap_StockholdersEquityAbstract_019f28d8-3bfe-7532-89de-f96bfad363e6" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockValue_019f28d8-3bfe-74d5-aa09-65f1308b103b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PreferredStockValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_019f28d8-3bfe-7532-89de-f96bfad363e6" xlink:to="loc_us-gaap_PreferredStockValue_019f28d8-3bfe-74d5-aa09-65f1308b103b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockValue_019f28d8-3bfe-7718-89eb-3601f11cded5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_019f28d8-3bfe-7532-89de-f96bfad363e6" xlink:to="loc_us-gaap_CommonStockValue_019f28d8-3bfe-7718-89eb-3601f11cded5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalCommonStock_019f28d8-3bfe-7b5b-bcc0-abdea59b2228" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AdditionalPaidInCapitalCommonStock"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_019f28d8-3bfe-7532-89de-f96bfad363e6" xlink:to="loc_us-gaap_AdditionalPaidInCapitalCommonStock_019f28d8-3bfe-7b5b-bcc0-abdea59b2228" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsAccumulatedDeficit_019f28d8-3bfe-7c79-8fd3-68ca97925589" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RetainedEarningsAccumulatedDeficit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_019f28d8-3bfe-7532-89de-f96bfad363e6" xlink:to="loc_us-gaap_RetainedEarningsAccumulatedDeficit_019f28d8-3bfe-7c79-8fd3-68ca97925589" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax_019f28d8-3bfe-71d7-8b8a-79ba76b393ef" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_019f28d8-3bfe-7532-89de-f96bfad363e6" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax_019f28d8-3bfe-71d7-8b8a-79ba76b393ef" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockValue_019f28d8-3bfe-76e1-ba2f-c231910ef890" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TreasuryStockValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_019f28d8-3bfe-7532-89de-f96bfad363e6" xlink:to="loc_us-gaap_TreasuryStockValue_019f28d8-3bfe-76e1-ba2f-c231910ef890" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_019f28d8-3bfe-7156-b3fb-fbd340676f94" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_019f28d8-3bfe-7532-89de-f96bfad363e6" xlink:to="loc_us-gaap_StockholdersEquity_019f28d8-3bfe-7156-b3fb-fbd340676f94" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesAndStockholdersEquity_019f28d8-3bfe-7aeb-8be4-f24b1daf3a1d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LiabilitiesAndStockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019f28d8-3bfe-7023-b3a7-d20528514262" xlink:to="loc_us-gaap_LiabilitiesAndStockholdersEquity_019f28d8-3bfe-7aeb-8be4-f24b1daf3a1d" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.archcapgroup.com/role/UnauditedConsolidatedBalanceSheetsParentheticals" xlink:type="simple" xlink:href="acgl-20260630.xsd#UnauditedConsolidatedBalanceSheetsParentheticals"/>
  <link:definitionLink xlink:role="http://www.archcapgroup.com/role/UnauditedConsolidatedBalanceSheetsParentheticals" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_019f28d8-3bfe-77c3-8046-b348744df486" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_019f28d8-3bfe-7c31-a276-85de6874329d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_019f28d8-3bfe-77c3-8046-b348744df486" xlink:to="loc_us-gaap_StatementTable_019f28d8-3bfe-7c31-a276-85de6874329d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_019f28d8-3bfe-75e7-8f29-2bc47dccfa65" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019f28d8-3bfe-7c31-a276-85de6874329d" xlink:to="loc_us-gaap_FinancialInstrumentAxis_019f28d8-3bfe-75e7-8f29-2bc47dccfa65" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3bfe-75e7-8f29-2bc47dccfa65_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019f28d8-3bfe-75e7-8f29-2bc47dccfa65" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3bfe-75e7-8f29-2bc47dccfa65_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3bfe-77d5-ac2b-204905854c79" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019f28d8-3bfe-75e7-8f29-2bc47dccfa65" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3bfe-77d5-ac2b-204905854c79" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FixedMaturitiesMember_019f28d8-3bfe-7fbb-a401-4a3bd00c1cb8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FixedMaturitiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3bfe-77d5-ac2b-204905854c79" xlink:to="loc_us-gaap_FixedMaturitiesMember_019f28d8-3bfe-7fbb-a401-4a3bd00c1cb8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortTermInvestmentsMember_019f28d8-3bfe-77c3-b6cb-f998e9dfc67c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShortTermInvestmentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3bfe-77d5-ac2b-204905854c79" xlink:to="loc_us-gaap_ShortTermInvestmentsMember_019f28d8-3bfe-77c3-b6cb-f998e9dfc67c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis_019f28d8-3bfe-7c11-9528-11c39b40253d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019f28d8-3bfe-77c3-8046-b348744df486" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis_019f28d8-3bfe-7c11-9528-11c39b40253d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleAllowanceForCreditLoss_019f28d8-3bfe-7a0f-9fb2-2b7d007e19d7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleAllowanceForCreditLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019f28d8-3bfe-77c3-8046-b348744df486" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleAllowanceForCreditLoss_019f28d8-3bfe-7a0f-9fb2-2b7d007e19d7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PremiumsReceivableAllowanceForDoubtfulAccounts_019f28d8-3bfe-7b83-900c-d62a6bac0d2c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PremiumsReceivableAllowanceForDoubtfulAccounts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019f28d8-3bfe-77c3-8046-b348744df486" xlink:to="loc_us-gaap_PremiumsReceivableAllowanceForDoubtfulAccounts_019f28d8-3bfe-7b83-900c-d62a6bac0d2c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReinsuranceRecoverablesAllowance_019f28d8-3bfe-7a43-9493-597d720c3efe" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ReinsuranceRecoverablesAllowance"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019f28d8-3bfe-77c3-8046-b348744df486" xlink:to="loc_us-gaap_ReinsuranceRecoverablesAllowance_019f28d8-3bfe-7a43-9493-597d720c3efe" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_ContractholderReceivablesAllowanceForCreditLoss_019f28d8-3bfe-73fb-8846-8f8ffbc5d5b5" xlink:href="acgl-20260630.xsd#acgl_ContractholderReceivablesAllowanceForCreditLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019f28d8-3bfe-77c3-8046-b348744df486" xlink:to="loc_acgl_ContractholderReceivablesAllowanceForCreditLoss_019f28d8-3bfe-73fb-8846-8f8ffbc5d5b5" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockParOrStatedValuePerShare_019f28d8-3bfe-7029-91be-cbb404a1ffc2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockParOrStatedValuePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019f28d8-3bfe-77c3-8046-b348744df486" xlink:to="loc_us-gaap_CommonStockParOrStatedValuePerShare_019f28d8-3bfe-7029-91be-cbb404a1ffc2" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesIssued_019f28d8-3bfe-7ea6-aebe-fcbe49038490" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockSharesIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019f28d8-3bfe-77c3-8046-b348744df486" xlink:to="loc_us-gaap_CommonStockSharesIssued_019f28d8-3bfe-7ea6-aebe-fcbe49038490" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockCommonShares_019f28d8-3bfe-72f5-9fc6-df5cc1c749db" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TreasuryStockCommonShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019f28d8-3bfe-77c3-8046-b348744df486" xlink:to="loc_us-gaap_TreasuryStockCommonShares_019f28d8-3bfe-72f5-9fc6-df5cc1c749db" xlink:type="arc" order="7"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.archcapgroup.com/role/UnauditedConsolidatedStatementsofChangesinShareholdersEquity" xlink:type="simple" xlink:href="acgl-20260630.xsd#UnauditedConsolidatedStatementsofChangesinShareholdersEquity"/>
  <link:definitionLink xlink:role="http://www.archcapgroup.com/role/UnauditedConsolidatedStatementsofChangesinShareholdersEquity" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_019f28d8-3bfe-7eed-a69e-dfb16178705d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_019f28d8-3bfe-7dc0-831d-04a5e8daa7ac" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_019f28d8-3bfe-7eed-a69e-dfb16178705d" xlink:to="loc_us-gaap_StatementTable_019f28d8-3bfe-7dc0-831d-04a5e8daa7ac" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_019f28d8-3bfe-7332-b10f-87810d401f03" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019f28d8-3bfe-7dc0-831d-04a5e8daa7ac" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_019f28d8-3bfe-7332-b10f-87810d401f03" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019f28d8-3bfe-7332-b10f-87810d401f03_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019f28d8-3bfe-7332-b10f-87810d401f03" xlink:to="loc_us-gaap_EquityComponentDomain_019f28d8-3bfe-7332-b10f-87810d401f03_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019f28d8-3bfe-7ea2-a20d-4cf4c7874602" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019f28d8-3bfe-7332-b10f-87810d401f03" xlink:to="loc_us-gaap_EquityComponentDomain_019f28d8-3bfe-7ea2-a20d-4cf4c7874602" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockMember_019f28d8-3bfe-7ac7-96a2-e5cdddf9af48" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019f28d8-3bfe-7ea2-a20d-4cf4c7874602" xlink:to="loc_us-gaap_PreferredStockMember_019f28d8-3bfe-7ac7-96a2-e5cdddf9af48" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_019f28d8-3bfe-7713-8add-a865b805a4e8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019f28d8-3bfe-7ea2-a20d-4cf4c7874602" xlink:to="loc_us-gaap_CommonStockMember_019f28d8-3bfe-7713-8add-a865b805a4e8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_019f28d8-3bfe-704f-8acb-ee5c4ae31de4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AdditionalPaidInCapitalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019f28d8-3bfe-7ea2-a20d-4cf4c7874602" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_019f28d8-3bfe-704f-8acb-ee5c4ae31de4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsMember_019f28d8-3bfe-757c-b5a9-ca3dff947f07" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RetainedEarningsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019f28d8-3bfe-7ea2-a20d-4cf4c7874602" xlink:to="loc_us-gaap_RetainedEarningsMember_019f28d8-3bfe-757c-b5a9-ca3dff947f07" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_019f28d8-3bfe-7383-b123-931cfea5a185" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019f28d8-3bfe-7ea2-a20d-4cf4c7874602" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_019f28d8-3bfe-7383-b123-931cfea5a185" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember_019f28d8-3bfe-76ec-a527-7f1cec03f122" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_019f28d8-3bfe-7383-b123-931cfea5a185" xlink:to="loc_us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember_019f28d8-3bfe-76ec-a527-7f1cec03f122" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedTranslationAdjustmentMember_019f28d8-3bfe-7112-a8ec-ce3ddcd5cca8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedTranslationAdjustmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_019f28d8-3bfe-7383-b123-931cfea5a185" xlink:to="loc_us-gaap_AccumulatedTranslationAdjustmentMember_019f28d8-3bfe-7112-a8ec-ce3ddcd5cca8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockCommonMember_019f28d8-3bfe-7614-831a-6f948755242c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TreasuryStockCommonMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019f28d8-3bfe-7ea2-a20d-4cf4c7874602" xlink:to="loc_us-gaap_TreasuryStockCommonMember_019f28d8-3bfe-7614-831a-6f948755242c" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_019f28d8-3bfe-7fa9-93c3-f9d53106cf3b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019f28d8-3bfe-7eed-a69e-dfb16178705d" xlink:to="loc_us-gaap_StockholdersEquity_019f28d8-3bfe-7fa9-93c3-f9d53106cf3b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_019f28d8-3bfe-7c8b-8376-64080174e90c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019f28d8-3bfe-7eed-a69e-dfb16178705d" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_019f28d8-3bfe-7c8b-8376-64080174e90c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalOther_019f28d8-3bfe-707f-a933-8ea798c5a040" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalOther"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019f28d8-3bfe-7eed-a69e-dfb16178705d" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalOther_019f28d8-3bfe-707f-a933-8ea798c5a040" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_019f28d8-3bfe-72d0-8113-bfdba5e327a3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019f28d8-3bfe-7eed-a69e-dfb16178705d" xlink:to="loc_us-gaap_NetIncomeLoss_019f28d8-3bfe-72d0-8113-bfdba5e327a3" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DividendsPreferredStockCash_019f28d8-3bfe-77b4-bdbe-29232d32201a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DividendsPreferredStockCash"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019f28d8-3bfe-7eed-a69e-dfb16178705d" xlink:to="loc_us-gaap_DividendsPreferredStockCash_019f28d8-3bfe-77b4-bdbe-29232d32201a" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_019f28d8-3bfe-79c7-86f7-2df3c3470794" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019f28d8-3bfe-7eed-a69e-dfb16178705d" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_019f28d8-3bfe-79c7-86f7-2df3c3470794" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockValueAcquiredCostMethod_019f28d8-3bfe-7462-b696-d5217f66f96a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TreasuryStockValueAcquiredCostMethod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019f28d8-3bfe-7eed-a69e-dfb16178705d" xlink:to="loc_us-gaap_TreasuryStockValueAcquiredCostMethod_019f28d8-3bfe-7462-b696-d5217f66f96a" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_019f28d8-3bfe-7d05-bfb6-7bdcae737f2a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquity"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.archcapgroup.com/role/AcquisitionSummaryofallocationofpurchasepriceDetails" xlink:type="simple" xlink:href="acgl-20260630.xsd#AcquisitionSummaryofallocationofpurchasepriceDetails"/>
  <link:definitionLink xlink:role="http://www.archcapgroup.com/role/AcquisitionSummaryofallocationofpurchasepriceDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_019f28d8-3bff-764a-a1ba-720b583dca0e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessAcquisitionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_019f28d8-3bff-7bdd-8e8a-5d26330fa7c8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019f28d8-3bff-764a-a1ba-720b583dca0e" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_019f28d8-3bff-7bdd-8e8a-5d26330fa7c8" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_019f28d8-3bff-7148-858c-615d3939e9fe" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_019f28d8-3bff-7bdd-8e8a-5d26330fa7c8" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_019f28d8-3bff-7148-858c-615d3939e9fe" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019f28d8-3bff-7148-858c-615d3939e9fe_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_019f28d8-3bff-7148-858c-615d3939e9fe" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019f28d8-3bff-7148-858c-615d3939e9fe_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019f28d8-3bff-7cb3-8a42-a18756f7e343" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_019f28d8-3bff-7148-858c-615d3939e9fe" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019f28d8-3bff-7cb3-8a42-a18756f7e343" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_USMidCorpAndEntertainmentMember_019f28d8-3bff-742d-a589-83e84dc90f02" xlink:href="acgl-20260630.xsd#acgl_USMidCorpAndEntertainmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019f28d8-3bff-7cb3-8a42-a18756f7e343" xlink:to="loc_acgl_USMidCorpAndEntertainmentMember_019f28d8-3bff-742d-a589-83e84dc90f02" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019f28d8-3bff-7411-87e2-545038aefee8" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_019f28d8-3bff-7bdd-8e8a-5d26330fa7c8" xlink:to="loc_srt_RangeAxis_019f28d8-3bff-7411-87e2-545038aefee8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019f28d8-3bff-7411-87e2-545038aefee8_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019f28d8-3bff-7411-87e2-545038aefee8" xlink:to="loc_srt_RangeMember_019f28d8-3bff-7411-87e2-545038aefee8_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019f28d8-3bff-7413-b0e6-202978d4c818" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019f28d8-3bff-7411-87e2-545038aefee8" xlink:to="loc_srt_RangeMember_019f28d8-3bff-7413-b0e6-202978d4c818" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019f28d8-3bff-7fbc-b180-e6ec84b0d995" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019f28d8-3bff-7413-b0e6-202978d4c818" xlink:to="loc_srt_MinimumMember_019f28d8-3bff-7fbc-b180-e6ec84b0d995" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019f28d8-3bff-7c3a-a34b-f670561b6ce3" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019f28d8-3bff-7413-b0e6-202978d4c818" xlink:to="loc_srt_MaximumMember_019f28d8-3bff-7c3a-a34b-f670561b6ce3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019f28d8-3bff-7cf3-8893-fd6af5fdb7f8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_019f28d8-3bff-7bdd-8e8a-5d26330fa7c8" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019f28d8-3bff-7cf3-8893-fd6af5fdb7f8" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019f28d8-3bff-7cf3-8893-fd6af5fdb7f8_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019f28d8-3bff-7cf3-8893-fd6af5fdb7f8" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019f28d8-3bff-7cf3-8893-fd6af5fdb7f8_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019f28d8-3bff-7ad5-9abb-d8c4ae6a635c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019f28d8-3bff-7cf3-8893-fd6af5fdb7f8" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019f28d8-3bff-7ad5-9abb-d8c4ae6a635c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DistributionRightsMember_019f28d8-3bff-7227-b386-3c62f3c703bc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DistributionRightsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019f28d8-3bff-7ad5-9abb-d8c4ae6a635c" xlink:to="loc_us-gaap_DistributionRightsMember_019f28d8-3bff-7227-b386-3c62f3c703bc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InsuranceContractsAcquiredInBusinessCombinationMember_019f28d8-3bff-77a0-9f5b-fe806209b8e5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InsuranceContractsAcquiredInBusinessCombinationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019f28d8-3bff-7ad5-9abb-d8c4ae6a635c" xlink:to="loc_us-gaap_InsuranceContractsAcquiredInBusinessCombinationMember_019f28d8-3bff-77a0-9f5b-fe806209b8e5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherIntangibleAssetsMember_019f28d8-3bff-7606-92f9-93e8023addab" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherIntangibleAssetsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019f28d8-3bff-7ad5-9abb-d8c4ae6a635c" xlink:to="loc_us-gaap_OtherIntangibleAssetsMember_019f28d8-3bff-7606-92f9-93e8023addab" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByLiabilityClassAxis_019f28d8-3bff-74ea-bae4-d8cdef57d93a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByLiabilityClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_019f28d8-3bff-7bdd-8e8a-5d26330fa7c8" xlink:to="loc_us-gaap_FairValueByLiabilityClassAxis_019f28d8-3bff-74ea-bae4-d8cdef57d93a" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_019f28d8-3bff-74ea-bae4-d8cdef57d93a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByLiabilityClassAxis_019f28d8-3bff-74ea-bae4-d8cdef57d93a" xlink:to="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_019f28d8-3bff-74ea-bae4-d8cdef57d93a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_019f28d8-3bff-79ea-b881-1837caa5e35c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByLiabilityClassAxis_019f28d8-3bff-74ea-bae4-d8cdef57d93a" xlink:to="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_019f28d8-3bff-79ea-b881-1837caa5e35c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_ReservesForLossesAndLossAdjustmentExpensesMember_019f28d8-3bff-7f94-9628-4d5503ea00db" xlink:href="acgl-20260630.xsd#acgl_ReservesForLossesAndLossAdjustmentExpensesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_019f28d8-3bff-79ea-b881-1837caa5e35c" xlink:to="loc_acgl_ReservesForLossesAndLossAdjustmentExpensesMember_019f28d8-3bff-7f94-9628-4d5503ea00db" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationConsiderationTransferredAbstract_019f28d8-3bff-70e9-9a0d-76823fc9a688" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessCombinationConsiderationTransferredAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019f28d8-3bff-764a-a1ba-720b583dca0e" xlink:to="loc_us-gaap_BusinessCombinationConsiderationTransferredAbstract_019f28d8-3bff-70e9-9a0d-76823fc9a688" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquireBusinessesGross_019f28d8-3bff-7e48-8f47-ba53c38d97b6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PaymentsToAcquireBusinessesGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationConsiderationTransferredAbstract_019f28d8-3bff-70e9-9a0d-76823fc9a688" xlink:to="loc_us-gaap_PaymentsToAcquireBusinessesGross_019f28d8-3bff-7e48-8f47-ba53c38d97b6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssetsAbstract_019f28d8-3bff-7d9c-b0df-f0dd9e7c8b31" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssetsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019f28d8-3bff-764a-a1ba-720b583dca0e" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssetsAbstract_019f28d8-3bff-7d9c-b0df-f0dd9e7c8b31" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndInvestments_019f28d8-3bff-7eb4-b9bc-3c2dd13bc97e" xlink:href="acgl-20260630.xsd#acgl_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndInvestments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssetsAbstract_019f28d8-3bff-7d9c-b0df-f0dd9e7c8b31" xlink:to="loc_acgl_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndInvestments_019f28d8-3bff-7eb4-b9bc-3c2dd13bc97e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPremiumsReceivable_019f28d8-3bff-79a6-9c40-3eedee9028e3" xlink:href="acgl-20260630.xsd#acgl_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPremiumsReceivable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssetsAbstract_019f28d8-3bff-7d9c-b0df-f0dd9e7c8b31" xlink:to="loc_acgl_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPremiumsReceivable_019f28d8-3bff-79a6-9c40-3eedee9028e3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles_019f28d8-3bff-71ee-9f00-6b498ea21f70" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssetsAbstract_019f28d8-3bff-7d9c-b0df-f0dd9e7c8b31" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles_019f28d8-3bff-71ee-9f00-6b498ea21f70" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife_019f28d8-3bff-73e1-8739-1cb4b9faa78f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles_019f28d8-3bff-71ee-9f00-6b498ea21f70" xlink:to="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife_019f28d8-3bff-73e1-8739-1cb4b9faa78f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOtherAssets_019f28d8-3bff-716e-b234-8587de952534" xlink:href="acgl-20260630.xsd#acgl_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOtherAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssetsAbstract_019f28d8-3bff-7d9c-b0df-f0dd9e7c8b31" xlink:to="loc_acgl_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOtherAssets_019f28d8-3bff-716e-b234-8587de952534" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssets_019f28d8-3bff-7692-9da7-87673a87bf4d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssetsAbstract_019f28d8-3bff-7d9c-b0df-f0dd9e7c8b31" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssets_019f28d8-3bff-7692-9da7-87673a87bf4d" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilitiesAbstract_019f28d8-3bff-71c7-836a-55854e405502" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019f28d8-3bff-764a-a1ba-720b583dca0e" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilitiesAbstract_019f28d8-3bff-71c7-836a-55854e405502" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedReservesForLossesAndLossAdjustmentExpenses_019f28d8-3bff-7a88-90ce-85de0bcc566a" xlink:href="acgl-20260630.xsd#acgl_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedReservesForLossesAndLossAdjustmentExpenses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilitiesAbstract_019f28d8-3bff-71c7-836a-55854e405502" xlink:to="loc_acgl_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedReservesForLossesAndLossAdjustmentExpenses_019f28d8-3bff-7a88-90ce-85de0bcc566a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesDeferredRevenue_019f28d8-3bff-7587-ae2a-1de704a2ce32" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesDeferredRevenue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilitiesAbstract_019f28d8-3bff-71c7-836a-55854e405502" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesDeferredRevenue_019f28d8-3bff-7587-ae2a-1de704a2ce32" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOtherLiabilities_019f28d8-3bff-7b54-9be1-42a3405cf4fa" xlink:href="acgl-20260630.xsd#acgl_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOtherLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilitiesAbstract_019f28d8-3bff-71c7-836a-55854e405502" xlink:to="loc_acgl_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOtherLiabilities_019f28d8-3bff-7b54-9be1-42a3405cf4fa" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities_019f28d8-3bff-7a67-9da3-4f17df2ec711" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilitiesAbstract_019f28d8-3bff-71c7-836a-55854e405502" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities_019f28d8-3bff-7a67-9da3-4f17df2ec711" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet_019f28d8-3bff-77db-81dd-c93c13152b6b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019f28d8-3bff-764a-a1ba-720b583dca0e" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet_019f28d8-3bff-77db-81dd-c93c13152b6b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_019f28d8-3bff-71bf-925d-76988f501c15" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_Goodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019f28d8-3bff-764a-a1ba-720b583dca0e" xlink:to="loc_us-gaap_Goodwill_019f28d8-3bff-71bf-925d-76988f501c15" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredTaxAssetsGoodwillAndIntangibleAssets_019f28d8-3bff-7907-ae82-16e800312431" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DeferredTaxAssetsGoodwillAndIntangibleAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019f28d8-3bff-764a-a1ba-720b583dca0e" xlink:to="loc_us-gaap_DeferredTaxAssetsGoodwillAndIntangibleAssets_019f28d8-3bff-7907-ae82-16e800312431" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsLossReservesAndUnearnedPremiums_019f28d8-3bff-7667-9a95-2a5e6e50e689" xlink:href="acgl-20260630.xsd#acgl_DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsLossReservesAndUnearnedPremiums"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019f28d8-3bff-764a-a1ba-720b583dca0e" xlink:to="loc_acgl_DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsLossReservesAndUnearnedPremiums_019f28d8-3bff-7667-9a95-2a5e6e50e689" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesFairValueAdjustment_019f28d8-3bff-7f0f-8878-c22811b3dc01" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LiabilitiesFairValueAdjustment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019f28d8-3bff-764a-a1ba-720b583dca0e" xlink:to="loc_us-gaap_LiabilitiesFairValueAdjustment_019f28d8-3bff-7f0f-8878-c22811b3dc01" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillPurchaseAccountingAdjustments_019f28d8-3bff-79aa-b3de-6d63e9af2e8e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_GoodwillPurchaseAccountingAdjustments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019f28d8-3bff-764a-a1ba-720b583dca0e" xlink:to="loc_us-gaap_GoodwillPurchaseAccountingAdjustments_019f28d8-3bff-79aa-b3de-6d63e9af2e8e" xlink:type="arc" order="8"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.archcapgroup.com/role/ShareTransactionsSharerepurchasesDetails" xlink:type="simple" xlink:href="acgl-20260630.xsd#ShareTransactionsSharerepurchasesDetails"/>
  <link:definitionLink xlink:role="http://www.archcapgroup.com/role/ShareTransactionsSharerepurchasesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockLineItems_019f28d8-3bff-779a-83d3-899d4cd32b70" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfStockLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfStockByClassTable_019f28d8-3bff-7bfc-85a3-3b196b0d29ab" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfStockByClassTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ClassOfStockLineItems_019f28d8-3bff-779a-83d3-899d4cd32b70" xlink:to="loc_us-gaap_ScheduleOfStockByClassTable_019f28d8-3bff-7bfc-85a3-3b196b0d29ab" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ShareRepurchaseProgramAxis_019f28d8-3bff-7f99-a19a-9e76b5e5f925" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ShareRepurchaseProgramAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_019f28d8-3bff-7bfc-85a3-3b196b0d29ab" xlink:to="loc_srt_ShareRepurchaseProgramAxis_019f28d8-3bff-7f99-a19a-9e76b5e5f925" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ShareRepurchaseProgramDomain_019f28d8-3bff-7f99-a19a-9e76b5e5f925_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ShareRepurchaseProgramDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ShareRepurchaseProgramAxis_019f28d8-3bff-7f99-a19a-9e76b5e5f925" xlink:to="loc_srt_ShareRepurchaseProgramDomain_019f28d8-3bff-7f99-a19a-9e76b5e5f925_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ShareRepurchaseProgramDomain_019f28d8-3bff-7c08-9e73-b05e02e0011d" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ShareRepurchaseProgramDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ShareRepurchaseProgramAxis_019f28d8-3bff-7f99-a19a-9e76b5e5f925" xlink:to="loc_srt_ShareRepurchaseProgramDomain_019f28d8-3bff-7c08-9e73-b05e02e0011d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_AuthorizedShareRepurchaseProgramMember_019f28d8-3bff-7d64-98a6-2c7160a91eac" xlink:href="acgl-20260630.xsd#acgl_AuthorizedShareRepurchaseProgramMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ShareRepurchaseProgramDomain_019f28d8-3bff-7c08-9e73-b05e02e0011d" xlink:to="loc_acgl_AuthorizedShareRepurchaseProgramMember_019f28d8-3bff-7d64-98a6-2c7160a91eac" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_019f28d8-3bff-7e7f-a4c8-ff14fae0ee56" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_019f28d8-3bff-7bfc-85a3-3b196b0d29ab" xlink:to="loc_us-gaap_StatementClassOfStockAxis_019f28d8-3bff-7e7f-a4c8-ff14fae0ee56" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019f28d8-3bff-7e7f-a4c8-ff14fae0ee56_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019f28d8-3bff-7e7f-a4c8-ff14fae0ee56" xlink:to="loc_us-gaap_ClassOfStockDomain_019f28d8-3bff-7e7f-a4c8-ff14fae0ee56_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019f28d8-3bff-7338-bc22-597b9f1fa24a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019f28d8-3bff-7e7f-a4c8-ff14fae0ee56" xlink:to="loc_us-gaap_ClassOfStockDomain_019f28d8-3bff-7338-bc22-597b9f1fa24a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_019f28d8-3bff-798e-a5df-34f8c87dfc56" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_019f28d8-3bff-7338-bc22-597b9f1fa24a" xlink:to="loc_us-gaap_CommonStockMember_019f28d8-3bff-798e-a5df-34f8c87dfc56" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_TreasuryStockSharesAcquiredCumulativeSinceInception_019f28d8-3bff-758d-ac92-d39578472cc3" xlink:href="acgl-20260630.xsd#acgl_TreasuryStockSharesAcquiredCumulativeSinceInception"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019f28d8-3bff-779a-83d3-899d4cd32b70" xlink:to="loc_acgl_TreasuryStockSharesAcquiredCumulativeSinceInception_019f28d8-3bff-758d-ac92-d39578472cc3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_TreasuryStockValueAcquiredCumulativeSinceInception_019f28d8-3bff-76c4-a4f6-c74090f24c5a" xlink:href="acgl-20260630.xsd#acgl_TreasuryStockValueAcquiredCumulativeSinceInception"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019f28d8-3bff-779a-83d3-899d4cd32b70" xlink:to="loc_acgl_TreasuryStockValueAcquiredCumulativeSinceInception_019f28d8-3bff-76c4-a4f6-c74090f24c5a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockSharesAcquired_019f28d8-3bff-73a5-9382-8dc343634928" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TreasuryStockSharesAcquired"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019f28d8-3bff-779a-83d3-899d4cd32b70" xlink:to="loc_us-gaap_TreasuryStockSharesAcquired_019f28d8-3bff-73a5-9382-8dc343634928" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockValueAcquiredCostMethod_019f28d8-3bff-7ae2-a160-7d32d3c3cd50" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TreasuryStockValueAcquiredCostMethod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019f28d8-3bff-779a-83d3-899d4cd32b70" xlink:to="loc_us-gaap_TreasuryStockValueAcquiredCostMethod_019f28d8-3bff-7ae2-a160-7d32d3c3cd50" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount1_019f28d8-3bff-72b3-b9bf-399a7d214a08" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019f28d8-3bff-779a-83d3-899d4cd32b70" xlink:to="loc_us-gaap_StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount1_019f28d8-3bff-72b3-b9bf-399a7d214a08" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_ShareRepurchaseProgramIncreaseAuthorizedAmount_019f28d8-3bff-72d4-9e7d-04537e4bb04c" xlink:href="acgl-20260630.xsd#acgl_ShareRepurchaseProgramIncreaseAuthorizedAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019f28d8-3bff-779a-83d3-899d4cd32b70" xlink:to="loc_acgl_ShareRepurchaseProgramIncreaseAuthorizedAmount_019f28d8-3bff-72d4-9e7d-04537e4bb04c" xlink:type="arc" order="5"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.archcapgroup.com/role/EarningsPerCommonShareDetails" xlink:type="simple" xlink:href="acgl-20260630.xsd#EarningsPerCommonShareDetails"/>
  <link:definitionLink xlink:role="http://www.archcapgroup.com/role/EarningsPerCommonShareDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_019f28d8-3cb1-7818-8b49-b3cfe6278013" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_019f28d8-3cb1-7e03-a9cf-1da0c4e9f0ac" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_019f28d8-3cb1-7818-8b49-b3cfe6278013" xlink:to="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_019f28d8-3cb1-7e03-a9cf-1da0c4e9f0ac" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_019f28d8-3cb1-7660-8c60-ab7af5a5f12c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_019f28d8-3cb1-7e03-a9cf-1da0c4e9f0ac" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_019f28d8-3cb1-7660-8c60-ab7af5a5f12c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019f28d8-3cb1-7660-8c60-ab7af5a5f12c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AntidilutiveSecuritiesNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_019f28d8-3cb1-7660-8c60-ab7af5a5f12c" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019f28d8-3cb1-7660-8c60-ab7af5a5f12c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019f28d8-3cb1-721e-bfc1-e0335c1f1d6e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AntidilutiveSecuritiesNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_019f28d8-3cb1-7660-8c60-ab7af5a5f12c" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019f28d8-3cb1-721e-bfc1-e0335c1f1d6e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_019f28d8-3cb1-7489-8eaf-cc53ff3428ad" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019f28d8-3cb1-721e-bfc1-e0335c1f1d6e" xlink:to="loc_us-gaap_EmployeeStockOptionMember_019f28d8-3cb1-7489-8eaf-cc53ff3428ad" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_019f28d8-3cb1-7462-94f4-1108eed44edf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_019f28d8-3cb1-7818-8b49-b3cfe6278013" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_019f28d8-3cb1-7462-94f4-1108eed44edf" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.archcapgroup.com/role/SegmentInformationSummaryofunderwritingincomeorlossbysegmentDetails" xlink:type="simple" xlink:href="acgl-20260630.xsd#SegmentInformationSummaryofunderwritingincomeorlossbysegmentDetails"/>
  <link:definitionLink xlink:role="http://www.archcapgroup.com/role/SegmentInformationSummaryofunderwritingincomeorlossbysegmentDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_019f28d8-3cb1-7d71-89d7-54e8caa672b1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019f28d8-3cb1-7f8b-a8f8-fb6f794c27f8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019f28d8-3cb1-7d71-89d7-54e8caa672b1" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019f28d8-3cb1-7f8b-a8f8-fb6f794c27f8" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsAxis_019f28d8-3cb1-7135-9ba0-03027b795a12" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidationItemsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019f28d8-3cb1-7f8b-a8f8-fb6f794c27f8" xlink:to="loc_srt_ConsolidationItemsAxis_019f28d8-3cb1-7135-9ba0-03027b795a12" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_019f28d8-3cb1-7135-9ba0-03027b795a12_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidationItemsAxis_019f28d8-3cb1-7135-9ba0-03027b795a12" xlink:to="loc_srt_ConsolidationItemsDomain_019f28d8-3cb1-7135-9ba0-03027b795a12_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_019f28d8-3cb1-7fa9-af34-69063ca13c63" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidationItemsAxis_019f28d8-3cb1-7135-9ba0-03027b795a12" xlink:to="loc_srt_ConsolidationItemsDomain_019f28d8-3cb1-7fa9-af34-69063ca13c63" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingSegmentsMember_019f28d8-3cb1-76f5-a2fe-d74b5238bc7f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingSegmentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_019f28d8-3cb1-7fa9-af34-69063ca13c63" xlink:to="loc_us-gaap_OperatingSegmentsMember_019f28d8-3cb1-76f5-a2fe-d74b5238bc7f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_019f28d8-3cb1-7cb9-8c6c-152e46f1f091" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019f28d8-3cb1-7f8b-a8f8-fb6f794c27f8" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_019f28d8-3cb1-7cb9-8c6c-152e46f1f091" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019f28d8-3cb1-7cb9-8c6c-152e46f1f091_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019f28d8-3cb1-7cb9-8c6c-152e46f1f091" xlink:to="loc_us-gaap_SegmentDomain_019f28d8-3cb1-7cb9-8c6c-152e46f1f091_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019f28d8-3cb1-725f-875d-378d6f68cb50" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019f28d8-3cb1-7cb9-8c6c-152e46f1f091" xlink:to="loc_us-gaap_SegmentDomain_019f28d8-3cb1-725f-875d-378d6f68cb50" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_InsuranceSegmentMember_019f28d8-3cb1-73c0-b79c-8a491792e86d" xlink:href="acgl-20260630.xsd#acgl_InsuranceSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019f28d8-3cb1-725f-875d-378d6f68cb50" xlink:to="loc_acgl_InsuranceSegmentMember_019f28d8-3cb1-73c0-b79c-8a491792e86d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_ReinsuranceSegmentMember_019f28d8-3cb1-7198-89b0-fa45e2d0a749" xlink:href="acgl-20260630.xsd#acgl_ReinsuranceSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019f28d8-3cb1-725f-875d-378d6f68cb50" xlink:to="loc_acgl_ReinsuranceSegmentMember_019f28d8-3cb1-7198-89b0-fa45e2d0a749" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_MortgageSegmentMember_019f28d8-3cb1-72f9-b5dc-66c1b7a2aef1" xlink:href="acgl-20260630.xsd#acgl_MortgageSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019f28d8-3cb1-725f-875d-378d6f68cb50" xlink:to="loc_acgl_MortgageSegmentMember_019f28d8-3cb1-72f9-b5dc-66c1b7a2aef1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NumberOfReportableSegments_019f28d8-3cb1-7bd6-8a34-ef5ea5f4d686" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NumberOfReportableSegments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019f28d8-3cb1-7d71-89d7-54e8caa672b1" xlink:to="loc_us-gaap_NumberOfReportableSegments_019f28d8-3cb1-7bd6-8a34-ef5ea5f4d686" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PremiumsWrittenGross_019f28d8-3cb1-7a0e-862f-24e319d6a00e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PremiumsWrittenGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019f28d8-3cb1-7d71-89d7-54e8caa672b1" xlink:to="loc_us-gaap_PremiumsWrittenGross_019f28d8-3cb1-7a0e-862f-24e319d6a00e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CededPremiumsWritten_019f28d8-3cb1-7ac2-91dc-2f30e5383539" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CededPremiumsWritten"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019f28d8-3cb1-7d71-89d7-54e8caa672b1" xlink:to="loc_us-gaap_CededPremiumsWritten_019f28d8-3cb1-7ac2-91dc-2f30e5383539" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PremiumsWrittenNet_019f28d8-3cb1-788d-8c3a-c5fe0d52818c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PremiumsWrittenNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019f28d8-3cb1-7d71-89d7-54e8caa672b1" xlink:to="loc_us-gaap_PremiumsWrittenNet_019f28d8-3cb1-788d-8c3a-c5fe0d52818c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInUnearnedPremiumsNet_019f28d8-3cb1-7335-a22f-625dfeeb6c92" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInUnearnedPremiumsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019f28d8-3cb1-7d71-89d7-54e8caa672b1" xlink:to="loc_us-gaap_IncreaseDecreaseInUnearnedPremiumsNet_019f28d8-3cb1-7335-a22f-625dfeeb6c92" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PremiumsEarnedNet_019f28d8-3cb1-7053-afaa-8195c0c44f2d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PremiumsEarnedNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019f28d8-3cb1-7d71-89d7-54e8caa672b1" xlink:to="loc_us-gaap_PremiumsEarnedNet_019f28d8-3cb1-7053-afaa-8195c0c44f2d" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherIncome_019f28d8-3cb1-7db1-a935-9aae5aa5b558" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherIncome"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019f28d8-3cb1-7d71-89d7-54e8caa672b1" xlink:to="loc_us-gaap_OtherIncome_019f28d8-3cb1-7db1-a935-9aae5aa5b558" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PolicyholderBenefitsAndClaimsIncurredNet_019f28d8-3cb1-711e-83d1-57d31514c1c3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PolicyholderBenefitsAndClaimsIncurredNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019f28d8-3cb1-7d71-89d7-54e8caa672b1" xlink:to="loc_us-gaap_PolicyholderBenefitsAndClaimsIncurredNet_019f28d8-3cb1-711e-83d1-57d31514c1c3" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredPolicyAcquisitionCostAmortizationExpense_019f28d8-3cb1-7bc7-8082-76344f8d22a2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DeferredPolicyAcquisitionCostAmortizationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019f28d8-3cb1-7d71-89d7-54e8caa672b1" xlink:to="loc_us-gaap_DeferredPolicyAcquisitionCostAmortizationExpense_019f28d8-3cb1-7bc7-8082-76344f8d22a2" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherExpenses_019f28d8-3cb1-7ad9-81ac-9cd00c3335d0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherExpenses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019f28d8-3cb1-7d71-89d7-54e8caa672b1" xlink:to="loc_us-gaap_OtherExpenses_019f28d8-3cb1-7ad9-81ac-9cd00c3335d0" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_UnderwritingIncomeLossSegment_019f28d8-3cb1-7839-90a7-0e176eefd289" xlink:href="acgl-20260630.xsd#acgl_UnderwritingIncomeLossSegment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019f28d8-3cb1-7d71-89d7-54e8caa672b1" xlink:to="loc_acgl_UnderwritingIncomeLossSegment_019f28d8-3cb1-7839-90a7-0e176eefd289" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentIncomeNet_019f28d8-3cb1-7023-a446-b4099c0714d8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InvestmentIncomeNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019f28d8-3cb1-7d71-89d7-54e8caa672b1" xlink:to="loc_us-gaap_InvestmentIncomeNet_019f28d8-3cb1-7023-a446-b4099c0714d8" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_NetRealizedGainsLosses_019f28d8-3cb1-72b7-aed4-69e327f68eb4" xlink:href="acgl-20260630.xsd#acgl_NetRealizedGainsLosses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019f28d8-3cb1-7d71-89d7-54e8caa672b1" xlink:to="loc_acgl_NetRealizedGainsLosses_019f28d8-3cb1-72b7-aed4-69e327f68eb4" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromEquityMethodInvestments_019f28d8-3cb1-7633-82fc-401b71f820ed" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncomeLossFromEquityMethodInvestments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019f28d8-3cb1-7d71-89d7-54e8caa672b1" xlink:to="loc_us-gaap_IncomeLossFromEquityMethodInvestments_019f28d8-3cb1-7633-82fc-401b71f820ed" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherOperatingIncomeExpenseNet_019f28d8-3cb1-7822-972c-b396c717cf11" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherOperatingIncomeExpenseNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019f28d8-3cb1-7d71-89d7-54e8caa672b1" xlink:to="loc_us-gaap_OtherOperatingIncomeExpenseNet_019f28d8-3cb1-7822-972c-b396c717cf11" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_SegmentOtherCostAndExpenseOperating_019f28d8-3cb1-754b-83a2-55e8cfd132e1" xlink:href="acgl-20260630.xsd#acgl_SegmentOtherCostAndExpenseOperating"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019f28d8-3cb1-7d71-89d7-54e8caa672b1" xlink:to="loc_acgl_SegmentOtherCostAndExpenseOperating_019f28d8-3cb1-754b-83a2-55e8cfd132e1" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationAcquisitionRelatedCosts_019f28d8-3cb1-7dab-adda-769dafacc58d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessCombinationAcquisitionRelatedCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019f28d8-3cb1-7d71-89d7-54e8caa672b1" xlink:to="loc_us-gaap_BusinessCombinationAcquisitionRelatedCosts_019f28d8-3cb1-7dab-adda-769dafacc58d" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AmortizationOfIntangibleAssets_019f28d8-3cb1-7816-b42e-01c18195bae2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AmortizationOfIntangibleAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019f28d8-3cb1-7d71-89d7-54e8caa672b1" xlink:to="loc_us-gaap_AmortizationOfIntangibleAssets_019f28d8-3cb1-7816-b42e-01c18195bae2" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpense_019f28d8-3cb1-79cf-94ba-8770268320f8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019f28d8-3cb1-7d71-89d7-54e8caa672b1" xlink:to="loc_us-gaap_InterestExpense_019f28d8-3cb1-79cf-94ba-8770268320f8" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForeignCurrencyTransactionGainLossBeforeTax_019f28d8-3cb1-71eb-ba6b-6a49d47e05fe" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ForeignCurrencyTransactionGainLossBeforeTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019f28d8-3cb1-7d71-89d7-54e8caa672b1" xlink:to="loc_us-gaap_ForeignCurrencyTransactionGainLossBeforeTax_019f28d8-3cb1-71eb-ba6b-6a49d47e05fe" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_IncomeLossFromContinuingOperationsBeforeEarningsFromAffiliatesIncomeTaxesNoncontrollingInterest_019f28d8-3cb1-7e80-b595-68ee853005ea" xlink:href="acgl-20260630.xsd#acgl_IncomeLossFromContinuingOperationsBeforeEarningsFromAffiliatesIncomeTaxesNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019f28d8-3cb1-7d71-89d7-54e8caa672b1" xlink:to="loc_acgl_IncomeLossFromContinuingOperationsBeforeEarningsFromAffiliatesIncomeTaxesNoncontrollingInterest_019f28d8-3cb1-7e80-b595-68ee853005ea" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxExpenseBenefit_019f28d8-3cb1-7cbd-9da6-f809923ef37e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncomeTaxExpenseBenefit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019f28d8-3cb1-7d71-89d7-54e8caa672b1" xlink:to="loc_us-gaap_IncomeTaxExpenseBenefit_019f28d8-3cb1-7cbd-9da6-f809923ef37e" xlink:type="arc" order="21"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_IncomeLossFromOperatingAffiliatesEquityMethod_019f28d8-3cb1-7368-9dda-a196995ec66c" xlink:href="acgl-20260630.xsd#acgl_IncomeLossFromOperatingAffiliatesEquityMethod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019f28d8-3cb1-7d71-89d7-54e8caa672b1" xlink:to="loc_acgl_IncomeLossFromOperatingAffiliatesEquityMethod_019f28d8-3cb1-7368-9dda-a196995ec66c" xlink:type="arc" order="22"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_019f28d8-3cb1-77d2-8848-f63b008f4b9c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019f28d8-3cb1-7d71-89d7-54e8caa672b1" xlink:to="loc_us-gaap_NetIncomeLoss_019f28d8-3cb1-77d2-8848-f63b008f4b9c" xlink:type="arc" order="23"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockDividendsIncomeStatementImpact_019f28d8-3cb1-7eec-b9eb-ea50642aac0d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PreferredStockDividendsIncomeStatementImpact"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019f28d8-3cb1-7d71-89d7-54e8caa672b1" xlink:to="loc_us-gaap_PreferredStockDividendsIncomeStatementImpact_019f28d8-3cb1-7eec-b9eb-ea50642aac0d" xlink:type="arc" order="24"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic_019f28d8-3cb1-7950-81a6-98017c89372b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019f28d8-3cb1-7d71-89d7-54e8caa672b1" xlink:to="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic_019f28d8-3cb1-7950-81a6-98017c89372b" xlink:type="arc" order="25"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InsuranceRatiosAbstract_019f28d8-3cb1-7aec-b775-cdee1e3262b6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InsuranceRatiosAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019f28d8-3cb1-7d71-89d7-54e8caa672b1" xlink:to="loc_us-gaap_InsuranceRatiosAbstract_019f28d8-3cb1-7aec-b775-cdee1e3262b6" xlink:type="arc" order="26"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossRatio_019f28d8-3cb1-7406-9d2d-141b79acdab3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LossRatio"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InsuranceRatiosAbstract_019f28d8-3cb1-7aec-b775-cdee1e3262b6" xlink:to="loc_us-gaap_LossRatio_019f28d8-3cb1-7406-9d2d-141b79acdab3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AcquisitionCostRatio_019f28d8-3cb1-7b71-9cc3-0ae91ad24fce" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AcquisitionCostRatio"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InsuranceRatiosAbstract_019f28d8-3cb1-7aec-b775-cdee1e3262b6" xlink:to="loc_us-gaap_AcquisitionCostRatio_019f28d8-3cb1-7b71-9cc3-0ae91ad24fce" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_OtherOperatingExpenseNetRatioPercentage_019f28d8-3cb1-7cd5-83d6-94905fa8a20d" xlink:href="acgl-20260630.xsd#acgl_OtherOperatingExpenseNetRatioPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InsuranceRatiosAbstract_019f28d8-3cb1-7aec-b775-cdee1e3262b6" xlink:to="loc_acgl_OtherOperatingExpenseNetRatioPercentage_019f28d8-3cb1-7cd5-83d6-94905fa8a20d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_CombinedRatioPercentage_019f28d8-3cb1-7d19-8ba4-88a160d443a1" xlink:href="acgl-20260630.xsd#acgl_CombinedRatioPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InsuranceRatiosAbstract_019f28d8-3cb1-7aec-b775-cdee1e3262b6" xlink:to="loc_acgl_CombinedRatioPercentage_019f28d8-3cb1-7d19-8ba4-88a160d443a1" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IntangibleAssetsNetIncludingGoodwill_019f28d8-3cb1-747f-ad7c-ea018a94c323" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IntangibleAssetsNetIncludingGoodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019f28d8-3cb1-7d71-89d7-54e8caa672b1" xlink:to="loc_us-gaap_IntangibleAssetsNetIncludingGoodwill_019f28d8-3cb1-747f-ad7c-ea018a94c323" xlink:type="arc" order="27"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.archcapgroup.com/role/ReserveforLossesandLossAdjustmentExpensesPrioryeardevelopmentDetails" xlink:type="simple" xlink:href="acgl-20260630.xsd#ReserveforLossesandLossAdjustmentExpensesPrioryeardevelopmentDetails"/>
  <link:definitionLink xlink:role="http://www.archcapgroup.com/role/ReserveforLossesandLossAdjustmentExpensesPrioryeardevelopmentDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CausesOfIncreaseDecreaseInLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseLineItems_019f28d8-3cb1-7b12-81fa-365cad8713da" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CausesOfIncreaseDecreaseInLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CausesOfIncreaseDecreaseInLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseTable_019f28d8-3cb1-7be9-a23f-5a7a29888680" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CausesOfIncreaseDecreaseInLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_CausesOfIncreaseDecreaseInLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseLineItems_019f28d8-3cb1-7b12-81fa-365cad8713da" xlink:to="loc_us-gaap_CausesOfIncreaseDecreaseInLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseTable_019f28d8-3cb1-7be9-a23f-5a7a29888680" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsAxis_019f28d8-3cb1-7c2c-a1e4-054ce0c2634c" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidationItemsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_CausesOfIncreaseDecreaseInLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseTable_019f28d8-3cb1-7be9-a23f-5a7a29888680" xlink:to="loc_srt_ConsolidationItemsAxis_019f28d8-3cb1-7c2c-a1e4-054ce0c2634c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_019f28d8-3cb1-7c2c-a1e4-054ce0c2634c_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidationItemsAxis_019f28d8-3cb1-7c2c-a1e4-054ce0c2634c" xlink:to="loc_srt_ConsolidationItemsDomain_019f28d8-3cb1-7c2c-a1e4-054ce0c2634c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_019f28d8-3cb1-7a9c-9ee7-67e3d4124112" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidationItemsAxis_019f28d8-3cb1-7c2c-a1e4-054ce0c2634c" xlink:to="loc_srt_ConsolidationItemsDomain_019f28d8-3cb1-7a9c-9ee7-67e3d4124112" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingSegmentsMember_019f28d8-3cb1-77ce-b239-c89fc4fb9a1c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingSegmentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_019f28d8-3cb1-7a9c-9ee7-67e3d4124112" xlink:to="loc_us-gaap_OperatingSegmentsMember_019f28d8-3cb1-77ce-b239-c89fc4fb9a1c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_019f28d8-3cb1-70ff-9b30-74775c64c5ff" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_CausesOfIncreaseDecreaseInLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseTable_019f28d8-3cb1-7be9-a23f-5a7a29888680" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_019f28d8-3cb1-70ff-9b30-74775c64c5ff" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019f28d8-3cb1-70ff-9b30-74775c64c5ff_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019f28d8-3cb1-70ff-9b30-74775c64c5ff" xlink:to="loc_us-gaap_SegmentDomain_019f28d8-3cb1-70ff-9b30-74775c64c5ff_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019f28d8-3cb1-78ca-b372-8fc4156af53b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019f28d8-3cb1-70ff-9b30-74775c64c5ff" xlink:to="loc_us-gaap_SegmentDomain_019f28d8-3cb1-78ca-b372-8fc4156af53b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_InsuranceSegmentMember_019f28d8-3cb1-781c-9c28-715e9fd6a2d2" xlink:href="acgl-20260630.xsd#acgl_InsuranceSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019f28d8-3cb1-78ca-b372-8fc4156af53b" xlink:to="loc_acgl_InsuranceSegmentMember_019f28d8-3cb1-781c-9c28-715e9fd6a2d2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_ReinsuranceSegmentMember_019f28d8-3cb1-754a-9353-ee840143c2b2" xlink:href="acgl-20260630.xsd#acgl_ReinsuranceSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019f28d8-3cb1-78ca-b372-8fc4156af53b" xlink:to="loc_acgl_ReinsuranceSegmentMember_019f28d8-3cb1-754a-9353-ee840143c2b2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_MortgageSegmentMember_019f28d8-3cb1-7fa2-b484-208909d194e8" xlink:href="acgl-20260630.xsd#acgl_MortgageSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019f28d8-3cb1-78ca-b372-8fc4156af53b" xlink:to="loc_acgl_MortgageSegmentMember_019f28d8-3cb1-7fa2-b484-208909d194e8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortdurationInsuranceContractsClaimDurationAxis_019f28d8-3cb1-7405-8375-fe75e4f17c54" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShortdurationInsuranceContractsClaimDurationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_CausesOfIncreaseDecreaseInLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseTable_019f28d8-3cb1-7be9-a23f-5a7a29888680" xlink:to="loc_us-gaap_ShortdurationInsuranceContractsClaimDurationAxis_019f28d8-3cb1-7405-8375-fe75e4f17c54" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortdurationInsuranceContractsClaimDurationDomain_019f28d8-3cb1-7405-8375-fe75e4f17c54_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShortdurationInsuranceContractsClaimDurationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ShortdurationInsuranceContractsClaimDurationAxis_019f28d8-3cb1-7405-8375-fe75e4f17c54" xlink:to="loc_us-gaap_ShortdurationInsuranceContractsClaimDurationDomain_019f28d8-3cb1-7405-8375-fe75e4f17c54_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortdurationInsuranceContractsClaimDurationDomain_019f28d8-3cb1-7fde-baac-e9599a5a3b43" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShortdurationInsuranceContractsClaimDurationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ShortdurationInsuranceContractsClaimDurationAxis_019f28d8-3cb1-7405-8375-fe75e4f17c54" xlink:to="loc_us-gaap_ShortdurationInsuranceContractsClaimDurationDomain_019f28d8-3cb1-7fde-baac-e9599a5a3b43" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_ShortTailedLinesMember_019f28d8-3cb1-7bf5-8272-342fb439434e" xlink:href="acgl-20260630.xsd#acgl_ShortTailedLinesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShortdurationInsuranceContractsClaimDurationDomain_019f28d8-3cb1-7fde-baac-e9599a5a3b43" xlink:to="loc_acgl_ShortTailedLinesMember_019f28d8-3cb1-7bf5-8272-342fb439434e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_LongTailedLinesMember_019f28d8-3cb1-7a7f-bb86-2b1a225e8469" xlink:href="acgl-20260630.xsd#acgl_LongTailedLinesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShortdurationInsuranceContractsClaimDurationDomain_019f28d8-3cb1-7fde-baac-e9599a5a3b43" xlink:to="loc_acgl_LongTailedLinesMember_019f28d8-3cb1-7a7f-bb86-2b1a225e8469" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_019f28d8-3cb1-7bb2-aa7a-a868178d830e" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_CausesOfIncreaseDecreaseInLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseTable_019f28d8-3cb1-7be9-a23f-5a7a29888680" xlink:to="loc_srt_ProductOrServiceAxis_019f28d8-3cb1-7bb2-aa7a-a868178d830e" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019f28d8-3cb1-7bb2-aa7a-a868178d830e_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_019f28d8-3cb1-7bb2-aa7a-a868178d830e" xlink:to="loc_srt_ProductsAndServicesDomain_019f28d8-3cb1-7bb2-aa7a-a868178d830e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019f28d8-3cb1-7fbd-92c7-5b0e8fca9cd2" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_019f28d8-3cb1-7bb2-aa7a-a868178d830e" xlink:to="loc_srt_ProductsAndServicesDomain_019f28d8-3cb1-7fbd-92c7-5b0e8fca9cd2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_TravelAndAccidentProductLineMember_019f28d8-3cb1-7d1a-8fbb-235b8a669e67" xlink:href="acgl-20260630.xsd#acgl_TravelAndAccidentProductLineMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019f28d8-3cb1-7fbd-92c7-5b0e8fca9cd2" xlink:to="loc_acgl_TravelAndAccidentProductLineMember_019f28d8-3cb1-7d1a-8fbb-235b8a669e67" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_PropertyOtherThanPropertyCatastropheMember_019f28d8-3cb1-73ab-9e4e-0482bffd767d" xlink:href="acgl-20260630.xsd#acgl_PropertyOtherThanPropertyCatastropheMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019f28d8-3cb1-7fbd-92c7-5b0e8fca9cd2" xlink:to="loc_acgl_PropertyOtherThanPropertyCatastropheMember_019f28d8-3cb1-73ab-9e4e-0482bffd767d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_PropertyCatastropheMember_019f28d8-3cb1-7c3b-8f70-78e207580e9f" xlink:href="acgl-20260630.xsd#acgl_PropertyCatastropheMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019f28d8-3cb1-7fbd-92c7-5b0e8fca9cd2" xlink:to="loc_acgl_PropertyCatastropheMember_019f28d8-3cb1-7c3b-8f70-78e207580e9f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_CasualtyMember_019f28d8-3cb1-74b3-852b-be3658438fda" xlink:href="acgl-20260630.xsd#acgl_CasualtyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019f28d8-3cb1-7fbd-92c7-5b0e8fca9cd2" xlink:to="loc_acgl_CasualtyMember_019f28d8-3cb1-74b3-852b-be3658438fda" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_PropertyEnergyMarineAndAviationProductLineMember_019fadc1-f760-7ecb-8839-4fdae1bfdcca" xlink:href="acgl-20260630.xsd#acgl_PropertyEnergyMarineAndAviationProductLineMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019f28d8-3cb1-7fbd-92c7-5b0e8fca9cd2" xlink:to="loc_acgl_PropertyEnergyMarineAndAviationProductLineMember_019fadc1-f760-7ecb-8839-4fdae1bfdcca" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_ProgramsLineOfBusinessMember_019fadcb-4057-7bfa-a64a-0ce8fa21abf6" xlink:href="acgl-20260630.xsd#acgl_ProgramsLineOfBusinessMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019f28d8-3cb1-7fbd-92c7-5b0e8fca9cd2" xlink:to="loc_acgl_ProgramsLineOfBusinessMember_019fadcb-4057-7bfa-a64a-0ce8fa21abf6" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPriorYearClaimsAndClaimsAdjustmentExpense_019f28d8-3cb1-7f0c-9b54-dc482378fb04" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPriorYearClaimsAndClaimsAdjustmentExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CausesOfIncreaseDecreaseInLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseLineItems_019f28d8-3cb1-7b12-81fa-365cad8713da" xlink:to="loc_us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPriorYearClaimsAndClaimsAdjustmentExpense_019f28d8-3cb1-7f0c-9b54-dc482378fb04" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.archcapgroup.com/role/AllowanceforExpectedCreditLossesCededcreditriskDetails" xlink:type="simple" xlink:href="acgl-20260630.xsd#AllowanceforExpectedCreditLossesCededcreditriskDetails"/>
  <link:definitionLink xlink:role="http://www.archcapgroup.com/role/AllowanceforExpectedCreditLossesCededcreditriskDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CededCreditRiskLineItems_019f28d8-3cb1-73d1-a6cd-5dc1c786f198" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CededCreditRiskLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CededCreditRiskTable_019f28d8-3cb1-7aa6-8d6b-f82017d1711d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CededCreditRiskTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_CededCreditRiskLineItems_019f28d8-3cb1-73d1-a6cd-5dc1c786f198" xlink:to="loc_us-gaap_CededCreditRiskTable_019f28d8-3cb1-7aa6-8d6b-f82017d1711d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByTypeAxis_019f28d8-3cb1-7146-9e04-2e246a6dee73" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_CededCreditRiskTable_019f28d8-3cb1-7aa6-8d6b-f82017d1711d" xlink:to="loc_us-gaap_ConcentrationRiskByTypeAxis_019f28d8-3cb1-7146-9e04-2e246a6dee73" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_019f28d8-3cb1-7146-9e04-2e246a6dee73_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_019f28d8-3cb1-7146-9e04-2e246a6dee73" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_019f28d8-3cb1-7146-9e04-2e246a6dee73_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_019f28d8-3cb1-7525-8d55-0f62c8017bb8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_019f28d8-3cb1-7146-9e04-2e246a6dee73" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_019f28d8-3cb1-7525-8d55-0f62c8017bb8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReinsurerConcentrationRiskMember_019f28d8-3cb1-77fb-ada4-1c22f35303fb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ReinsurerConcentrationRiskMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskTypeDomain_019f28d8-3cb1-7525-8d55-0f62c8017bb8" xlink:to="loc_us-gaap_ReinsurerConcentrationRiskMember_019f28d8-3cb1-77fb-ada4-1c22f35303fb" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019f28d8-3cb1-7911-85d3-a367d6c0c55c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskByBenchmarkAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_CededCreditRiskTable_019f28d8-3cb1-7aa6-8d6b-f82017d1711d" xlink:to="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019f28d8-3cb1-7911-85d3-a367d6c0c55c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019f28d8-3cb1-7911-85d3-a367d6c0c55c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019f28d8-3cb1-7911-85d3-a367d6c0c55c" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019f28d8-3cb1-7911-85d3-a367d6c0c55c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019f28d8-3cb1-7ace-8ab1-59e5ed92277c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019f28d8-3cb1-7911-85d3-a367d6c0c55c" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019f28d8-3cb1-7ace-8ab1-59e5ed92277c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReinsuranceRecoverableForPaidAndUnpaidClaimsAndClaimsAdjustmentsMember_019f28d8-3cb1-728f-99d6-2023b0633bed" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ReinsuranceRecoverableForPaidAndUnpaidClaimsAndClaimsAdjustmentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019f28d8-3cb1-7ace-8ab1-59e5ed92277c" xlink:to="loc_us-gaap_ReinsuranceRecoverableForPaidAndUnpaidClaimsAndClaimsAdjustmentsMember_019f28d8-3cb1-728f-99d6-2023b0633bed" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquityTotalMember_019f28d8-3cb1-732a-98dd-1b35c38c1ec2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquityTotalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019f28d8-3cb1-7ace-8ab1-59e5ed92277c" xlink:to="loc_us-gaap_StockholdersEquityTotalMember_019f28d8-3cb1-732a-98dd-1b35c38c1ec2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CreditRatingAMBestAxis_019f28d8-3cb1-7d26-92f9-5e92779cb881" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_CreditRatingAMBestAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_CededCreditRiskTable_019f28d8-3cb1-7aa6-8d6b-f82017d1711d" xlink:to="loc_srt_CreditRatingAMBestAxis_019f28d8-3cb1-7d26-92f9-5e92779cb881" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CreditRatingAMBestDomain_019f28d8-3cb1-7d26-92f9-5e92779cb881_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_CreditRatingAMBestDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_CreditRatingAMBestAxis_019f28d8-3cb1-7d26-92f9-5e92779cb881" xlink:to="loc_srt_CreditRatingAMBestDomain_019f28d8-3cb1-7d26-92f9-5e92779cb881_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CreditRatingAMBestDomain_019f28d8-3cb1-78a0-8312-499e5c708e66" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_CreditRatingAMBestDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_CreditRatingAMBestAxis_019f28d8-3cb1-7d26-92f9-5e92779cb881" xlink:to="loc_srt_CreditRatingAMBestDomain_019f28d8-3cb1-78a0-8312-499e5c708e66" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_AmBestAminusOrBetterRatingMember_019f28d8-3cb1-7a25-8baa-0a3165308d4f" xlink:href="acgl-20260630.xsd#acgl_AmBestAminusOrBetterRatingMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_CreditRatingAMBestDomain_019f28d8-3cb1-78a0-8312-499e5c708e66" xlink:to="loc_acgl_AmBestAminusOrBetterRatingMember_019f28d8-3cb1-7a25-8baa-0a3165308d4f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_NotRatedMember_019f28d8-3cb1-7150-bbd6-32754a396ea1" xlink:href="acgl-20260630.xsd#acgl_NotRatedMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_CreditRatingAMBestDomain_019f28d8-3cb1-78a0-8312-499e5c708e66" xlink:to="loc_acgl_NotRatedMember_019f28d8-3cb1-7150-bbd6-32754a396ea1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CededCreditRiskAxis_019f28d8-3cb1-7814-95c9-a96c4071ce42" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CededCreditRiskAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_CededCreditRiskTable_019f28d8-3cb1-7aa6-8d6b-f82017d1711d" xlink:to="loc_us-gaap_CededCreditRiskAxis_019f28d8-3cb1-7814-95c9-a96c4071ce42" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CededCreditRiskReinsurerDomain_019f28d8-3cb1-7814-95c9-a96c4071ce42_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CededCreditRiskReinsurerDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CededCreditRiskAxis_019f28d8-3cb1-7814-95c9-a96c4071ce42" xlink:to="loc_us-gaap_CededCreditRiskReinsurerDomain_019f28d8-3cb1-7814-95c9-a96c4071ce42_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CededCreditRiskReinsurerDomain_019f28d8-3cb1-7681-b5cd-f9b233d3f0d0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CededCreditRiskReinsurerDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CededCreditRiskAxis_019f28d8-3cb1-7814-95c9-a96c4071ce42" xlink:to="loc_us-gaap_CededCreditRiskReinsurerDomain_019f28d8-3cb1-7681-b5cd-f9b233d3f0d0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_AllOtherCarriersMember_019f28d8-3cb1-79fc-a825-5b56d22dcad1" xlink:href="acgl-20260630.xsd#acgl_AllOtherCarriersMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CededCreditRiskReinsurerDomain_019f28d8-3cb1-7681-b5cd-f9b233d3f0d0" xlink:to="loc_acgl_AllOtherCarriersMember_019f28d8-3cb1-79fc-a825-5b56d22dcad1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_LargestBalanceDueFromAnyOneCarrierMember_019f28d8-3cb1-7ffb-9479-63b4d3ef0c2d" xlink:href="acgl-20260630.xsd#acgl_LargestBalanceDueFromAnyOneCarrierMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CededCreditRiskReinsurerDomain_019f28d8-3cb1-7681-b5cd-f9b233d3f0d0" xlink:to="loc_acgl_LargestBalanceDueFromAnyOneCarrierMember_019f28d8-3cb1-7ffb-9479-63b4d3ef0c2d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CededCreditRiskCollateralizationAxis_019f28d8-3cb1-7a64-9f30-517c4fd3db27" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CededCreditRiskCollateralizationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_CededCreditRiskTable_019f28d8-3cb1-7aa6-8d6b-f82017d1711d" xlink:to="loc_us-gaap_CededCreditRiskCollateralizationAxis_019f28d8-3cb1-7a64-9f30-517c4fd3db27" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CededCreditRiskCollateralizationDomain_019f28d8-3cb1-7a64-9f30-517c4fd3db27_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CededCreditRiskCollateralizationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CededCreditRiskCollateralizationAxis_019f28d8-3cb1-7a64-9f30-517c4fd3db27" xlink:to="loc_us-gaap_CededCreditRiskCollateralizationDomain_019f28d8-3cb1-7a64-9f30-517c4fd3db27_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CededCreditRiskCollateralizationDomain_019f28d8-3cb1-7f53-8c69-98767bb08d59" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CededCreditRiskCollateralizationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CededCreditRiskCollateralizationAxis_019f28d8-3cb1-7a64-9f30-517c4fd3db27" xlink:to="loc_us-gaap_CededCreditRiskCollateralizationDomain_019f28d8-3cb1-7f53-8c69-98767bb08d59" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CededCreditRiskSecuredMember_019f28d8-3cb1-747f-b161-d99920800c39" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CededCreditRiskSecuredMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CededCreditRiskCollateralizationDomain_019f28d8-3cb1-7f53-8c69-98767bb08d59" xlink:to="loc_us-gaap_CededCreditRiskSecuredMember_019f28d8-3cb1-747f-b161-d99920800c39" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReinsuranceRecoverablesOnPaidAndUnpaidLosses_019f28d8-3cb1-753e-9b3f-f41c86a01bda" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ReinsuranceRecoverablesOnPaidAndUnpaidLosses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CededCreditRiskLineItems_019f28d8-3cb1-73d1-a6cd-5dc1c786f198" xlink:to="loc_us-gaap_ReinsuranceRecoverablesOnPaidAndUnpaidLosses_019f28d8-3cb1-753e-9b3f-f41c86a01bda" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskPercentage1_019f28d8-3cb1-7a8e-83b3-a9ef17ab06f2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskPercentage1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CededCreditRiskLineItems_019f28d8-3cb1-73d1-a6cd-5dc1c786f198" xlink:to="loc_us-gaap_ConcentrationRiskPercentage1_019f28d8-3cb1-7a8e-83b3-a9ef17ab06f2" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.archcapgroup.com/role/InvestmentInformationSummaryofavailableforsalesecuritiesDetails" xlink:type="simple" xlink:href="acgl-20260630.xsd#InvestmentInformationSummaryofavailableforsalesecuritiesDetails"/>
  <link:definitionLink xlink:role="http://www.archcapgroup.com/role/InvestmentInformationSummaryofavailableforsalesecuritiesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019f28d8-3cb1-7978-9f16-d83dffb966bc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_019f28d8-3cb1-723e-999c-0cf2cb0d4acb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019f28d8-3cb1-7978-9f16-d83dffb966bc" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_019f28d8-3cb1-723e-999c-0cf2cb0d4acb" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_019f28d8-3cb1-7215-aba6-99ca11079c52" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_019f28d8-3cb1-723e-999c-0cf2cb0d4acb" xlink:to="loc_us-gaap_FinancialInstrumentAxis_019f28d8-3cb1-7215-aba6-99ca11079c52" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb1-7215-aba6-99ca11079c52_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019f28d8-3cb1-7215-aba6-99ca11079c52" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb1-7215-aba6-99ca11079c52_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb1-795e-852e-35d7011881ab" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019f28d8-3cb1-7215-aba6-99ca11079c52" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb1-795e-852e-35d7011881ab" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FixedMaturitiesMember_019f28d8-3cb1-75aa-98f8-522e1811995b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FixedMaturitiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb1-795e-852e-35d7011881ab" xlink:to="loc_us-gaap_FixedMaturitiesMember_019f28d8-3cb1-75aa-98f8-522e1811995b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateDebtSecuritiesMember_019f28d8-3cb1-7350-a4ea-66e01ef2a5ee" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CorporateDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FixedMaturitiesMember_019f28d8-3cb1-75aa-98f8-522e1811995b" xlink:to="loc_us-gaap_CorporateDebtSecuritiesMember_019f28d8-3cb1-7350-a4ea-66e01ef2a5ee" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USTreasuryAndGovernmentMember_019f28d8-3cb1-70ea-ad20-d3eb3440df1a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_USTreasuryAndGovernmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FixedMaturitiesMember_019f28d8-3cb1-75aa-98f8-522e1811995b" xlink:to="loc_us-gaap_USTreasuryAndGovernmentMember_019f28d8-3cb1-70ea-ad20-d3eb3440df1a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetBackedSecuritiesMember_019f28d8-3cb1-7144-8780-e66278f1d78d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AssetBackedSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FixedMaturitiesMember_019f28d8-3cb1-75aa-98f8-522e1811995b" xlink:to="loc_us-gaap_AssetBackedSecuritiesMember_019f28d8-3cb1-7144-8780-e66278f1d78d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForeignGovernmentDebtSecuritiesMember_019f28d8-3cb1-7661-99bb-cd82d849297c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ForeignGovernmentDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FixedMaturitiesMember_019f28d8-3cb1-75aa-98f8-522e1811995b" xlink:to="loc_us-gaap_ForeignGovernmentDebtSecuritiesMember_019f28d8-3cb1-7661-99bb-cd82d849297c" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResidentialMortgageBackedSecuritiesMember_019f28d8-3cb1-7418-8347-220600987de7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ResidentialMortgageBackedSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FixedMaturitiesMember_019f28d8-3cb1-75aa-98f8-522e1811995b" xlink:to="loc_us-gaap_ResidentialMortgageBackedSecuritiesMember_019f28d8-3cb1-7418-8347-220600987de7" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialMortgageBackedSecuritiesMember_019f28d8-3cb1-76c6-9448-10ebe1577bf0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommercialMortgageBackedSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FixedMaturitiesMember_019f28d8-3cb1-75aa-98f8-522e1811995b" xlink:to="loc_us-gaap_CommercialMortgageBackedSecuritiesMember_019f28d8-3cb1-76c6-9448-10ebe1577bf0" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MunicipalBondsMember_019f28d8-3cb1-7d58-bf5f-b97f2d62e5fa" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MunicipalBondsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FixedMaturitiesMember_019f28d8-3cb1-75aa-98f8-522e1811995b" xlink:to="loc_us-gaap_MunicipalBondsMember_019f28d8-3cb1-7d58-bf5f-b97f2d62e5fa" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortTermInvestmentsMember_019f28d8-3cb1-7f02-bd4e-252d4a36976a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShortTermInvestmentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb1-795e-852e-35d7011881ab" xlink:to="loc_us-gaap_ShortTermInvestmentsMember_019f28d8-3cb1-7f02-bd4e-252d4a36976a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_019f28d8-3cb1-768b-a5e9-b3b88f3a3b33" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleSecuritiesDebtSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019f28d8-3cb1-7978-9f16-d83dffb966bc" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_019f28d8-3cb1-768b-a5e9-b3b88f3a3b33" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax_019f28d8-3cb1-7330-8c16-cf5a1ac72e10" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019f28d8-3cb1-7978-9f16-d83dffb966bc" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax_019f28d8-3cb1-7330-8c16-cf5a1ac72e10" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax_019f28d8-3cb1-708b-9be3-41d7f649a4f8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019f28d8-3cb1-7978-9f16-d83dffb966bc" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax_019f28d8-3cb1-708b-9be3-41d7f649a4f8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleAllowanceForCreditLoss_019f28d8-3cb1-7bf2-937b-c6b3be185ae7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleAllowanceForCreditLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019f28d8-3cb1-7978-9f16-d83dffb966bc" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleAllowanceForCreditLoss_019f28d8-3cb1-7bf2-937b-c6b3be185ae7" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis_019f28d8-3cb1-723f-95c6-3bf0d5a1ac1a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019f28d8-3cb1-7978-9f16-d83dffb966bc" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis_019f28d8-3cb1-723f-95c6-3bf0d5a1ac1a" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.archcapgroup.com/role/InvestmentInformationAgingofavailableforsalesecuritiesinanunrealizedlosspositionDetails" xlink:type="simple" xlink:href="acgl-20260630.xsd#InvestmentInformationAgingofavailableforsalesecuritiesinanunrealizedlosspositionDetails"/>
  <link:definitionLink xlink:role="http://www.archcapgroup.com/role/InvestmentInformationAgingofavailableforsalesecuritiesinanunrealizedlosspositionDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019f28d8-3cb1-7cd0-b572-5616413a3295" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_019f28d8-3cb1-7151-b4ec-e48f15df3e8a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019f28d8-3cb1-7cd0-b572-5616413a3295" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_019f28d8-3cb1-7151-b4ec-e48f15df3e8a" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_019f28d8-3cb1-7984-9276-2e526cb63286" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_019f28d8-3cb1-7151-b4ec-e48f15df3e8a" xlink:to="loc_us-gaap_FinancialInstrumentAxis_019f28d8-3cb1-7984-9276-2e526cb63286" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb1-7984-9276-2e526cb63286_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019f28d8-3cb1-7984-9276-2e526cb63286" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb1-7984-9276-2e526cb63286_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb1-743c-89bd-fa6889b4f8a5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019f28d8-3cb1-7984-9276-2e526cb63286" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb1-743c-89bd-fa6889b4f8a5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FixedMaturitiesMember_019f28d8-3cb1-7706-8f2a-e325406bce49" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FixedMaturitiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb1-743c-89bd-fa6889b4f8a5" xlink:to="loc_us-gaap_FixedMaturitiesMember_019f28d8-3cb1-7706-8f2a-e325406bce49" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateDebtSecuritiesMember_019f28d8-3cb1-70e9-8857-3260eeff0c0d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CorporateDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FixedMaturitiesMember_019f28d8-3cb1-7706-8f2a-e325406bce49" xlink:to="loc_us-gaap_CorporateDebtSecuritiesMember_019f28d8-3cb1-70e9-8857-3260eeff0c0d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USTreasuryAndGovernmentMember_019f28d8-3cb1-7c75-a8ca-d487947e1fae" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_USTreasuryAndGovernmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FixedMaturitiesMember_019f28d8-3cb1-7706-8f2a-e325406bce49" xlink:to="loc_us-gaap_USTreasuryAndGovernmentMember_019f28d8-3cb1-7c75-a8ca-d487947e1fae" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForeignGovernmentDebtSecuritiesMember_019f28d8-3cb1-7387-ae66-4424f804a2ed" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ForeignGovernmentDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FixedMaturitiesMember_019f28d8-3cb1-7706-8f2a-e325406bce49" xlink:to="loc_us-gaap_ForeignGovernmentDebtSecuritiesMember_019f28d8-3cb1-7387-ae66-4424f804a2ed" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetBackedSecuritiesMember_019f28d8-3cb1-7bf8-be1c-91a42ed8d3cd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AssetBackedSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FixedMaturitiesMember_019f28d8-3cb1-7706-8f2a-e325406bce49" xlink:to="loc_us-gaap_AssetBackedSecuritiesMember_019f28d8-3cb1-7bf8-be1c-91a42ed8d3cd" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResidentialMortgageBackedSecuritiesMember_019f28d8-3cb1-799c-82e6-5edff4ba78dc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ResidentialMortgageBackedSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FixedMaturitiesMember_019f28d8-3cb1-7706-8f2a-e325406bce49" xlink:to="loc_us-gaap_ResidentialMortgageBackedSecuritiesMember_019f28d8-3cb1-799c-82e6-5edff4ba78dc" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialMortgageBackedSecuritiesMember_019f28d8-3cb1-7618-a157-bacdb10e08fb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommercialMortgageBackedSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FixedMaturitiesMember_019f28d8-3cb1-7706-8f2a-e325406bce49" xlink:to="loc_us-gaap_CommercialMortgageBackedSecuritiesMember_019f28d8-3cb1-7618-a157-bacdb10e08fb" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MunicipalBondsMember_019f28d8-3cb1-7ebd-80f5-97a24d61b23b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MunicipalBondsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FixedMaturitiesMember_019f28d8-3cb1-7706-8f2a-e325406bce49" xlink:to="loc_us-gaap_MunicipalBondsMember_019f28d8-3cb1-7ebd-80f5-97a24d61b23b" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortTermInvestmentsMember_019f28d8-3cb1-7001-a51a-d03298eddaca" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShortTermInvestmentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb1-743c-89bd-fa6889b4f8a5" xlink:to="loc_us-gaap_ShortTermInvestmentsMember_019f28d8-3cb1-7001-a51a-d03298eddaca" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableforSaleUnrealizedLossPositionAbstract_019f28d8-3cb1-7e54-95f0-1f270a48663c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableforSaleUnrealizedLossPositionAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019f28d8-3cb1-7cd0-b572-5616413a3295" xlink:to="loc_us-gaap_DebtSecuritiesAvailableforSaleUnrealizedLossPositionAbstract_019f28d8-3cb1-7e54-95f0-1f270a48663c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12Months_019f28d8-3cb1-7021-8eff-4275050c9ea6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12Months"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtSecuritiesAvailableforSaleUnrealizedLossPositionAbstract_019f28d8-3cb1-7e54-95f0-1f270a48663c" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12Months_019f28d8-3cb1-7021-8eff-4275050c9ea6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLonger_019f28d8-3cb1-7cfc-b219-e9a685151b57" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLonger"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtSecuritiesAvailableforSaleUnrealizedLossPositionAbstract_019f28d8-3cb1-7e54-95f0-1f270a48663c" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLonger_019f28d8-3cb1-7cfc-b219-e9a685151b57" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPosition_019f28d8-3cb1-786c-9cf2-1a2156d5035f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPosition"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtSecuritiesAvailableforSaleUnrealizedLossPositionAbstract_019f28d8-3cb1-7e54-95f0-1f270a48663c" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPosition_019f28d8-3cb1-786c-9cf2-1a2156d5035f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLossAbstract_019f28d8-3cb1-76cf-a77c-5821c879fbc6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLossAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019f28d8-3cb1-7cd0-b572-5616413a3295" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLossAbstract_019f28d8-3cb1-76cf-a77c-5821c879fbc6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss_019f28d8-3cb1-7650-acb6-7bdab8b468e0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLossAbstract_019f28d8-3cb1-76cf-a77c-5821c879fbc6" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss_019f28d8-3cb1-7650-acb6-7bdab8b468e0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss_019f28d8-3cb1-7abf-99be-e76a467cc877" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLossAbstract_019f28d8-3cb1-76cf-a77c-5821c879fbc6" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss_019f28d8-3cb1-7abf-99be-e76a467cc877" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss_019f28d8-3cb1-755e-bb26-c59fc70078f7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLossAbstract_019f28d8-3cb1-76cf-a77c-5821c879fbc6" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss_019f28d8-3cb1-755e-bb26-c59fc70078f7" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.archcapgroup.com/role/InvestmentInformationMaturityprofileofavailableforsalesecuritiesDetails" xlink:type="simple" xlink:href="acgl-20260630.xsd#InvestmentInformationMaturityprofileofavailableforsalesecuritiesDetails"/>
  <link:definitionLink xlink:role="http://www.archcapgroup.com/role/InvestmentInformationMaturityprofileofavailableforsalesecuritiesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019f28d8-3cb1-7de7-9bfa-46411a534e7c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_019f28d8-3cb1-771c-b065-169f17d4a478" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019f28d8-3cb1-7de7-9bfa-46411a534e7c" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_019f28d8-3cb1-771c-b065-169f17d4a478" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_019f28d8-3cb1-73f4-9052-3d5332bbd648" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_019f28d8-3cb1-771c-b065-169f17d4a478" xlink:to="loc_us-gaap_FinancialInstrumentAxis_019f28d8-3cb1-73f4-9052-3d5332bbd648" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb1-73f4-9052-3d5332bbd648_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019f28d8-3cb1-73f4-9052-3d5332bbd648" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb1-73f4-9052-3d5332bbd648_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb1-72c3-bf6f-5a0cec00d097" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019f28d8-3cb1-73f4-9052-3d5332bbd648" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb1-72c3-bf6f-5a0cec00d097" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FixedMaturitiesMember_019f28d8-3cb1-79fa-8a8b-069ebb6099be" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FixedMaturitiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb1-72c3-bf6f-5a0cec00d097" xlink:to="loc_us-gaap_FixedMaturitiesMember_019f28d8-3cb1-79fa-8a8b-069ebb6099be" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResidentialMortgageBackedSecuritiesMember_019f28d8-3cb1-7bd2-988d-6388516882c6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ResidentialMortgageBackedSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FixedMaturitiesMember_019f28d8-3cb1-79fa-8a8b-069ebb6099be" xlink:to="loc_us-gaap_ResidentialMortgageBackedSecuritiesMember_019f28d8-3cb1-7bd2-988d-6388516882c6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialMortgageBackedSecuritiesMember_019f28d8-3cb1-70da-8c2f-45c2fcfc5c9c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommercialMortgageBackedSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FixedMaturitiesMember_019f28d8-3cb1-79fa-8a8b-069ebb6099be" xlink:to="loc_us-gaap_CommercialMortgageBackedSecuritiesMember_019f28d8-3cb1-70da-8c2f-45c2fcfc5c9c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetBackedSecuritiesMember_019f28d8-3cb1-7ab7-a17d-4b849f6c864f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AssetBackedSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FixedMaturitiesMember_019f28d8-3cb1-79fa-8a8b-069ebb6099be" xlink:to="loc_us-gaap_AssetBackedSecuritiesMember_019f28d8-3cb1-7ab7-a17d-4b849f6c864f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesFairValueAbstract_019f28d8-3cb1-72cd-9b5d-00c3e9dc0029" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesFairValueAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019f28d8-3cb1-7de7-9bfa-46411a534e7c" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesFairValueAbstract_019f28d8-3cb1-72cd-9b5d-00c3e9dc0029" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue_019f28d8-3cb1-756c-a606-c9a719486fff" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesFairValueAbstract_019f28d8-3cb1-72cd-9b5d-00c3e9dc0029" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue_019f28d8-3cb1-756c-a606-c9a719486fff" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue_019f28d8-3cb1-7c83-aec0-8df414f300ea" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesFairValueAbstract_019f28d8-3cb1-72cd-9b5d-00c3e9dc0029" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue_019f28d8-3cb1-7c83-aec0-8df414f300ea" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue_019f28d8-3cb1-7782-8124-baed8cb27579" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesFairValueAbstract_019f28d8-3cb1-72cd-9b5d-00c3e9dc0029" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue_019f28d8-3cb1-7782-8124-baed8cb27579" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsFairValue_019f28d8-3cb1-7780-a959-9e18a945eb7d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesFairValueAbstract_019f28d8-3cb1-72cd-9b5d-00c3e9dc0029" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsFairValue_019f28d8-3cb1-7780-a959-9e18a945eb7d" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDate_019f28d8-3cb1-7e78-ba20-7ffb2df02f80" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesFairValueAbstract_019f28d8-3cb1-72cd-9b5d-00c3e9dc0029" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDate_019f28d8-3cb1-7e78-ba20-7ffb2df02f80" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateFairValue_019f28d8-3cb1-74e2-a626-d51147c754f9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesFairValueAbstract_019f28d8-3cb1-72cd-9b5d-00c3e9dc0029" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateFairValue_019f28d8-3cb1-74e2-a626-d51147c754f9" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_019f28d8-3cb1-7b17-be81-3ee8b6480c9c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleSecuritiesDebtSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesFairValueAbstract_019f28d8-3cb1-72cd-9b5d-00c3e9dc0029" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_019f28d8-3cb1-7b17-be81-3ee8b6480c9c" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAmortizedCostAbstract_019f28d8-3cb1-7f46-b68f-d5f45867b76f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAmortizedCostAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019f28d8-3cb1-7de7-9bfa-46411a534e7c" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAmortizedCostAbstract_019f28d8-3cb1-7f46-b68f-d5f45867b76f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasisAbstract_019f28d8-3cb1-781d-ac78-47f3257a9f04" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasisAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAmortizedCostAbstract_019f28d8-3cb1-7f46-b68f-d5f45867b76f" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasisAbstract_019f28d8-3cb1-781d-ac78-47f3257a9f04" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearAmortizedCost_019f28d8-3cb1-7bb3-ba8a-8e6968ef2906" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearAmortizedCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasisAbstract_019f28d8-3cb1-781d-ac78-47f3257a9f04" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearAmortizedCost_019f28d8-3cb1-7bb3-ba8a-8e6968ef2906" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsAmortizedCost_019f28d8-3cb1-7d3c-8bba-de545b742ce2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsAmortizedCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasisAbstract_019f28d8-3cb1-781d-ac78-47f3257a9f04" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsAmortizedCost_019f28d8-3cb1-7d3c-8bba-de545b742ce2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsAmortizedCost_019f28d8-3cb1-79dd-a469-490536ed3d9d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsAmortizedCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasisAbstract_019f28d8-3cb1-781d-ac78-47f3257a9f04" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsAmortizedCost_019f28d8-3cb1-79dd-a469-490536ed3d9d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsAmortizedCost_019f28d8-3cb1-7c98-8eb3-087d8c5a3409" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsAmortizedCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasisAbstract_019f28d8-3cb1-781d-ac78-47f3257a9f04" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsAmortizedCost_019f28d8-3cb1-7c98-8eb3-087d8c5a3409" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasis_019f28d8-3cb1-7f96-bd3a-e33a022abd28" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasisAbstract_019f28d8-3cb1-781d-ac78-47f3257a9f04" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasis_019f28d8-3cb1-7f96-bd3a-e33a022abd28" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateAmortizedCost_019f28d8-3cb1-75aa-a721-57b78d29f845" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateAmortizedCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAmortizedCostAbstract_019f28d8-3cb1-7f46-b68f-d5f45867b76f" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateAmortizedCost_019f28d8-3cb1-75aa-a721-57b78d29f845" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis_019f28d8-3cb1-78ad-89d4-77f5238c2d68" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAmortizedCostAbstract_019f28d8-3cb1-7f46-b68f-d5f45867b76f" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis_019f28d8-3cb1-78ad-89d4-77f5238c2d68" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.archcapgroup.com/role/InvestmentInformationOtherinvestmentsatfairvalueDetails" xlink:type="simple" xlink:href="acgl-20260630.xsd#InvestmentInformationOtherinvestmentsatfairvalueDetails"/>
  <link:definitionLink xlink:role="http://www.archcapgroup.com/role/InvestmentInformationOtherinvestmentsatfairvalueDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_acgl_ScheduleOfOtherInvestmentsLineItems_019f28d8-3cb1-7f63-8012-ad3235d5c4b4" xlink:href="acgl-20260630.xsd#acgl_ScheduleOfOtherInvestmentsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_ScheduleOfOtherInvestmentsTable_019f28d8-3cb1-70f2-b0be-fb97f8652ca2" xlink:href="acgl-20260630.xsd#acgl_ScheduleOfOtherInvestmentsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_acgl_ScheduleOfOtherInvestmentsLineItems_019f28d8-3cb1-7f63-8012-ad3235d5c4b4" xlink:to="loc_acgl_ScheduleOfOtherInvestmentsTable_019f28d8-3cb1-70f2-b0be-fb97f8652ca2" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_019f28d8-3cb1-7490-83a0-196621d0f681" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_acgl_ScheduleOfOtherInvestmentsTable_019f28d8-3cb1-70f2-b0be-fb97f8652ca2" xlink:to="loc_us-gaap_FinancialInstrumentAxis_019f28d8-3cb1-7490-83a0-196621d0f681" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb1-7490-83a0-196621d0f681_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019f28d8-3cb1-7490-83a0-196621d0f681" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb1-7490-83a0-196621d0f681_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb1-7df7-9233-9e75a1d66e31" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019f28d8-3cb1-7490-83a0-196621d0f681" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb1-7df7-9233-9e75a1d66e31" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_OtherInvestmentsFairValueOptionMember_019f28d8-3cb1-7a2b-af68-9da51abd894a" xlink:href="acgl-20260630.xsd#acgl_OtherInvestmentsFairValueOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb1-7df7-9233-9e75a1d66e31" xlink:to="loc_acgl_OtherInvestmentsFairValueOptionMember_019f28d8-3cb1-7a2b-af68-9da51abd894a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_InvestmentGradeFixedIncomeMember_019f28d8-3cb1-7379-b9bf-3f1d6a9a3897" xlink:href="acgl-20260630.xsd#acgl_InvestmentGradeFixedIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_acgl_OtherInvestmentsFairValueOptionMember_019f28d8-3cb1-7a2b-af68-9da51abd894a" xlink:to="loc_acgl_InvestmentGradeFixedIncomeMember_019f28d8-3cb1-7379-b9bf-3f1d6a9a3897" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PrivateEquityFundsMember_019f28d8-3cb1-78f8-a8f1-5f0c32ffca34" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PrivateEquityFundsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_acgl_OtherInvestmentsFairValueOptionMember_019f28d8-3cb1-7a2b-af68-9da51abd894a" xlink:to="loc_us-gaap_PrivateEquityFundsMember_019f28d8-3cb1-78f8-a8f1-5f0c32ffca34" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_OtherInvestmentsLendingMember_019f28d8-3cb1-7f5f-9be0-05262d5de45b" xlink:href="acgl-20260630.xsd#acgl_OtherInvestmentsLendingMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_acgl_OtherInvestmentsFairValueOptionMember_019f28d8-3cb1-7a2b-af68-9da51abd894a" xlink:to="loc_acgl_OtherInvestmentsLendingMember_019f28d8-3cb1-7f5f-9be0-05262d5de45b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_TermLoanInvestmentsMember_019f28d8-3cb1-7de3-865a-a8da4d4e982a" xlink:href="acgl-20260630.xsd#acgl_TermLoanInvestmentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_acgl_OtherInvestmentsFairValueOptionMember_019f28d8-3cb1-7a2b-af68-9da51abd894a" xlink:to="loc_acgl_TermLoanInvestmentsMember_019f28d8-3cb1-7de3-865a-a8da4d4e982a" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_CreditRelatedFundsMember_019f28d8-3cb1-7337-8119-741a72b2bdf5" xlink:href="acgl-20260630.xsd#acgl_CreditRelatedFundsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_acgl_OtherInvestmentsFairValueOptionMember_019f28d8-3cb1-7a2b-af68-9da51abd894a" xlink:to="loc_acgl_CreditRelatedFundsMember_019f28d8-3cb1-7337-8119-741a72b2bdf5" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_FairValueOptionEquitiesMember_019f28d8-3cb1-7db9-86d1-1196ef776e5a" xlink:href="acgl-20260630.xsd#acgl_FairValueOptionEquitiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_acgl_OtherInvestmentsFairValueOptionMember_019f28d8-3cb1-7a2b-af68-9da51abd894a" xlink:to="loc_acgl_FairValueOptionEquitiesMember_019f28d8-3cb1-7db9-86d1-1196ef776e5a" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_OtherInvestmentsEnergyMember_019f28d8-3cb1-7417-bf19-d5b8fe3f53a4" xlink:href="acgl-20260630.xsd#acgl_OtherInvestmentsEnergyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_acgl_OtherInvestmentsFairValueOptionMember_019f28d8-3cb1-7a2b-af68-9da51abd894a" xlink:to="loc_acgl_OtherInvestmentsEnergyMember_019f28d8-3cb1-7417-bf19-d5b8fe3f53a4" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FixedIncomeInvestmentsMember_019f28d8-3cb1-78c2-8a06-186d877c83c6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FixedIncomeInvestmentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb1-7df7-9233-9e75a1d66e31" xlink:to="loc_us-gaap_FixedIncomeInvestmentsMember_019f28d8-3cb1-78c2-8a06-186d877c83c6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_ShortTermInvestmentsFairValueOptionMember_019f28d8-3cb1-7f11-9b35-0aead42c58a6" xlink:href="acgl-20260630.xsd#acgl_ShortTermInvestmentsFairValueOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb1-7df7-9233-9e75a1d66e31" xlink:to="loc_acgl_ShortTermInvestmentsFairValueOptionMember_019f28d8-3cb1-7f11-9b35-0aead42c58a6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesMember_019f28d8-3cb1-7222-880c-b4c5ec5b379b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquitySecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb1-7df7-9233-9e75a1d66e31" xlink:to="loc_us-gaap_EquitySecuritiesMember_019f28d8-3cb1-7222-880c-b4c5ec5b379b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherInvestments_019f28d8-3cb1-715d-b437-f3574026d5a9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherInvestments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_acgl_ScheduleOfOtherInvestmentsLineItems_019f28d8-3cb1-7f63-8012-ad3235d5c4b4" xlink:to="loc_us-gaap_OtherInvestments_019f28d8-3cb1-715d-b437-f3574026d5a9" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.archcapgroup.com/role/InvestmentInformationNetinvestmentincomeDetails" xlink:type="simple" xlink:href="acgl-20260630.xsd#InvestmentInformationNetinvestmentincomeDetails"/>
  <link:definitionLink xlink:role="http://www.archcapgroup.com/role/InvestmentInformationNetinvestmentincomeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfInvestmentIncomeReportedAmountsByCategoryLineItems_019f28d8-3cb2-7695-901d-852c0653c4e0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfInvestmentIncomeReportedAmountsByCategoryLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfInvestmentIncomeReportedAmountsByCategoryTable_019f28d8-3cb1-78a9-b87f-a4a378d21a23" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfInvestmentIncomeReportedAmountsByCategoryTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ScheduleOfInvestmentIncomeReportedAmountsByCategoryLineItems_019f28d8-3cb2-7695-901d-852c0653c4e0" xlink:to="loc_us-gaap_ScheduleOfInvestmentIncomeReportedAmountsByCategoryTable_019f28d8-3cb1-78a9-b87f-a4a378d21a23" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_019f28d8-3cb1-7eec-b2b7-705ea075d0f3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfInvestmentIncomeReportedAmountsByCategoryTable_019f28d8-3cb1-78a9-b87f-a4a378d21a23" xlink:to="loc_us-gaap_FinancialInstrumentAxis_019f28d8-3cb1-7eec-b2b7-705ea075d0f3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb1-7eec-b2b7-705ea075d0f3_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019f28d8-3cb1-7eec-b2b7-705ea075d0f3" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb1-7eec-b2b7-705ea075d0f3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb1-7138-8170-e328f8da87c8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019f28d8-3cb1-7eec-b2b7-705ea075d0f3" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb1-7138-8170-e328f8da87c8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FixedMaturitiesMember_019f28d8-3cb2-728f-9aba-94736b86e26a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FixedMaturitiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb1-7138-8170-e328f8da87c8" xlink:to="loc_us-gaap_FixedMaturitiesMember_019f28d8-3cb2-728f-9aba-94736b86e26a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortTermInvestmentsMember_019f28d8-3cb2-7f04-8e1f-a0c6c8386a5c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShortTermInvestmentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb1-7138-8170-e328f8da87c8" xlink:to="loc_us-gaap_ShortTermInvestmentsMember_019f28d8-3cb2-7f04-8e1f-a0c6c8386a5c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesMember_019f28d8-3cb2-7c56-996c-81cb4cd1b8c6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquitySecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb1-7138-8170-e328f8da87c8" xlink:to="loc_us-gaap_EquitySecuritiesMember_019f28d8-3cb2-7c56-996c-81cb4cd1b8c6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_OtherInvestmentIncomeMember_019f28d8-3cb2-7a86-8a00-71337f495388" xlink:href="acgl-20260630.xsd#acgl_OtherInvestmentIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb1-7138-8170-e328f8da87c8" xlink:to="loc_acgl_OtherInvestmentIncomeMember_019f28d8-3cb2-7a86-8a00-71337f495388" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentIncomeNetAbstract_019f28d8-3cb2-7d1f-907a-217f84d7d3be" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InvestmentIncomeNetAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfInvestmentIncomeReportedAmountsByCategoryLineItems_019f28d8-3cb2-7695-901d-852c0653c4e0" xlink:to="loc_us-gaap_InvestmentIncomeNetAbstract_019f28d8-3cb2-7d1f-907a-217f84d7d3be" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentIncomeInterestAndDividend_019f28d8-3cb2-73d5-a621-8988fd9838b0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InvestmentIncomeInterestAndDividend"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentIncomeNetAbstract_019f28d8-3cb2-7d1f-907a-217f84d7d3be" xlink:to="loc_us-gaap_InvestmentIncomeInterestAndDividend_019f28d8-3cb2-73d5-a621-8988fd9838b0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentIncomeInvestmentExpense_019f28d8-3cb2-704f-9896-97b84f414457" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InvestmentIncomeInvestmentExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentIncomeNetAbstract_019f28d8-3cb2-7d1f-907a-217f84d7d3be" xlink:to="loc_us-gaap_InvestmentIncomeInvestmentExpense_019f28d8-3cb2-704f-9896-97b84f414457" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentIncomeNet_019f28d8-3cb2-7737-bd31-58536beaae9c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InvestmentIncomeNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentIncomeNetAbstract_019f28d8-3cb2-7d1f-907a-217f84d7d3be" xlink:to="loc_us-gaap_InvestmentIncomeNet_019f28d8-3cb2-7737-bd31-58536beaae9c" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.archcapgroup.com/role/InvestmentInformationNetrealizedgainsandlossesDetails" xlink:type="simple" xlink:href="acgl-20260630.xsd#InvestmentInformationNetrealizedgainsandlossesDetails"/>
  <link:definitionLink xlink:role="http://www.archcapgroup.com/role/InvestmentInformationNetrealizedgainsandlossesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfGainLossOnInvestmentsIncludingMarketableSecuritiesAndInvestmentsHeldAtCostIncomeStatementReportedAmountsSummaryLineItems_019f28d8-3cb2-70b3-ad68-02ef9e3b93bb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfGainLossOnInvestmentsIncludingMarketableSecuritiesAndInvestmentsHeldAtCostIncomeStatementReportedAmountsSummaryLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfGainLossOnInvestmentsTable_019f28d8-3cb2-7a03-a6a4-a745c21c37c6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfGainLossOnInvestmentsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ScheduleOfGainLossOnInvestmentsIncludingMarketableSecuritiesAndInvestmentsHeldAtCostIncomeStatementReportedAmountsSummaryLineItems_019f28d8-3cb2-70b3-ad68-02ef9e3b93bb" xlink:to="loc_us-gaap_ScheduleOfGainLossOnInvestmentsTable_019f28d8-3cb2-7a03-a6a4-a745c21c37c6" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_019f28d8-3cb2-7152-a471-a35a03c05d30" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfGainLossOnInvestmentsTable_019f28d8-3cb2-7a03-a6a4-a745c21c37c6" xlink:to="loc_us-gaap_FinancialInstrumentAxis_019f28d8-3cb2-7152-a471-a35a03c05d30" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb2-7152-a471-a35a03c05d30_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019f28d8-3cb2-7152-a471-a35a03c05d30" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb2-7152-a471-a35a03c05d30_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb2-75c5-94f9-e8df4b034a2a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019f28d8-3cb2-7152-a471-a35a03c05d30" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb2-75c5-94f9-e8df4b034a2a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FixedMaturitiesMember_019f28d8-3cb2-7699-96a9-f8c0f4093af3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FixedMaturitiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb2-75c5-94f9-e8df4b034a2a" xlink:to="loc_us-gaap_FixedMaturitiesMember_019f28d8-3cb2-7699-96a9-f8c0f4093af3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_OtherInvestmentsFairValueOptionMember_019f28d8-3cb2-7825-b915-e953fd0601d2" xlink:href="acgl-20260630.xsd#acgl_OtherInvestmentsFairValueOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb2-75c5-94f9-e8df4b034a2a" xlink:to="loc_acgl_OtherInvestmentsFairValueOptionMember_019f28d8-3cb2-7825-b915-e953fd0601d2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_ShortTermInvestmentsFairValueOptionMember_019f673e-187c-7e86-b880-08107c3f4973" xlink:href="acgl-20260630.xsd#acgl_ShortTermInvestmentsFairValueOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb2-75c5-94f9-e8df4b034a2a" xlink:to="loc_acgl_ShortTermInvestmentsFairValueOptionMember_019f673e-187c-7e86-b880-08107c3f4973" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GainLossOnInvestmentsExcludingOtherThanTemporaryImpairmentsAbstract_019f28d8-3cb2-7f72-805c-ca234ec534b6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_GainLossOnInvestmentsExcludingOtherThanTemporaryImpairmentsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfGainLossOnInvestmentsIncludingMarketableSecuritiesAndInvestmentsHeldAtCostIncomeStatementReportedAmountsSummaryLineItems_019f28d8-3cb2-70b3-ad68-02ef9e3b93bb" xlink:to="loc_us-gaap_GainLossOnInvestmentsExcludingOtherThanTemporaryImpairmentsAbstract_019f28d8-3cb2-7f72-805c-ca234ec534b6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleGainLossAbstract_019f28d8-3cb2-70da-8f96-f1f8bc546d5c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleGainLossAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GainLossOnInvestmentsExcludingOtherThanTemporaryImpairmentsAbstract_019f28d8-3cb2-7f72-805c-ca234ec534b6" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleGainLossAbstract_019f28d8-3cb2-70da-8f96-f1f8bc546d5c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleRealizedGain_019f28d8-3cb2-7ee9-a565-c70a11500530" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleRealizedGain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtSecuritiesAvailableForSaleGainLossAbstract_019f28d8-3cb2-70da-8f96-f1f8bc546d5c" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleRealizedGain_019f28d8-3cb2-7ee9-a565-c70a11500530" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleRealizedLoss_019f28d8-3cb2-7284-bcda-8852d4cf1333" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleRealizedLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtSecuritiesAvailableForSaleGainLossAbstract_019f28d8-3cb2-70da-8f96-f1f8bc546d5c" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleRealizedLoss_019f28d8-3cb2-7284-bcda-8852d4cf1333" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueOptionChangesInFairValueGainLoss1_019f28d8-3cb2-7042-ac15-009522ba75c2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueOptionChangesInFairValueGainLoss1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GainLossOnInvestmentsExcludingOtherThanTemporaryImpairmentsAbstract_019f28d8-3cb2-7f72-805c-ca234ec534b6" xlink:to="loc_us-gaap_FairValueOptionChangesInFairValueGainLoss1_019f28d8-3cb2-7042-ac15-009522ba75c2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesFvNiGainLossAbstract_019f28d8-3cb2-74ca-82bf-759ce77b08ae" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquitySecuritiesFvNiGainLossAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GainLossOnInvestmentsExcludingOtherThanTemporaryImpairmentsAbstract_019f28d8-3cb2-7f72-805c-ca234ec534b6" xlink:to="loc_us-gaap_EquitySecuritiesFvNiGainLossAbstract_019f28d8-3cb2-74ca-82bf-759ce77b08ae" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesFvNiRealizedGainLoss_019f28d8-3cb2-7996-873d-9ff597457be8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquitySecuritiesFvNiRealizedGainLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesFvNiGainLossAbstract_019f28d8-3cb2-74ca-82bf-759ce77b08ae" xlink:to="loc_us-gaap_EquitySecuritiesFvNiRealizedGainLoss_019f28d8-3cb2-7996-873d-9ff597457be8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesFvNiUnrealizedGainLoss_019f28d8-3cb2-7201-85cf-aea0538ba060" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquitySecuritiesFvNiUnrealizedGainLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesFvNiGainLossAbstract_019f28d8-3cb2-74ca-82bf-759ce77b08ae" xlink:to="loc_us-gaap_EquitySecuritiesFvNiUnrealizedGainLoss_019f28d8-3cb2-7201-85cf-aea0538ba060" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllowanceForCreditLossAbstract_019f28d8-3cb2-7ef8-9959-a57e25271dd0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AllowanceForCreditLossAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GainLossOnInvestmentsExcludingOtherThanTemporaryImpairmentsAbstract_019f28d8-3cb2-7f72-805c-ca234ec534b6" xlink:to="loc_us-gaap_AllowanceForCreditLossAbstract_019f28d8-3cb2-7ef8-9959-a57e25271dd0" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleAllowanceForCreditLossPeriodIncreaseDecrease_019f28d8-3cb2-74cf-9a4b-0a461ce4d297" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleAllowanceForCreditLossPeriodIncreaseDecrease"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AllowanceForCreditLossAbstract_019f28d8-3cb2-7ef8-9959-a57e25271dd0" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleAllowanceForCreditLossPeriodIncreaseDecrease_019f28d8-3cb2-74cf-9a4b-0a461ce4d297" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_UnderwritingRelatedAllowanceForCreditLossesCreditLossExpenseReversal_019f28d8-3cb2-7ce6-94aa-6f99b88e9873" xlink:href="acgl-20260630.xsd#acgl_UnderwritingRelatedAllowanceForCreditLossesCreditLossExpenseReversal"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AllowanceForCreditLossAbstract_019f28d8-3cb2-7ef8-9959-a57e25271dd0" xlink:to="loc_acgl_UnderwritingRelatedAllowanceForCreditLossesCreditLossExpenseReversal_019f28d8-3cb2-7ce6-94aa-6f99b88e9873" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeGainLossOnDerivativeNet_019f28d8-3cb2-7df7-afe0-1d7e40b9bc88" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeGainLossOnDerivativeNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GainLossOnInvestmentsExcludingOtherThanTemporaryImpairmentsAbstract_019f28d8-3cb2-7f72-805c-ca234ec534b6" xlink:to="loc_us-gaap_DerivativeGainLossOnDerivativeNet_019f28d8-3cb2-7df7-afe0-1d7e40b9bc88" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_OtherRealizedAndUnrealizedGainsAndLosses_019f28d8-3cb2-7d79-a24c-8641822b7421" xlink:href="acgl-20260630.xsd#acgl_OtherRealizedAndUnrealizedGainsAndLosses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GainLossOnInvestmentsExcludingOtherThanTemporaryImpairmentsAbstract_019f28d8-3cb2-7f72-805c-ca234ec534b6" xlink:to="loc_acgl_OtherRealizedAndUnrealizedGainsAndLosses_019f28d8-3cb2-7d79-a24c-8641822b7421" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_NetRealizedGainsLosses_019f28d8-3cb2-7350-8275-98deb8c24527" xlink:href="acgl-20260630.xsd#acgl_NetRealizedGainsLosses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GainLossOnInvestmentsExcludingOtherThanTemporaryImpairmentsAbstract_019f28d8-3cb2-7f72-805c-ca234ec534b6" xlink:to="loc_acgl_NetRealizedGainsLosses_019f28d8-3cb2-7350-8275-98deb8c24527" xlink:type="arc" order="7"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.archcapgroup.com/role/InvestmentInformationInvestmentsaccountedforusingtheequitymethodDetails" xlink:type="simple" xlink:href="acgl-20260630.xsd#InvestmentInformationInvestmentsaccountedforusingtheequitymethodDetails"/>
  <link:definitionLink xlink:role="http://www.archcapgroup.com/role/InvestmentInformationInvestmentsaccountedforusingtheequitymethodDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_019f28d8-3cb2-7b07-9a75-4ee02fc3cd87" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfEquityMethodInvestmentsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEquityMethodInvestmentsTable_019f28d8-3cb2-79a4-83ab-ae70885cfdfb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfEquityMethodInvestmentsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_019f28d8-3cb2-7b07-9a75-4ee02fc3cd87" xlink:to="loc_us-gaap_ScheduleOfEquityMethodInvestmentsTable_019f28d8-3cb2-79a4-83ab-ae70885cfdfb" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_019f28d8-3cb2-71a3-b731-be17a23f190a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsTable_019f28d8-3cb2-79a4-83ab-ae70885cfdfb" xlink:to="loc_us-gaap_FinancialInstrumentAxis_019f28d8-3cb2-71a3-b731-be17a23f190a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb2-71a3-b731-be17a23f190a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019f28d8-3cb2-71a3-b731-be17a23f190a" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb2-71a3-b731-be17a23f190a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb2-7382-be67-df1839f49e9f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019f28d8-3cb2-71a3-b731-be17a23f190a" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb2-7382-be67-df1839f49e9f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PrivateEquityFundsMember_019f28d8-3cb2-7cc1-96b5-44d75f7bb7ae" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PrivateEquityFundsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb2-7382-be67-df1839f49e9f" xlink:to="loc_us-gaap_PrivateEquityFundsMember_019f28d8-3cb2-7cc1-96b5-44d75f7bb7ae" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_CreditRelatedFundsMember_019f28d8-3cb2-73cb-b6d7-e9a36eddf4a1" xlink:href="acgl-20260630.xsd#acgl_CreditRelatedFundsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb2-7382-be67-df1839f49e9f" xlink:to="loc_acgl_CreditRelatedFundsMember_019f28d8-3cb2-73cb-b6d7-e9a36eddf4a1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RealEstateFundsMember_019f28d8-3cb2-76b5-b607-28d7f48ec9dc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RealEstateFundsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb2-7382-be67-df1839f49e9f" xlink:to="loc_us-gaap_RealEstateFundsMember_019f28d8-3cb2-76b5-b607-28d7f48ec9dc" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_OtherInvestmentsLendingMember_019f28d8-3cb2-7af8-b834-5fdea4bf0966" xlink:href="acgl-20260630.xsd#acgl_OtherInvestmentsLendingMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb2-7382-be67-df1839f49e9f" xlink:to="loc_acgl_OtherInvestmentsLendingMember_019f28d8-3cb2-7af8-b834-5fdea4bf0966" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FixedIncomeInvestmentsMember_019f28d8-3cb2-7486-b16b-c40547a68dd8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FixedIncomeInvestmentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb2-7382-be67-df1839f49e9f" xlink:to="loc_us-gaap_FixedIncomeInvestmentsMember_019f28d8-3cb2-7486-b16b-c40547a68dd8" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_OtherInvestmentsInfrastructureMember_019f28d8-3cb2-7549-8027-5d6b01913b18" xlink:href="acgl-20260630.xsd#acgl_OtherInvestmentsInfrastructureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb2-7382-be67-df1839f49e9f" xlink:to="loc_acgl_OtherInvestmentsInfrastructureMember_019f28d8-3cb2-7549-8027-5d6b01913b18" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesMember_019f28d8-3cb2-7f60-a519-c5cf9067b409" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquitySecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb2-7382-be67-df1839f49e9f" xlink:to="loc_us-gaap_EquitySecuritiesMember_019f28d8-3cb2-7f60-a519-c5cf9067b409" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_OtherInvestmentsEnergyMember_019f28d8-3cb2-7998-936a-b8e355e08a1a" xlink:href="acgl-20260630.xsd#acgl_OtherInvestmentsEnergyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb2-7382-be67-df1839f49e9f" xlink:to="loc_acgl_OtherInvestmentsEnergyMember_019f28d8-3cb2-7998-936a-b8e355e08a1a" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityMethodInvestments_019f28d8-3cb2-78d7-b3d2-5811808ef1c4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityMethodInvestments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_019f28d8-3cb2-7b07-9a75-4ee02fc3cd87" xlink:to="loc_us-gaap_EquityMethodInvestments_019f28d8-3cb2-78d7-b3d2-5811808ef1c4" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.archcapgroup.com/role/InvestmentInformationLimitedpartnershipinterestsDetails" xlink:type="simple" xlink:href="acgl-20260630.xsd#InvestmentInformationLimitedpartnershipinterestsDetails"/>
  <link:definitionLink xlink:role="http://www.archcapgroup.com/role/InvestmentInformationLimitedpartnershipinterestsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_019f28d8-3cb2-7d60-a5e8-fad6d6bdbd2c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfEquityMethodInvestmentsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEquityMethodInvestmentsTable_019f28d8-3cb2-75ec-9fe1-977b4fb89726" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfEquityMethodInvestmentsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_019f28d8-3cb2-7d60-a5e8-fad6d6bdbd2c" xlink:to="loc_us-gaap_ScheduleOfEquityMethodInvestmentsTable_019f28d8-3cb2-75ec-9fe1-977b4fb89726" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentTypeAxis_019f28d8-3cb2-7fc3-bbbc-346b912a3333" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InvestmentTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsTable_019f28d8-3cb2-75ec-9fe1-977b4fb89726" xlink:to="loc_us-gaap_InvestmentTypeAxis_019f28d8-3cb2-7fc3-bbbc-346b912a3333" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentTypeCategorizationMember_019f28d8-3cb2-7fc3-bbbc-346b912a3333_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InvestmentTypeCategorizationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_InvestmentTypeAxis_019f28d8-3cb2-7fc3-bbbc-346b912a3333" xlink:to="loc_us-gaap_InvestmentTypeCategorizationMember_019f28d8-3cb2-7fc3-bbbc-346b912a3333_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentTypeCategorizationMember_019f28d8-3cb2-7bcd-8e78-d1f5b3264966" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InvestmentTypeCategorizationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_InvestmentTypeAxis_019f28d8-3cb2-7fc3-bbbc-346b912a3333" xlink:to="loc_us-gaap_InvestmentTypeCategorizationMember_019f28d8-3cb2-7bcd-8e78-d1f5b3264966" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityMethodInvestmentsMember_019f28d8-3cb2-7502-8785-494ec80f4dc0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityMethodInvestmentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentTypeCategorizationMember_019f28d8-3cb2-7bcd-8e78-d1f5b3264966" xlink:to="loc_us-gaap_EquityMethodInvestmentsMember_019f28d8-3cb2-7502-8785-494ec80f4dc0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_FairValueOptionMember_019f28d8-3cb2-718c-a1cf-1f0479b909ba" xlink:href="acgl-20260630.xsd#acgl_FairValueOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentTypeCategorizationMember_019f28d8-3cb2-7bcd-8e78-d1f5b3264966" xlink:to="loc_acgl_FairValueOptionMember_019f28d8-3cb2-718c-a1cf-1f0479b909ba" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableInterestEntityEntityMaximumLossExposureAmount_019f28d8-3cb2-7689-ad3d-6c6c1c3e9c65" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_VariableInterestEntityEntityMaximumLossExposureAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_019f28d8-3cb2-7d60-a5e8-fad6d6bdbd2c" xlink:to="loc_us-gaap_VariableInterestEntityEntityMaximumLossExposureAmount_019f28d8-3cb2-7689-ad3d-6c6c1c3e9c65" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_InvestmentCommitments_019f28d8-3cb2-7b5e-af9d-87665954fe2f" xlink:href="acgl-20260630.xsd#acgl_InvestmentCommitments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_019f28d8-3cb2-7d60-a5e8-fad6d6bdbd2c" xlink:to="loc_acgl_InvestmentCommitments_019f28d8-3cb2-7b5e-af9d-87665954fe2f" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.archcapgroup.com/role/InvestmentInformationInvestmentinoperatingaffiliatesDetails" xlink:type="simple" xlink:href="acgl-20260630.xsd#InvestmentInformationInvestmentinoperatingaffiliatesDetails"/>
  <link:definitionLink xlink:role="http://www.archcapgroup.com/role/InvestmentInformationInvestmentinoperatingaffiliatesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_019f28d8-3cb2-7f3f-9ebc-85fed8117ed9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfEquityMethodInvestmentsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEquityMethodInvestmentsTable_019f28d8-3cb2-7c9a-abce-f6ae4fef9ce7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfEquityMethodInvestmentsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_019f28d8-3cb2-7f3f-9ebc-85fed8117ed9" xlink:to="loc_us-gaap_ScheduleOfEquityMethodInvestmentsTable_019f28d8-3cb2-7c9a-abce-f6ae4fef9ce7" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_019f28d8-3cb2-76ce-a36e-f2a858bda7f3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsTable_019f28d8-3cb2-7c9a-abce-f6ae4fef9ce7" xlink:to="loc_us-gaap_StatementClassOfStockAxis_019f28d8-3cb2-76ce-a36e-f2a858bda7f3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019f28d8-3cb2-76ce-a36e-f2a858bda7f3_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019f28d8-3cb2-76ce-a36e-f2a858bda7f3" xlink:to="loc_us-gaap_ClassOfStockDomain_019f28d8-3cb2-76ce-a36e-f2a858bda7f3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019f28d8-3cb2-760f-95df-139e6adaaddc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019f28d8-3cb2-76ce-a36e-f2a858bda7f3" xlink:to="loc_us-gaap_ClassOfStockDomain_019f28d8-3cb2-760f-95df-139e6adaaddc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_IssuedSharesExcludingTreasuryStockMember_019f28d8-3cb2-7240-b890-767c7b92c0df" xlink:href="acgl-20260630.xsd#acgl_IssuedSharesExcludingTreasuryStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_019f28d8-3cb2-760f-95df-139e6adaaddc" xlink:to="loc_acgl_IssuedSharesExcludingTreasuryStockMember_019f28d8-3cb2-7240-b890-767c7b92c0df" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_019f28d8-3cb2-7851-846f-3d7ead3bbd53" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsTable_019f28d8-3cb2-7c9a-abce-f6ae4fef9ce7" xlink:to="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_019f28d8-3cb2-7851-846f-3d7ead3bbd53" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_EquityMethodInvesteeNameDomain_019f28d8-3cb2-7851-846f-3d7ead3bbd53_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_EquityMethodInvesteeNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_019f28d8-3cb2-7851-846f-3d7ead3bbd53" xlink:to="loc_srt_EquityMethodInvesteeNameDomain_019f28d8-3cb2-7851-846f-3d7ead3bbd53_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_EquityMethodInvesteeNameDomain_019f28d8-3cb2-7255-804e-32c144b952ff" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_EquityMethodInvesteeNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_019f28d8-3cb2-7851-846f-3d7ead3bbd53" xlink:to="loc_srt_EquityMethodInvesteeNameDomain_019f28d8-3cb2-7255-804e-32c144b952ff" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_CofaceMember_019f28d8-3cb2-7820-b76a-71f5162e11cd" xlink:href="acgl-20260630.xsd#acgl_CofaceMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_EquityMethodInvesteeNameDomain_019f28d8-3cb2-7255-804e-32c144b952ff" xlink:to="loc_acgl_CofaceMember_019f28d8-3cb2-7820-b76a-71f5162e11cd" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_GreysbridgeHoldingsLtdMember_019f28d8-3cb2-7c52-acff-98c1fbb267bc" xlink:href="acgl-20260630.xsd#acgl_GreysbridgeHoldingsLtdMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_EquityMethodInvesteeNameDomain_019f28d8-3cb2-7255-804e-32c144b952ff" xlink:to="loc_acgl_GreysbridgeHoldingsLtdMember_019f28d8-3cb2-7c52-acff-98c1fbb267bc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_EquityMethodInvestmentsReportingLag_019f28d8-3cb2-7db6-b6b6-0ddfd907c76b" xlink:href="acgl-20260630.xsd#acgl_EquityMethodInvestmentsReportingLag"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_019f28d8-3cb2-7f3f-9ebc-85fed8117ed9" xlink:to="loc_acgl_EquityMethodInvestmentsReportingLag_019f28d8-3cb2-7db6-b6b6-0ddfd907c76b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityMethodInvestmentOwnershipPercentage_019f28d8-3cb2-783a-8394-9418bca8fa5a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityMethodInvestmentOwnershipPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_019f28d8-3cb2-7f3f-9ebc-85fed8117ed9" xlink:to="loc_us-gaap_EquityMethodInvestmentOwnershipPercentage_019f28d8-3cb2-783a-8394-9418bca8fa5a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_InvestmentInOperatingAffiliateEquityMethod_019f28d8-3cb2-70d3-9332-e4e8577098a1" xlink:href="acgl-20260630.xsd#acgl_InvestmentInOperatingAffiliateEquityMethod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_019f28d8-3cb2-7f3f-9ebc-85fed8117ed9" xlink:to="loc_acgl_InvestmentInOperatingAffiliateEquityMethod_019f28d8-3cb2-70d3-9332-e4e8577098a1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_IncomeLossFromOperatingAffiliatesEquityMethod_019f28d8-3cb2-7d34-b2d8-16ef5fa737e9" xlink:href="acgl-20260630.xsd#acgl_IncomeLossFromOperatingAffiliatesEquityMethod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_019f28d8-3cb2-7f3f-9ebc-85fed8117ed9" xlink:to="loc_acgl_IncomeLossFromOperatingAffiliatesEquityMethod_019f28d8-3cb2-7d34-b2d8-16ef5fa737e9" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.archcapgroup.com/role/InvestmentInformationAllowanceforexpectedcreditlossesDetails" xlink:type="simple" xlink:href="acgl-20260630.xsd#InvestmentInformationAllowanceforexpectedcreditlossesDetails"/>
  <link:definitionLink xlink:role="http://www.archcapgroup.com/role/InvestmentInformationAllowanceforexpectedcreditlossesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableforsaleAllowanceForCreditLossLineItems_019f28d8-3cb2-72c3-aa0e-0fce9c79e693" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableforsaleAllowanceForCreditLossLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleAllowanceForCreditLossTable_019f28d8-3cb2-7a41-86ab-db1c63dec592" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleAllowanceForCreditLossTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtSecuritiesAvailableforsaleAllowanceForCreditLossLineItems_019f28d8-3cb2-72c3-aa0e-0fce9c79e693" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleAllowanceForCreditLossTable_019f28d8-3cb2-7a41-86ab-db1c63dec592" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_019f28d8-3cb2-7be7-9cdf-6ba52943bcea" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtSecuritiesAvailableForSaleAllowanceForCreditLossTable_019f28d8-3cb2-7a41-86ab-db1c63dec592" xlink:to="loc_us-gaap_FinancialInstrumentAxis_019f28d8-3cb2-7be7-9cdf-6ba52943bcea" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb2-7be7-9cdf-6ba52943bcea_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019f28d8-3cb2-7be7-9cdf-6ba52943bcea" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb2-7be7-9cdf-6ba52943bcea_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb2-7167-a323-85bc02acc480" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019f28d8-3cb2-7be7-9cdf-6ba52943bcea" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb2-7167-a323-85bc02acc480" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FixedMaturitiesMember_019f28d8-3cb2-7de6-a069-ef5c8462dcb1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FixedMaturitiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb2-7167-a323-85bc02acc480" xlink:to="loc_us-gaap_FixedMaturitiesMember_019f28d8-3cb2-7de6-a069-ef5c8462dcb1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CollateralizedSecuritiesMember_019f28d8-3cb2-7afc-989d-af3e936d9fef" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CollateralizedSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FixedMaturitiesMember_019f28d8-3cb2-7de6-a069-ef5c8462dcb1" xlink:to="loc_us-gaap_CollateralizedSecuritiesMember_019f28d8-3cb2-7afc-989d-af3e936d9fef" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateDebtSecuritiesMember_019f28d8-3cb2-7e08-b923-a6a3f45ca43b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CorporateDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FixedMaturitiesMember_019f28d8-3cb2-7de6-a069-ef5c8462dcb1" xlink:to="loc_us-gaap_CorporateDebtSecuritiesMember_019f28d8-3cb2-7e08-b923-a6a3f45ca43b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForeignGovernmentDebtSecuritiesMember_019f28d8-3cb2-7340-8726-c695cb8092d6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ForeignGovernmentDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FixedMaturitiesMember_019f28d8-3cb2-7de6-a069-ef5c8462dcb1" xlink:to="loc_us-gaap_ForeignGovernmentDebtSecuritiesMember_019f28d8-3cb2-7340-8726-c695cb8092d6" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleAllowanceForCreditLossRollForward_019f28d8-3cb2-747a-9d29-37cad4b7ac92" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleAllowanceForCreditLossRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtSecuritiesAvailableforsaleAllowanceForCreditLossLineItems_019f28d8-3cb2-72c3-aa0e-0fce9c79e693" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleAllowanceForCreditLossRollForward_019f28d8-3cb2-747a-9d29-37cad4b7ac92" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleAllowanceForCreditLoss_019f28d8-3cb2-7b56-af0a-1e277c8837b4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleAllowanceForCreditLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtSecuritiesAvailableForSaleAllowanceForCreditLossRollForward_019f28d8-3cb2-747a-9d29-37cad4b7ac92" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleAllowanceForCreditLoss_019f28d8-3cb2-7b56-af0a-1e277c8837b4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleAllowanceForCreditLossNotPreviouslyRecorded_019f28d8-3cb2-7550-ae52-749b69f988c2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleAllowanceForCreditLossNotPreviouslyRecorded"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtSecuritiesAvailableForSaleAllowanceForCreditLossRollForward_019f28d8-3cb2-747a-9d29-37cad4b7ac92" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleAllowanceForCreditLossNotPreviouslyRecorded_019f28d8-3cb2-7550-ae52-749b69f988c2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleAllowanceForCreditLossNotToSellBeforeRecoveryCreditLossPreviouslyRecordedExpenseReversal_019f28d8-3cb2-7957-82c5-7e147186f212" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleAllowanceForCreditLossNotToSellBeforeRecoveryCreditLossPreviouslyRecordedExpenseReversal"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtSecuritiesAvailableForSaleAllowanceForCreditLossRollForward_019f28d8-3cb2-747a-9d29-37cad4b7ac92" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleAllowanceForCreditLossNotToSellBeforeRecoveryCreditLossPreviouslyRecordedExpenseReversal_019f28d8-3cb2-7957-82c5-7e147186f212" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleAllowanceForCreditLossSecuritiesSold_019f28d8-3cb2-7ce0-b0c1-fbb7a01ab77d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleAllowanceForCreditLossSecuritiesSold"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtSecuritiesAvailableForSaleAllowanceForCreditLossRollForward_019f28d8-3cb2-747a-9d29-37cad4b7ac92" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleAllowanceForCreditLossSecuritiesSold_019f28d8-3cb2-7ce0-b0c1-fbb7a01ab77d" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleAllowanceForCreditLoss_019f28d8-3cb2-703a-863c-d78e8f17fa7a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleAllowanceForCreditLoss"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.archcapgroup.com/role/InvestmentInformationRestrictedassetsDetails" xlink:type="simple" xlink:href="acgl-20260630.xsd#InvestmentInformationRestrictedassetsDetails"/>
  <link:definitionLink xlink:role="http://www.archcapgroup.com/role/InvestmentInformationRestrictedassetsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_acgl_RestrictedAssetsLineItems_019f28d8-3cb2-7673-9abd-e3b34901e314" xlink:href="acgl-20260630.xsd#acgl_RestrictedAssetsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_ScheduleOfRestrictedAssetsTable_019f28d8-3cb2-7a6e-8f1f-8cdbdf71cb40" xlink:href="acgl-20260630.xsd#acgl_ScheduleOfRestrictedAssetsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_acgl_RestrictedAssetsLineItems_019f28d8-3cb2-7673-9abd-e3b34901e314" xlink:to="loc_acgl_ScheduleOfRestrictedAssetsTable_019f28d8-3cb2-7a6e-8f1f-8cdbdf71cb40" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_RestrictedAssetsRestrictionTypeAxis_019f28d8-3cb2-73c6-9e4c-8f973767fa84" xlink:href="acgl-20260630.xsd#acgl_RestrictedAssetsRestrictionTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_acgl_ScheduleOfRestrictedAssetsTable_019f28d8-3cb2-7a6e-8f1f-8cdbdf71cb40" xlink:to="loc_acgl_RestrictedAssetsRestrictionTypeAxis_019f28d8-3cb2-73c6-9e4c-8f973767fa84" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_RestrictedAssetsRestrictionTypeDomain_019f28d8-3cb2-73c6-9e4c-8f973767fa84_default" xlink:href="acgl-20260630.xsd#acgl_RestrictedAssetsRestrictionTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_acgl_RestrictedAssetsRestrictionTypeAxis_019f28d8-3cb2-73c6-9e4c-8f973767fa84" xlink:to="loc_acgl_RestrictedAssetsRestrictionTypeDomain_019f28d8-3cb2-73c6-9e4c-8f973767fa84_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_RestrictedAssetsRestrictionTypeDomain_019f28d8-3cb2-7df2-98e5-377f5f83258c" xlink:href="acgl-20260630.xsd#acgl_RestrictedAssetsRestrictionTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_acgl_RestrictedAssetsRestrictionTypeAxis_019f28d8-3cb2-73c6-9e4c-8f973767fa84" xlink:to="loc_acgl_RestrictedAssetsRestrictionTypeDomain_019f28d8-3cb2-7df2-98e5-377f5f83258c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_CollateralOrGuaranteesAffiliatedTransactionsMember_019f28d8-3cb2-7853-97c4-906b96d4028e" xlink:href="acgl-20260630.xsd#acgl_CollateralOrGuaranteesAffiliatedTransactionsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_acgl_RestrictedAssetsRestrictionTypeDomain_019f28d8-3cb2-7df2-98e5-377f5f83258c" xlink:to="loc_acgl_CollateralOrGuaranteesAffiliatedTransactionsMember_019f28d8-3cb2-7853-97c4-906b96d4028e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_CollateralOrGuaranteesThirdPartyAgreementsMember_019f28d8-3cb2-7430-b175-b39199af2281" xlink:href="acgl-20260630.xsd#acgl_CollateralOrGuaranteesThirdPartyAgreementsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_acgl_RestrictedAssetsRestrictionTypeDomain_019f28d8-3cb2-7df2-98e5-377f5f83258c" xlink:to="loc_acgl_CollateralOrGuaranteesThirdPartyAgreementsMember_019f28d8-3cb2-7430-b175-b39199af2281" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_DepositsWithUsRegulatoryAuthoritiesMember_019f28d8-3cb2-7abc-8518-128f264e534d" xlink:href="acgl-20260630.xsd#acgl_DepositsWithUsRegulatoryAuthoritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_acgl_RestrictedAssetsRestrictionTypeDomain_019f28d8-3cb2-7df2-98e5-377f5f83258c" xlink:to="loc_acgl_DepositsWithUsRegulatoryAuthoritiesMember_019f28d8-3cb2-7abc-8518-128f264e534d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_OtherRestrictedAssetsMember_019f28d8-3cb2-7af3-ba92-31731efb53dd" xlink:href="acgl-20260630.xsd#acgl_OtherRestrictedAssetsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_acgl_RestrictedAssetsRestrictionTypeDomain_019f28d8-3cb2-7df2-98e5-377f5f83258c" xlink:to="loc_acgl_OtherRestrictedAssetsMember_019f28d8-3cb2-7af3-ba92-31731efb53dd" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_RestrictedAssets_019f28d8-3cb2-7169-8005-d6fe1f7fe88a" xlink:href="acgl-20260630.xsd#acgl_RestrictedAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_acgl_RestrictedAssetsLineItems_019f28d8-3cb2-7673-9abd-e3b34901e314" xlink:to="loc_acgl_RestrictedAssets_019f28d8-3cb2-7169-8005-d6fe1f7fe88a" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.archcapgroup.com/role/InvestmentInformationNarrativeDetails" xlink:type="simple" xlink:href="acgl-20260630.xsd#InvestmentInformationNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.archcapgroup.com/role/InvestmentInformationNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_019f28d8-3cb2-741e-9ba1-6fd2b6c8df01" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019f28d8-3cb2-7b64-ace2-d40124190722" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019f28d8-3cb2-741e-9ba1-6fd2b6c8df01" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019f28d8-3cb2-7b64-ace2-d40124190722" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019f28d8-3cb2-7362-b4c5-6618c03d5790" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019f28d8-3cb2-7b64-ace2-d40124190722" xlink:to="loc_srt_RangeAxis_019f28d8-3cb2-7362-b4c5-6618c03d5790" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019f28d8-3cb2-7362-b4c5-6618c03d5790_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019f28d8-3cb2-7362-b4c5-6618c03d5790" xlink:to="loc_srt_RangeMember_019f28d8-3cb2-7362-b4c5-6618c03d5790_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019f28d8-3cb2-7061-abcb-6cbf464d4aec" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019f28d8-3cb2-7362-b4c5-6618c03d5790" xlink:to="loc_srt_RangeMember_019f28d8-3cb2-7061-abcb-6cbf464d4aec" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019f28d8-3cb2-7563-b1db-1875e718c84d" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019f28d8-3cb2-7061-abcb-6cbf464d4aec" xlink:to="loc_srt_MinimumMember_019f28d8-3cb2-7563-b1db-1875e718c84d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019f28d8-3cb2-7b96-839c-4228d74b90b5" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019f28d8-3cb2-7061-abcb-6cbf464d4aec" xlink:to="loc_srt_MaximumMember_019f28d8-3cb2-7b96-839c-4228d74b90b5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesFvNiCurrentAndNoncurrent_019f28d8-3cb2-7ef0-b06d-ccf5a1db3d43" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquitySecuritiesFvNiCurrentAndNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019f28d8-3cb2-741e-9ba1-6fd2b6c8df01" xlink:to="loc_us-gaap_EquitySecuritiesFvNiCurrentAndNoncurrent_019f28d8-3cb2-7ef0-b06d-ccf5a1db3d43" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromEquityMethodInvestments_019f28d8-3cb2-7084-95de-bcef54733596" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncomeLossFromEquityMethodInvestments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019f28d8-3cb2-741e-9ba1-6fd2b6c8df01" xlink:to="loc_us-gaap_IncomeLossFromEquityMethodInvestments_019f28d8-3cb2-7084-95de-bcef54733596" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_EquityMethodInvestmentsReportingLag_019f28d8-3cb2-7aa5-b63b-d4fd554b1564" xlink:href="acgl-20260630.xsd#acgl_EquityMethodInvestmentsReportingLag"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019f28d8-3cb2-741e-9ba1-6fd2b6c8df01" xlink:to="loc_acgl_EquityMethodInvestmentsReportingLag_019f28d8-3cb2-7aa5-b63b-d4fd554b1564" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.archcapgroup.com/role/FairValueFairValueHierarchyDetails" xlink:type="simple" xlink:href="acgl-20260630.xsd#FairValueFairValueHierarchyDetails"/>
  <link:definitionLink xlink:role="http://www.archcapgroup.com/role/FairValueFairValueHierarchyDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019f28d8-3cb2-7fca-ba54-5c4abc48b882" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019f28d8-3cb2-784e-9939-95f1fe357daa" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019f28d8-3cb2-7fca-ba54-5c4abc48b882" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019f28d8-3cb2-784e-9939-95f1fe357daa" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByMeasurementBasisAxis_019f28d8-3cb2-7255-bb89-e87a7f85c4b7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByMeasurementBasisAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019f28d8-3cb2-784e-9939-95f1fe357daa" xlink:to="loc_us-gaap_FairValueByMeasurementBasisAxis_019f28d8-3cb2-7255-bb89-e87a7f85c4b7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_019f28d8-3cb2-7255-bb89-e87a7f85c4b7_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PortionAtFairValueFairValueDisclosureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByMeasurementBasisAxis_019f28d8-3cb2-7255-bb89-e87a7f85c4b7" xlink:to="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_019f28d8-3cb2-7255-bb89-e87a7f85c4b7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueDisclosureItemAmountsDomain_019f28d8-3cb2-7a6a-962d-170b61d99406" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueDisclosureItemAmountsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByMeasurementBasisAxis_019f28d8-3cb2-7255-bb89-e87a7f85c4b7" xlink:to="loc_us-gaap_FairValueDisclosureItemAmountsDomain_019f28d8-3cb2-7a6a-962d-170b61d99406" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_019f28d8-3cb2-7531-aac9-04f24750637d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PortionAtFairValueFairValueDisclosureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueDisclosureItemAmountsDomain_019f28d8-3cb2-7a6a-962d-170b61d99406" xlink:to="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_019f28d8-3cb2-7531-aac9-04f24750637d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EstimateOfFairValueFairValueDisclosureMember_019f28d8-3cb2-7f08-94eb-05ab74b5e543" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EstimateOfFairValueFairValueDisclosureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueDisclosureItemAmountsDomain_019f28d8-3cb2-7a6a-962d-170b61d99406" xlink:to="loc_us-gaap_EstimateOfFairValueFairValueDisclosureMember_019f28d8-3cb2-7f08-94eb-05ab74b5e543" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019f28d8-3cb2-7b0f-b830-7ff11152bd52" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019f28d8-3cb2-784e-9939-95f1fe357daa" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019f28d8-3cb2-7b0f-b830-7ff11152bd52" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019f28d8-3cb2-7b0f-b830-7ff11152bd52_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019f28d8-3cb2-7b0f-b830-7ff11152bd52" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019f28d8-3cb2-7b0f-b830-7ff11152bd52_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019f28d8-3cb2-78c7-bb84-38db29eb9406" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019f28d8-3cb2-7b0f-b830-7ff11152bd52" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019f28d8-3cb2-78c7-bb84-38db29eb9406" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel1Member_019f28d8-3cb2-7cf4-900e-d8538490beb6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel1Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019f28d8-3cb2-78c7-bb84-38db29eb9406" xlink:to="loc_us-gaap_FairValueInputsLevel1Member_019f28d8-3cb2-7cf4-900e-d8538490beb6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_019f28d8-3cb2-76c4-befc-a98c0f6a318c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019f28d8-3cb2-78c7-bb84-38db29eb9406" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_019f28d8-3cb2-76c4-befc-a98c0f6a318c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_019f28d8-3cb2-7049-9d9b-6593084a9ae8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019f28d8-3cb2-78c7-bb84-38db29eb9406" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_019f28d8-3cb2-7049-9d9b-6593084a9ae8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasuredAtNetAssetValuePerShareMember_019f28d8-3cb2-7638-b05c-eeba8a7a0a06" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasuredAtNetAssetValuePerShareMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019f28d8-3cb2-78c7-bb84-38db29eb9406" xlink:to="loc_us-gaap_FairValueMeasuredAtNetAssetValuePerShareMember_019f28d8-3cb2-7638-b05c-eeba8a7a0a06" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentTypeAxis_019f28d8-3cb2-76dc-b30f-9cd11a4eefb3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InvestmentTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019f28d8-3cb2-784e-9939-95f1fe357daa" xlink:to="loc_us-gaap_InvestmentTypeAxis_019f28d8-3cb2-76dc-b30f-9cd11a4eefb3" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentTypeCategorizationMember_019f28d8-3cb2-76dc-b30f-9cd11a4eefb3_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InvestmentTypeCategorizationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_InvestmentTypeAxis_019f28d8-3cb2-76dc-b30f-9cd11a4eefb3" xlink:to="loc_us-gaap_InvestmentTypeCategorizationMember_019f28d8-3cb2-76dc-b30f-9cd11a4eefb3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentTypeCategorizationMember_019f28d8-3cb2-7921-a121-7a6defca3fe9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InvestmentTypeCategorizationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_InvestmentTypeAxis_019f28d8-3cb2-76dc-b30f-9cd11a4eefb3" xlink:to="loc_us-gaap_InvestmentTypeCategorizationMember_019f28d8-3cb2-7921-a121-7a6defca3fe9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FixedMaturitiesMember_019f28d8-3cb2-71bb-bc75-8c1038403d98" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FixedMaturitiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentTypeCategorizationMember_019f28d8-3cb2-7921-a121-7a6defca3fe9" xlink:to="loc_us-gaap_FixedMaturitiesMember_019f28d8-3cb2-71bb-bc75-8c1038403d98" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortTermInvestmentsMember_019f28d8-3cb2-7e3e-bed3-a24ea1354a4a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShortTermInvestmentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentTypeCategorizationMember_019f28d8-3cb2-7921-a121-7a6defca3fe9" xlink:to="loc_us-gaap_ShortTermInvestmentsMember_019f28d8-3cb2-7e3e-bed3-a24ea1354a4a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_019f28d8-3cb2-7806-aef0-cffd891938fa" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019f28d8-3cb2-784e-9939-95f1fe357daa" xlink:to="loc_us-gaap_FinancialInstrumentAxis_019f28d8-3cb2-7806-aef0-cffd891938fa" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb2-7806-aef0-cffd891938fa_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019f28d8-3cb2-7806-aef0-cffd891938fa" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb2-7806-aef0-cffd891938fa_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb2-7dd5-8103-64bc11684f48" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019f28d8-3cb2-7806-aef0-cffd891938fa" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb2-7dd5-8103-64bc11684f48" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateDebtSecuritiesMember_019f28d8-3cb2-7002-aa89-a9196fab8291" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CorporateDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb2-7dd5-8103-64bc11684f48" xlink:to="loc_us-gaap_CorporateDebtSecuritiesMember_019f28d8-3cb2-7002-aa89-a9196fab8291" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USTreasuryAndGovernmentMember_019f28d8-3cb2-73da-8452-0140921d1c4d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_USTreasuryAndGovernmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb2-7dd5-8103-64bc11684f48" xlink:to="loc_us-gaap_USTreasuryAndGovernmentMember_019f28d8-3cb2-73da-8452-0140921d1c4d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetBackedSecuritiesMember_019f28d8-3cb2-7efe-8563-9380b1269a8f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AssetBackedSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb2-7dd5-8103-64bc11684f48" xlink:to="loc_us-gaap_AssetBackedSecuritiesMember_019f28d8-3cb2-7efe-8563-9380b1269a8f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForeignGovernmentDebtSecuritiesMember_019f28d8-3cb2-7193-bf9b-d37488924681" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ForeignGovernmentDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb2-7dd5-8103-64bc11684f48" xlink:to="loc_us-gaap_ForeignGovernmentDebtSecuritiesMember_019f28d8-3cb2-7193-bf9b-d37488924681" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResidentialMortgageBackedSecuritiesMember_019f28d8-3cb2-757f-8d3e-662251ffc98a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ResidentialMortgageBackedSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb2-7dd5-8103-64bc11684f48" xlink:to="loc_us-gaap_ResidentialMortgageBackedSecuritiesMember_019f28d8-3cb2-757f-8d3e-662251ffc98a" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialMortgageBackedSecuritiesMember_019f28d8-3cb2-7413-8537-743af7d18781" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommercialMortgageBackedSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb2-7dd5-8103-64bc11684f48" xlink:to="loc_us-gaap_CommercialMortgageBackedSecuritiesMember_019f28d8-3cb2-7413-8537-743af7d18781" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MunicipalBondsMember_019f28d8-3cb2-75e1-bbed-f5bfc514a927" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MunicipalBondsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb2-7dd5-8103-64bc11684f48" xlink:to="loc_us-gaap_MunicipalBondsMember_019f28d8-3cb2-75e1-bbed-f5bfc514a927" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_ShortTermInvestmentsFairValueOptionMember_019f28d8-3cb2-7467-a886-2359c5bac84d" xlink:href="acgl-20260630.xsd#acgl_ShortTermInvestmentsFairValueOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb2-7dd5-8103-64bc11684f48" xlink:to="loc_acgl_ShortTermInvestmentsFairValueOptionMember_019f28d8-3cb2-7467-a886-2359c5bac84d" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResidentialMortgageMember_019f28d8-3cb2-742c-9959-958a1ad59f0a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ResidentialMortgageMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb2-7dd5-8103-64bc11684f48" xlink:to="loc_us-gaap_ResidentialMortgageMember_019f28d8-3cb2-742c-9959-958a1ad59f0a" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_OtherInvestmentsFairValueOptionMember_019f28d8-3cb2-737e-bf5d-4d16e983e0d5" xlink:href="acgl-20260630.xsd#acgl_OtherInvestmentsFairValueOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb2-7dd5-8103-64bc11684f48" xlink:to="loc_acgl_OtherInvestmentsFairValueOptionMember_019f28d8-3cb2-737e-bf5d-4d16e983e0d5" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasuredAtNetAssetValuePerShareMember_019f28d8-3cb2-7756-b628-ac35446ec4d8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasuredAtNetAssetValuePerShareMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb2-7dd5-8103-64bc11684f48" xlink:to="loc_us-gaap_FairValueMeasuredAtNetAssetValuePerShareMember_019f28d8-3cb2-7756-b628-ac35446ec4d8" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesMember_019f28d8-3cb2-7cf4-ad9f-b4f95350200c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquitySecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb2-7dd5-8103-64bc11684f48" xlink:to="loc_us-gaap_EquitySecuritiesMember_019f28d8-3cb2-7cf4-ad9f-b4f95350200c" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_019f28d8-3cb2-7154-8a6a-b8c69c80b8f1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleSecuritiesDebtSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019f28d8-3cb2-7fca-ba54-5c4abc48b882" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_019f28d8-3cb2-7154-8a6a-b8c69c80b8f1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesFvNiCurrentAndNoncurrent_019f28d8-3cb2-74de-a75d-dcb1346c4bec" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquitySecuritiesFvNiCurrentAndNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019f28d8-3cb2-7fca-ba54-5c4abc48b882" xlink:to="loc_us-gaap_EquitySecuritiesFvNiCurrentAndNoncurrent_019f28d8-3cb2-74de-a75d-dcb1346c4bec" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeAssets_019f28d8-3cb2-7607-8da8-d9f2165f495c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019f28d8-3cb2-7fca-ba54-5c4abc48b882" xlink:to="loc_us-gaap_DerivativeAssets_019f28d8-3cb2-7607-8da8-d9f2165f495c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherAssets_019f28d8-3cb2-7ece-a16e-73b9bbcc2d73" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019f28d8-3cb2-7fca-ba54-5c4abc48b882" xlink:to="loc_us-gaap_OtherAssets_019f28d8-3cb2-7ece-a16e-73b9bbcc2d73" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherInvestments_019f28d8-3cb2-74d2-abb3-476d1bc250a9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherInvestments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019f28d8-3cb2-7fca-ba54-5c4abc48b882" xlink:to="loc_us-gaap_OtherInvestments_019f28d8-3cb2-74d2-abb3-476d1bc250a9" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsFairValueDisclosure_019f28d8-3cb2-766d-8f7b-7c26c39bdb17" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AssetsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019f28d8-3cb2-7fca-ba54-5c4abc48b882" xlink:to="loc_us-gaap_AssetsFairValueDisclosure_019f28d8-3cb2-766d-8f7b-7c26c39bdb17" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherLiabilities_019f28d8-3cb2-760c-bec3-65d6bf561684" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019f28d8-3cb2-7fca-ba54-5c4abc48b882" xlink:to="loc_us-gaap_OtherLiabilities_019f28d8-3cb2-760c-bec3-65d6bf561684" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeLiabilities_019f28d8-3cb2-7602-bcea-4fc63fb5d06d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019f28d8-3cb2-7fca-ba54-5c4abc48b882" xlink:to="loc_us-gaap_DerivativeLiabilities_019f28d8-3cb2-7602-bcea-4fc63fb5d06d" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesFairValueDisclosure_019f28d8-3cb2-777c-9b21-bc3cbd1e64df" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LiabilitiesFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019f28d8-3cb2-7fca-ba54-5c4abc48b882" xlink:to="loc_us-gaap_LiabilitiesFairValueDisclosure_019f28d8-3cb2-777c-9b21-bc3cbd1e64df" xlink:type="arc" order="8"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.archcapgroup.com/role/FairValueRollforwardofLevel3assetsandliabilitiesDetails" xlink:type="simple" xlink:href="acgl-20260630.xsd#FairValueRollforwardofLevel3assetsandliabilitiesDetails"/>
  <link:definitionLink xlink:role="http://www.archcapgroup.com/role/FairValueRollforwardofLevel3assetsandliabilitiesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_019f28d8-3cb2-72c9-b003-68d18f485413" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationTable_019f28d8-3cb2-7de9-98f8-56f7b85a1b56" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_019f28d8-3cb2-72c9-b003-68d18f485413" xlink:to="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationTable_019f28d8-3cb2-7de9-98f8-56f7b85a1b56" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentTypeAxis_019f28d8-3cb2-74cf-aefc-d62a93614886" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InvestmentTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationTable_019f28d8-3cb2-7de9-98f8-56f7b85a1b56" xlink:to="loc_us-gaap_InvestmentTypeAxis_019f28d8-3cb2-74cf-aefc-d62a93614886" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentTypeCategorizationMember_019f28d8-3cb2-74cf-aefc-d62a93614886_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InvestmentTypeCategorizationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_InvestmentTypeAxis_019f28d8-3cb2-74cf-aefc-d62a93614886" xlink:to="loc_us-gaap_InvestmentTypeCategorizationMember_019f28d8-3cb2-74cf-aefc-d62a93614886_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentTypeCategorizationMember_019f28d8-3cb2-70a5-b58a-c3bda2c1d04f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InvestmentTypeCategorizationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_InvestmentTypeAxis_019f28d8-3cb2-74cf-aefc-d62a93614886" xlink:to="loc_us-gaap_InvestmentTypeCategorizationMember_019f28d8-3cb2-70a5-b58a-c3bda2c1d04f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableforsaleSecuritiesMember_019f28d8-3cb2-79c0-ac40-536b506475fe" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableforsaleSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentTypeCategorizationMember_019f28d8-3cb2-70a5-b58a-c3bda2c1d04f" xlink:to="loc_us-gaap_AvailableforsaleSecuritiesMember_019f28d8-3cb2-79c0-ac40-536b506475fe" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_FairValueOptionMember_019f28d8-3cb2-7f24-9082-4938fc2a96fb" xlink:href="acgl-20260630.xsd#acgl_FairValueOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentTypeCategorizationMember_019f28d8-3cb2-70a5-b58a-c3bda2c1d04f" xlink:to="loc_acgl_FairValueOptionMember_019f28d8-3cb2-7f24-9082-4938fc2a96fb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesMember_019f28d8-3cb2-7128-ab20-1865ffae6eb5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquitySecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentTypeCategorizationMember_019f28d8-3cb2-70a5-b58a-c3bda2c1d04f" xlink:to="loc_us-gaap_EquitySecuritiesMember_019f28d8-3cb2-7128-ab20-1865ffae6eb5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_019f28d8-3cb2-76c6-806c-c388e8a84f37" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationTable_019f28d8-3cb2-7de9-98f8-56f7b85a1b56" xlink:to="loc_us-gaap_FinancialInstrumentAxis_019f28d8-3cb2-76c6-806c-c388e8a84f37" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb2-76c6-806c-c388e8a84f37_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019f28d8-3cb2-76c6-806c-c388e8a84f37" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb2-76c6-806c-c388e8a84f37_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb2-75dd-8a76-44617683b5f9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019f28d8-3cb2-76c6-806c-c388e8a84f37" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb2-75dd-8a76-44617683b5f9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CollateralizedSecuritiesMember_019f28d8-3cb2-7ae4-9b5d-116e703290af" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CollateralizedSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb2-75dd-8a76-44617683b5f9" xlink:to="loc_us-gaap_CollateralizedSecuritiesMember_019f28d8-3cb2-7ae4-9b5d-116e703290af" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateDebtSecuritiesMember_019f28d8-3cb2-72ee-967f-96ec3e71e9f3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CorporateDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb2-75dd-8a76-44617683b5f9" xlink:to="loc_us-gaap_CorporateDebtSecuritiesMember_019f28d8-3cb2-72ee-967f-96ec3e71e9f3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_OtherInvestmentsFairValueOptionMember_019f28d8-3cb2-7d9c-b0fa-2faf397c0600" xlink:href="acgl-20260630.xsd#acgl_OtherInvestmentsFairValueOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb2-75dd-8a76-44617683b5f9" xlink:to="loc_acgl_OtherInvestmentsFairValueOptionMember_019f28d8-3cb2-7d9c-b0fa-2faf397c0600" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_ShortTermInvestmentsFairValueOptionMember_019f28d8-3cb2-75af-81ba-92e61fa6a482" xlink:href="acgl-20260630.xsd#acgl_ShortTermInvestmentsFairValueOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb2-75dd-8a76-44617683b5f9" xlink:to="loc_acgl_ShortTermInvestmentsFairValueOptionMember_019f28d8-3cb2-75af-81ba-92e61fa6a482" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesMember_019f28d8-3cb2-73fe-b749-1a1c42a46bae" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquitySecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f28d8-3cb2-75dd-8a76-44617683b5f9" xlink:to="loc_us-gaap_EquitySecuritiesMember_019f28d8-3cb2-73fe-b749-1a1c42a46bae" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_019f28d8-3cb2-74e8-af8f-cca8f73b1922" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_019f28d8-3cb2-72c9-b003-68d18f485413" xlink:to="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_019f28d8-3cb2-74e8-af8f-cca8f73b1922" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue_019f28d8-3cb2-7523-873f-83463ac0ba1d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_019f28d8-3cb2-74e8-af8f-cca8f73b1922" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue_019f28d8-3cb2-7523-873f-83463ac0ba1d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1_019f28d8-3cb2-7f3e-9dc1-59da8b20d081" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_019f28d8-3cb2-74e8-af8f-cca8f73b1922" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1_019f28d8-3cb2-7f3e-9dc1-59da8b20d081" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss_019f28d8-3cb2-755a-831a-c59b31ee5e30" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_019f28d8-3cb2-74e8-af8f-cca8f73b1922" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss_019f28d8-3cb2-755a-831a-c59b31ee5e30" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchasesSalesIssuancesSettlementsAbstract_019f28d8-3cb2-7849-bc40-c4beef7b0ed2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchasesSalesIssuancesSettlementsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_019f28d8-3cb2-74e8-af8f-cca8f73b1922" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchasesSalesIssuancesSettlementsAbstract_019f28d8-3cb2-7849-bc40-c4beef7b0ed2" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases_019f28d8-3cb2-7223-a1b2-9ba40de8b620" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchasesSalesIssuancesSettlementsAbstract_019f28d8-3cb2-7849-bc40-c4beef7b0ed2" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases_019f28d8-3cb2-7223-a1b2-9ba40de8b620" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetIssues_019f28d8-3cb2-7838-a8d6-ae969c6697f9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetIssues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchasesSalesIssuancesSettlementsAbstract_019f28d8-3cb2-7849-bc40-c4beef7b0ed2" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetIssues_019f28d8-3cb2-7838-a8d6-ae969c6697f9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales_019f28d8-3cb2-7fe5-831a-4de00c5c73a7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchasesSalesIssuancesSettlementsAbstract_019f28d8-3cb2-7849-bc40-c4beef7b0ed2" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales_019f28d8-3cb2-7fe5-831a-4de00c5c73a7" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements_019f28d8-3cb2-7ce9-9495-b9746e10e747" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchasesSalesIssuancesSettlementsAbstract_019f28d8-3cb2-7849-bc40-c4beef7b0ed2" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements_019f28d8-3cb2-7ce9-9495-b9746e10e747" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersNet_019f28d8-3cb2-7400-9b10-ae2002aec097" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_019f28d8-3cb2-74e8-af8f-cca8f73b1922" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersNet_019f28d8-3cb2-7400-9b10-ae2002aec097" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue_019f28d8-3cb2-7db6-acc3-f44941b6105e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_019f28d8-3cb2-7884-9c5f-759957b85f6d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_019f28d8-3cb2-72c9-b003-68d18f485413" xlink:to="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_019f28d8-3cb2-7884-9c5f-759957b85f6d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue_019f28d8-3cb2-796c-afea-e2ed484eb928" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_019f28d8-3cb2-7884-9c5f-759957b85f6d" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue_019f28d8-3cb2-796c-afea-e2ed484eb928" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings_019f28d8-3cb2-7f09-a3d6-a14b9ea2d3c3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_019f28d8-3cb2-7884-9c5f-759957b85f6d" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings_019f28d8-3cb2-7f09-a3d6-a14b9ea2d3c3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInOtherComprehensiveIncome_019f28d8-3cb2-7c09-9892-c7c4a4b4a13f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInOtherComprehensiveIncome"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_019f28d8-3cb2-7884-9c5f-759957b85f6d" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInOtherComprehensiveIncome_019f28d8-3cb2-7c09-9892-c7c4a4b4a13f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchasesSalesIssuancesSettlementsAbstract_019f28d8-3cb2-7b34-a45d-0ff315c74d6e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchasesSalesIssuancesSettlementsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_019f28d8-3cb2-7884-9c5f-759957b85f6d" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchasesSalesIssuancesSettlementsAbstract_019f28d8-3cb2-7b34-a45d-0ff315c74d6e" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchases_019f28d8-3cb2-7b50-95e7-ba1f6c362477" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchases"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchasesSalesIssuancesSettlementsAbstract_019f28d8-3cb2-7b34-a45d-0ff315c74d6e" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchases_019f28d8-3cb2-7b50-95e7-ba1f6c362477" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityIssues_019f28d8-3cb2-773e-8a9f-42975b979f95" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityIssues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchasesSalesIssuancesSettlementsAbstract_019f28d8-3cb2-7b34-a45d-0ff315c74d6e" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityIssues_019f28d8-3cb2-773e-8a9f-42975b979f95" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySales_019f28d8-3cb2-71e2-a9f0-e54b911ee84c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySales"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchasesSalesIssuancesSettlementsAbstract_019f28d8-3cb2-7b34-a45d-0ff315c74d6e" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySales_019f28d8-3cb2-71e2-a9f0-e54b911ee84c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements_019f28d8-3cb2-71a5-96aa-dc68c1c56e92" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchasesSalesIssuancesSettlementsAbstract_019f28d8-3cb2-7b34-a45d-0ff315c74d6e" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements_019f28d8-3cb2-71a5-96aa-dc68c1c56e92" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityTransfersNet_019f28d8-3cb2-7ae3-aa53-63096581745f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityTransfersNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_019f28d8-3cb2-7884-9c5f-759957b85f6d" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityTransfersNet_019f28d8-3cb2-7ae3-aa53-63096581745f" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue_019f28d8-3cb2-76a2-bad4-9b2d9d4afdea" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.archcapgroup.com/role/DerivativeInstrumentsFairvalueandnotionalamountofderivativesDetails" xlink:type="simple" xlink:href="acgl-20260630.xsd#DerivativeInstrumentsFairvalueandnotionalamountofderivativesDetails"/>
  <link:definitionLink xlink:role="http://www.archcapgroup.com/role/DerivativeInstrumentsFairvalueandnotionalamountofderivativesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeLineItems_019f28d8-3cb2-7aa8-9b11-95e8efe5792a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_019f28d8-3cb2-7f72-9b06-ae2eaa92644c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DerivativeLineItems_019f28d8-3cb2-7aa8-9b11-95e8efe5792a" xlink:to="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_019f28d8-3cb2-7f72-9b06-ae2eaa92644c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationAxis_019f28d8-3cb2-710f-a1a6-57c0bd2ba8b4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HedgingDesignationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_019f28d8-3cb2-7f72-9b06-ae2eaa92644c" xlink:to="loc_us-gaap_HedgingDesignationAxis_019f28d8-3cb2-710f-a1a6-57c0bd2ba8b4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationDomain_019f28d8-3cb2-710f-a1a6-57c0bd2ba8b4_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HedgingDesignationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_HedgingDesignationAxis_019f28d8-3cb2-710f-a1a6-57c0bd2ba8b4" xlink:to="loc_us-gaap_HedgingDesignationDomain_019f28d8-3cb2-710f-a1a6-57c0bd2ba8b4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationDomain_019f28d8-3cb2-7703-afc4-71148fa4a802" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HedgingDesignationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_HedgingDesignationAxis_019f28d8-3cb2-710f-a1a6-57c0bd2ba8b4" xlink:to="loc_us-gaap_HedgingDesignationDomain_019f28d8-3cb2-7703-afc4-71148fa4a802" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NondesignatedMember_019f28d8-3cb2-7551-b07f-5b5b957cd6da" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NondesignatedMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HedgingDesignationDomain_019f28d8-3cb2-7703-afc4-71148fa4a802" xlink:to="loc_us-gaap_NondesignatedMember_019f28d8-3cb2-7551-b07f-5b5b957cd6da" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeInstrumentRiskAxis_019f28d8-3cb2-75f6-ad26-40c624884021" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeInstrumentRiskAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_019f28d8-3cb2-7f72-9b06-ae2eaa92644c" xlink:to="loc_us-gaap_DerivativeInstrumentRiskAxis_019f28d8-3cb2-75f6-ad26-40c624884021" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_019f28d8-3cb2-75f6-ad26-40c624884021_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_019f28d8-3cb2-75f6-ad26-40c624884021" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_019f28d8-3cb2-75f6-ad26-40c624884021_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_019f28d8-3cb2-7ca1-afa9-b3d1031786e3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_019f28d8-3cb2-75f6-ad26-40c624884021" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_019f28d8-3cb2-7ca1-afa9-b3d1031786e3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FutureMember_019f28d8-3cb2-7bb2-bf09-868cd0080c65" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FutureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_019f28d8-3cb2-7ca1-afa9-b3d1031786e3" xlink:to="loc_us-gaap_FutureMember_019f28d8-3cb2-7bb2-bf09-868cd0080c65" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForeignExchangeForwardMember_019f28d8-3cb2-73a0-898b-97f897fc0a5f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ForeignExchangeForwardMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_019f28d8-3cb2-7ca1-afa9-b3d1031786e3" xlink:to="loc_us-gaap_ForeignExchangeForwardMember_019f28d8-3cb2-73a0-898b-97f897fc0a5f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherContractMember_019f28d8-3cb2-7316-8a0a-fb63d99aad1a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherContractMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_019f28d8-3cb2-7ca1-afa9-b3d1031786e3" xlink:to="loc_us-gaap_OtherContractMember_019f28d8-3cb2-7316-8a0a-fb63d99aad1a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeAssets_019f28d8-3cb2-7890-a67b-10d5ce6d5cd8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeLineItems_019f28d8-3cb2-7aa8-9b11-95e8efe5792a" xlink:to="loc_us-gaap_DerivativeAssets_019f28d8-3cb2-7890-a67b-10d5ce6d5cd8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeLiabilities_019f28d8-3cb2-7666-9f1e-13679bcede4e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeLineItems_019f28d8-3cb2-7aa8-9b11-95e8efe5792a" xlink:to="loc_us-gaap_DerivativeLiabilities_019f28d8-3cb2-7666-9f1e-13679bcede4e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeNotionalAmount_019f28d8-3cb2-72aa-b1f1-b1d4efd9b834" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeNotionalAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeLineItems_019f28d8-3cb2-7aa8-9b11-95e8efe5792a" xlink:to="loc_us-gaap_DerivativeNotionalAmount_019f28d8-3cb2-72aa-b1f1-b1d4efd9b834" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OffsettingAbstract_019f28d8-3cb2-7697-bdd2-79961b78e9c2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OffsettingAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeLineItems_019f28d8-3cb2-7aa8-9b11-95e8efe5792a" xlink:to="loc_us-gaap_OffsettingAbstract_019f28d8-3cb2-7697-bdd2-79961b78e9c2" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeFairValueOfDerivativeAsset_019f28d8-3cb2-7026-85c3-eafba07f3187" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeFairValueOfDerivativeAsset"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OffsettingAbstract_019f28d8-3cb2-7697-bdd2-79961b78e9c2" xlink:to="loc_us-gaap_DerivativeFairValueOfDerivativeAsset_019f28d8-3cb2-7026-85c3-eafba07f3187" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeFairValueOfDerivativeLiability_019f28d8-3cb2-7617-9f2e-799a922ce1d3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeFairValueOfDerivativeLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OffsettingAbstract_019f28d8-3cb2-7697-bdd2-79961b78e9c2" xlink:to="loc_us-gaap_DerivativeFairValueOfDerivativeLiability_019f28d8-3cb2-7617-9f2e-799a922ce1d3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeAssetStatementOfFinancialPositionExtensibleEnumeration_019f28d8-3cb2-7d48-ab31-0eaf0057533e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeAssetStatementOfFinancialPositionExtensibleEnumeration"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeLineItems_019f28d8-3cb2-7aa8-9b11-95e8efe5792a" xlink:to="loc_us-gaap_DerivativeAssetStatementOfFinancialPositionExtensibleEnumeration_019f28d8-3cb2-7d48-ab31-0eaf0057533e" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeLiabilityStatementOfFinancialPositionExtensibleEnumeration_019f28d8-3cb2-752f-81c4-0d998746f52a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeLiabilityStatementOfFinancialPositionExtensibleEnumeration"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeLineItems_019f28d8-3cb2-7aa8-9b11-95e8efe5792a" xlink:to="loc_us-gaap_DerivativeLiabilityStatementOfFinancialPositionExtensibleEnumeration_019f28d8-3cb2-752f-81c4-0d998746f52a" xlink:type="arc" order="5"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.archcapgroup.com/role/DerivativeInstrumentsSummaryofnetrealizedgainslossesrecordedintheconsolidatedstatementsofincomeDetails" xlink:type="simple" xlink:href="acgl-20260630.xsd#DerivativeInstrumentsSummaryofnetrealizedgainslossesrecordedintheconsolidatedstatementsofincomeDetails"/>
  <link:definitionLink xlink:role="http://www.archcapgroup.com/role/DerivativeInstrumentsSummaryofnetrealizedgainslossesrecordedintheconsolidatedstatementsofincomeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeLineItems_019f28d8-3cb2-7038-bfa8-1b512b57b4ae" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeTable_019f28d8-3cb2-7e25-9709-a24ad44a371a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DerivativeLineItems_019f28d8-3cb2-7038-bfa8-1b512b57b4ae" xlink:to="loc_us-gaap_DerivativeTable_019f28d8-3cb2-7e25-9709-a24ad44a371a" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationAxis_019f28d8-3cb2-725b-85a3-9bf901b1433c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HedgingDesignationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DerivativeTable_019f28d8-3cb2-7e25-9709-a24ad44a371a" xlink:to="loc_us-gaap_HedgingDesignationAxis_019f28d8-3cb2-725b-85a3-9bf901b1433c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationDomain_019f28d8-3cb2-725b-85a3-9bf901b1433c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HedgingDesignationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_HedgingDesignationAxis_019f28d8-3cb2-725b-85a3-9bf901b1433c" xlink:to="loc_us-gaap_HedgingDesignationDomain_019f28d8-3cb2-725b-85a3-9bf901b1433c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationDomain_019f28d8-3cb2-7e9f-a80f-75b3d00a016d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HedgingDesignationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_HedgingDesignationAxis_019f28d8-3cb2-725b-85a3-9bf901b1433c" xlink:to="loc_us-gaap_HedgingDesignationDomain_019f28d8-3cb2-7e9f-a80f-75b3d00a016d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NondesignatedMember_019f28d8-3cb2-7a8e-83b0-76e232fac679" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NondesignatedMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HedgingDesignationDomain_019f28d8-3cb2-7e9f-a80f-75b3d00a016d" xlink:to="loc_us-gaap_NondesignatedMember_019f28d8-3cb2-7a8e-83b0-76e232fac679" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeInstrumentRiskAxis_019f28d8-3cb2-70c8-a8af-7ea589fc03dd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeInstrumentRiskAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DerivativeTable_019f28d8-3cb2-7e25-9709-a24ad44a371a" xlink:to="loc_us-gaap_DerivativeInstrumentRiskAxis_019f28d8-3cb2-70c8-a8af-7ea589fc03dd" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_019f28d8-3cb2-70c8-a8af-7ea589fc03dd_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_019f28d8-3cb2-70c8-a8af-7ea589fc03dd" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_019f28d8-3cb2-70c8-a8af-7ea589fc03dd_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_019f28d8-3cb2-74ca-b55d-28a053087570" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_019f28d8-3cb2-70c8-a8af-7ea589fc03dd" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_019f28d8-3cb2-74ca-b55d-28a053087570" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FutureMember_019f28d8-3cb2-721d-a435-61acfd74275a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FutureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_019f28d8-3cb2-74ca-b55d-28a053087570" xlink:to="loc_us-gaap_FutureMember_019f28d8-3cb2-721d-a435-61acfd74275a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForeignExchangeForwardMember_019f28d8-3cb2-7323-80c8-d2981329254d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ForeignExchangeForwardMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_019f28d8-3cb2-74ca-b55d-28a053087570" xlink:to="loc_us-gaap_ForeignExchangeForwardMember_019f28d8-3cb2-7323-80c8-d2981329254d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherContractMember_019f28d8-3cb2-7de7-b5be-70f3e954ad14" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherContractMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_019f28d8-3cb2-74ca-b55d-28a053087570" xlink:to="loc_us-gaap_OtherContractMember_019f28d8-3cb2-7de7-b5be-70f3e954ad14" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeGainLossOnDerivativeNetAbstract_019f28d8-3cb2-71e0-8cd9-0ab97a46da20" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeGainLossOnDerivativeNetAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeLineItems_019f28d8-3cb2-7038-bfa8-1b512b57b4ae" xlink:to="loc_us-gaap_DerivativeGainLossOnDerivativeNetAbstract_019f28d8-3cb2-71e0-8cd9-0ab97a46da20" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeGainLossOnDerivativeNet_019f28d8-3cb2-7dbd-9eb5-eb55eeb30d36" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeGainLossOnDerivativeNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeGainLossOnDerivativeNetAbstract_019f28d8-3cb2-71e0-8cd9-0ab97a46da20" xlink:to="loc_us-gaap_DerivativeGainLossOnDerivativeNet_019f28d8-3cb2-7dbd-9eb5-eb55eeb30d36" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.archcapgroup.com/role/CommitmentsandContingenciesSeniorNotesDetails" xlink:type="simple" xlink:href="acgl-20260630.xsd#CommitmentsandContingenciesSeniorNotesDetails"/>
  <link:definitionLink xlink:role="http://www.archcapgroup.com/role/CommitmentsandContingenciesSeniorNotesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_019fae2f-765a-7802-91e1-438bff8168e1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_019fae2f-765a-71f3-b324-cf830859e791" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019fae2f-765a-7802-91e1-438bff8168e1" xlink:to="loc_us-gaap_DebtInstrumentTable_019fae2f-765a-71f3-b324-cf830859e791" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019fae2f-765a-7f33-bc6b-7ed90e1cc13e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019fae2f-765a-71f3-b324-cf830859e791" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019fae2f-765a-7f33-bc6b-7ed90e1cc13e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019fae2f-765a-7f33-bc6b-7ed90e1cc13e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019fae2f-765a-7f33-bc6b-7ed90e1cc13e" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019fae2f-765a-7f33-bc6b-7ed90e1cc13e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019fae2f-765a-7836-827a-d909599e802e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019fae2f-765a-7f33-bc6b-7ed90e1cc13e" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019fae2f-765a-7836-827a-d909599e802e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SeniorNotesMember_019fae2f-765a-7dc2-b28e-920f6aba48c4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SeniorNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019fae2f-765a-7836-827a-d909599e802e" xlink:to="loc_us-gaap_SeniorNotesMember_019fae2f-765a-7dc2-b28e-920f6aba48c4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019fae2f-765a-76d7-9aa7-8e9647b4e80c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019fae2f-765a-71f3-b324-cf830859e791" xlink:to="loc_us-gaap_DebtInstrumentAxis_019fae2f-765a-76d7-9aa7-8e9647b4e80c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019fae2f-765a-76d7-9aa7-8e9647b4e80c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019fae2f-765a-76d7-9aa7-8e9647b4e80c" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019fae2f-765a-76d7-9aa7-8e9647b4e80c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019fae2f-765a-7e03-9ec5-969d79231c8c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019fae2f-765a-76d7-9aa7-8e9647b4e80c" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019fae2f-765a-7e03-9ec5-969d79231c8c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_SeniorNotesDue2036Member_019fae2f-765a-7af7-b45a-eb9baf3382b5" xlink:href="acgl-20260630.xsd#acgl_SeniorNotesDue2036Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019fae2f-765a-7e03-9ec5-969d79231c8c" xlink:to="loc_acgl_SeniorNotesDue2036Member_019fae2f-765a-7af7-b45a-eb9baf3382b5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_SeniorNotesDue2056Member_019fae2f-765a-7599-8317-669ff3966d6b" xlink:href="acgl-20260630.xsd#acgl_SeniorNotesDue2056Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019fae2f-765a-7e03-9ec5-969d79231c8c" xlink:to="loc_acgl_SeniorNotesDue2056Member_019fae2f-765a-7599-8317-669ff3966d6b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_SeniorNotesDue2026Member_019fae2f-765a-71f1-ba9e-3012fedc684d" xlink:href="acgl-20260630.xsd#acgl_SeniorNotesDue2026Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019fae2f-765a-7e03-9ec5-969d79231c8c" xlink:to="loc_acgl_SeniorNotesDue2026Member_019fae2f-765a-71f1-ba9e-3012fedc684d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_SeniorNotesDue2043Member_019fae2f-765a-74a9-bb49-2c61b82881b8" xlink:href="acgl-20260630.xsd#acgl_SeniorNotesDue2043Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019fae2f-765a-7e03-9ec5-969d79231c8c" xlink:to="loc_acgl_SeniorNotesDue2043Member_019fae2f-765a-74a9-bb49-2c61b82881b8" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_SeniorNotesDue2046Member_019fae2f-765a-7b11-848a-3fa2028fede3" xlink:href="acgl-20260630.xsd#acgl_SeniorNotesDue2046Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019fae2f-765a-7e03-9ec5-969d79231c8c" xlink:to="loc_acgl_SeniorNotesDue2046Member_019fae2f-765a-7b11-848a-3fa2028fede3" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LegalEntityAxis_019fae2f-765a-7e8d-a6d1-fb6a8c1609cd" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_LegalEntityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019fae2f-765a-71f3-b324-cf830859e791" xlink:to="loc_dei_LegalEntityAxis_019fae2f-765a-7e8d-a6d1-fb6a8c1609cd" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019fae2f-765a-7e8d-a6d1-fb6a8c1609cd_default" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_019fae2f-765a-7e8d-a6d1-fb6a8c1609cd" xlink:to="loc_dei_EntityDomain_019fae2f-765a-7e8d-a6d1-fb6a8c1609cd_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019fae2f-765a-7bf9-9552-1b0b4666b3e1" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_019fae2f-765a-7e8d-a6d1-fb6a8c1609cd" xlink:to="loc_dei_EntityDomain_019fae2f-765a-7bf9-9552-1b0b4666b3e1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_ArchCapitalFinanceLLCMember_019fae2f-765a-719f-982c-ad0dc5f42f1a" xlink:href="acgl-20260630.xsd#acgl_ArchCapitalFinanceLLCMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_019fae2f-765a-7bf9-9552-1b0b4666b3e1" xlink:to="loc_acgl_ArchCapitalFinanceLLCMember_019fae2f-765a-719f-982c-ad0dc5f42f1a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_ArchCapitalGroupUSIncMember_019fae2f-765a-7781-b36e-9fe56771ceb5" xlink:href="acgl-20260630.xsd#acgl_ArchCapitalGroupUSIncMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_019fae2f-765a-7bf9-9552-1b0b4666b3e1" xlink:to="loc_acgl_ArchCapitalGroupUSIncMember_019fae2f-765a-7781-b36e-9fe56771ceb5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentFaceAmount_019fae2f-765a-74f9-be84-4b32b55fba29" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentFaceAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019fae2f-765a-7802-91e1-438bff8168e1" xlink:to="loc_us-gaap_DebtInstrumentFaceAmount_019fae2f-765a-74f9-be84-4b32b55fba29" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019fae2f-765a-7bd1-a1db-96a9ec07ade7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019fae2f-765a-7802-91e1-438bff8168e1" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019fae2f-765a-7bd1-a1db-96a9ec07ade7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentRedemptionPricePercentage_019fae2f-765a-7a3d-99e8-411cc5dbc12a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentRedemptionPricePercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019fae2f-765a-7802-91e1-438bff8168e1" xlink:to="loc_us-gaap_DebtInstrumentRedemptionPricePercentage_019fae2f-765a-7a3d-99e8-411cc5dbc12a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentRepurchaseAmount_019fae2f-765a-720e-8571-6fd78d02368e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentRepurchaseAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019fae2f-765a-7802-91e1-438bff8168e1" xlink:to="loc_us-gaap_DebtInstrumentRepurchaseAmount_019fae2f-765a-720e-8571-6fd78d02368e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GainsLossesOnExtinguishmentOfDebt_019fae2f-765a-7011-9235-9b8c4783c26b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_GainsLossesOnExtinguishmentOfDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019fae2f-765a-7802-91e1-438bff8168e1" xlink:to="loc_us-gaap_GainsLossesOnExtinguishmentOfDebt_019fae2f-765a-7011-9235-9b8c4783c26b" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.archcapgroup.com/role/VariableInterestEntitiesDetails" xlink:type="simple" xlink:href="acgl-20260630.xsd#VariableInterestEntitiesDetails"/>
  <link:definitionLink xlink:role="http://www.archcapgroup.com/role/VariableInterestEntitiesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableInterestEntityLineItems_019f28d8-3cb2-7dff-861b-e40f0ce839ea" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_VariableInterestEntityLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfVariableInterestEntitiesTable_019f28d8-3cb2-732a-b570-f715c1dff24e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfVariableInterestEntitiesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_VariableInterestEntityLineItems_019f28d8-3cb2-7dff-861b-e40f0ce839ea" xlink:to="loc_us-gaap_ScheduleOfVariableInterestEntitiesTable_019f28d8-3cb2-732a-b570-f715c1dff24e" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesAxis_019f28d8-3cb2-72e9-a8e6-0b8783462859" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidatedEntitiesAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfVariableInterestEntitiesTable_019f28d8-3cb2-732a-b570-f715c1dff24e" xlink:to="loc_srt_ConsolidatedEntitiesAxis_019f28d8-3cb2-72e9-a8e6-0b8783462859" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesDomain_019f28d8-3cb2-72e9-a8e6-0b8783462859_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidatedEntitiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidatedEntitiesAxis_019f28d8-3cb2-72e9-a8e6-0b8783462859" xlink:to="loc_srt_ConsolidatedEntitiesDomain_019f28d8-3cb2-72e9-a8e6-0b8783462859_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesDomain_019f28d8-3cb2-779c-90de-330f97d0c223" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidatedEntitiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidatedEntitiesAxis_019f28d8-3cb2-72e9-a8e6-0b8783462859" xlink:to="loc_srt_ConsolidatedEntitiesDomain_019f28d8-3cb2-779c-90de-330f97d0c223" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableInterestEntityNotPrimaryBeneficiaryMember_019f28d8-3cb2-7125-8384-fd5fb3c481fc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_VariableInterestEntityNotPrimaryBeneficiaryMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidatedEntitiesDomain_019f28d8-3cb2-779c-90de-330f97d0c223" xlink:to="loc_us-gaap_VariableInterestEntityNotPrimaryBeneficiaryMember_019f28d8-3cb2-7125-8384-fd5fb3c481fc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LegalEntityAxis_019f28d8-3cb2-7761-b95f-5fcd7175437a" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_LegalEntityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfVariableInterestEntitiesTable_019f28d8-3cb2-732a-b570-f715c1dff24e" xlink:to="loc_dei_LegalEntityAxis_019f28d8-3cb2-7761-b95f-5fcd7175437a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019f28d8-3cb2-7761-b95f-5fcd7175437a_default" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_019f28d8-3cb2-7761-b95f-5fcd7175437a" xlink:to="loc_dei_EntityDomain_019f28d8-3cb2-7761-b95f-5fcd7175437a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019f28d8-3cb2-72ad-b4c5-55f123d90c1f" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_019f28d8-3cb2-7761-b95f-5fcd7175437a" xlink:to="loc_dei_EntityDomain_019f28d8-3cb2-72ad-b4c5-55f123d90c1f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_Bellemeade20213LtdMember_019f28d8-3cb2-7e72-8983-c2463e970b16" xlink:href="acgl-20260630.xsd#acgl_Bellemeade20213LtdMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_019f28d8-3cb2-72ad-b4c5-55f123d90c1f" xlink:to="loc_acgl_Bellemeade20213LtdMember_019f28d8-3cb2-7e72-8983-c2463e970b16" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_Bellemeade20221LtdMember_019f28d8-3cb2-7dd4-8183-f927f2dd5d8a" xlink:href="acgl-20260630.xsd#acgl_Bellemeade20221LtdMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_019f28d8-3cb2-72ad-b4c5-55f123d90c1f" xlink:to="loc_acgl_Bellemeade20221LtdMember_019f28d8-3cb2-7dd4-8183-f927f2dd5d8a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_Bellemeade20222LtdMember_019f28d8-3cb2-7ead-8505-c63bb7bbca32" xlink:href="acgl-20260630.xsd#acgl_Bellemeade20222LtdMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_019f28d8-3cb2-72ad-b4c5-55f123d90c1f" xlink:to="loc_acgl_Bellemeade20222LtdMember_019f28d8-3cb2-7ead-8505-c63bb7bbca32" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_Bellemeade20231Ltd.Member_019f28d8-3cb2-7546-b708-54df8244f35e" xlink:href="acgl-20260630.xsd#acgl_Bellemeade20231Ltd.Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_019f28d8-3cb2-72ad-b4c5-55f123d90c1f" xlink:to="loc_acgl_Bellemeade20231Ltd.Member_019f28d8-3cb2-7546-b708-54df8244f35e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_BellemeadeRe20241Member_019f28d8-3cb2-7fbf-92dd-13cb21547b3c" xlink:href="acgl-20260630.xsd#acgl_BellemeadeRe20241Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_019f28d8-3cb2-72ad-b4c5-55f123d90c1f" xlink:to="loc_acgl_BellemeadeRe20241Member_019f28d8-3cb2-7fbf-92dd-13cb21547b3c" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_BellemeadeRe20251LtdMember_019f28d8-3cb2-7071-a705-7b2f2b758433" xlink:href="acgl-20260630.xsd#acgl_BellemeadeRe20251LtdMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_019f28d8-3cb2-72ad-b4c5-55f123d90c1f" xlink:to="loc_acgl_BellemeadeRe20251LtdMember_019f28d8-3cb2-7071-a705-7b2f2b758433" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Assets_019f28d8-3cb2-70f4-a4d9-20070a80ed11" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_Assets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableInterestEntityLineItems_019f28d8-3cb2-7dff-861b-e40f0ce839ea" xlink:to="loc_us-gaap_Assets_019f28d8-3cb2-70f4-a4d9-20070a80ed11" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_RemainingCoverageAmount_019f28d8-3cb2-7e00-a221-5f7087c50546" xlink:href="acgl-20260630.xsd#acgl_RemainingCoverageAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableInterestEntityLineItems_019f28d8-3cb2-7dff-861b-e40f0ce839ea" xlink:to="loc_acgl_RemainingCoverageAmount_019f28d8-3cb2-7e00-a221-5f7087c50546" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.archcapgroup.com/role/OtherComprehensiveIncomeLossAmountsreclassifiedfromaccumulatedothercomprehensiveincomeDetails" xlink:type="simple" xlink:href="acgl-20260630.xsd#OtherComprehensiveIncomeLossAmountsreclassifiedfromaccumulatedothercomprehensiveincomeDetails"/>
  <link:definitionLink xlink:role="http://www.archcapgroup.com/role/OtherComprehensiveIncomeLossAmountsreclassifiedfromaccumulatedothercomprehensiveincomeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems_019f28d8-3cb2-72f9-b4e3-1172d4b45e1a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeTable_019f28d8-3cb2-700c-9d9d-4f9d20be5493" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems_019f28d8-3cb2-72f9-b4e3-1172d4b45e1a" xlink:to="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeTable_019f28d8-3cb2-700c-9d9d-4f9d20be5493" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis_019f28d8-3cb2-78e4-8e0d-bb670e158ba6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeTable_019f28d8-3cb2-700c-9d9d-4f9d20be5493" xlink:to="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis_019f28d8-3cb2-78e4-8e0d-bb670e158ba6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain_019f28d8-3cb2-78e4-8e0d-bb670e158ba6_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis_019f28d8-3cb2-78e4-8e0d-bb670e158ba6" xlink:to="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain_019f28d8-3cb2-78e4-8e0d-bb670e158ba6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain_019f28d8-3cb2-7646-865f-88157de16052" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis_019f28d8-3cb2-78e4-8e0d-bb670e158ba6" xlink:to="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain_019f28d8-3cb2-7646-865f-88157de16052" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember_019f28d8-3cb2-75fc-b0c9-aec5bd65b433" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain_019f28d8-3cb2-7646-865f-88157de16052" xlink:to="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember_019f28d8-3cb2-75fc-b0c9-aec5bd65b433" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_019f28d8-3cb2-71dc-9086-716adac0b103" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeTable_019f28d8-3cb2-700c-9d9d-4f9d20be5493" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_019f28d8-3cb2-71dc-9086-716adac0b103" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019f28d8-3cb2-71dc-9086-716adac0b103_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019f28d8-3cb2-71dc-9086-716adac0b103" xlink:to="loc_us-gaap_EquityComponentDomain_019f28d8-3cb2-71dc-9086-716adac0b103_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019f28d8-3cb2-7122-9224-5f9c29ce6015" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019f28d8-3cb2-71dc-9086-716adac0b103" xlink:to="loc_us-gaap_EquityComponentDomain_019f28d8-3cb2-7122-9224-5f9c29ce6015" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember_019f28d8-3cb2-7f62-9455-fabb8f0a87f7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019f28d8-3cb2-7122-9224-5f9c29ce6015" xlink:to="loc_us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember_019f28d8-3cb2-7f62-9455-fabb8f0a87f7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_NetRealizedGainsLosses_019f28d8-3cb2-714b-bf2b-8299b3c2dfbe" xlink:href="acgl-20260630.xsd#acgl_NetRealizedGainsLosses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems_019f28d8-3cb2-72f9-b4e3-1172d4b45e1a" xlink:to="loc_acgl_NetRealizedGainsLosses_019f28d8-3cb2-714b-bf2b-8299b3c2dfbe" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleAllowanceForCreditLossPeriodIncreaseDecrease_019f28d8-3cb2-7022-ba05-9fa1852ad0e0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleAllowanceForCreditLossPeriodIncreaseDecrease"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems_019f28d8-3cb2-72f9-b4e3-1172d4b45e1a" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleAllowanceForCreditLossPeriodIncreaseDecrease_019f28d8-3cb2-7022-ba05-9fa1852ad0e0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_IncomeLossFromContinuingOperationsBeforeEarningsFromAffiliatesIncomeTaxesNoncontrollingInterest_019f28d8-3cb2-72ca-a0c4-358388d829e4" xlink:href="acgl-20260630.xsd#acgl_IncomeLossFromContinuingOperationsBeforeEarningsFromAffiliatesIncomeTaxesNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems_019f28d8-3cb2-72f9-b4e3-1172d4b45e1a" xlink:to="loc_acgl_IncomeLossFromContinuingOperationsBeforeEarningsFromAffiliatesIncomeTaxesNoncontrollingInterest_019f28d8-3cb2-72ca-a0c4-358388d829e4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxExpenseBenefit_019f28d8-3cb2-73f8-b0ae-3249e0b20cb0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncomeTaxExpenseBenefit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems_019f28d8-3cb2-72f9-b4e3-1172d4b45e1a" xlink:to="loc_us-gaap_IncomeTaxExpenseBenefit_019f28d8-3cb2-73f8-b0ae-3249e0b20cb0" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_019f28d8-3cb2-7699-9b35-2a4e3bd97a72" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems_019f28d8-3cb2-72f9-b4e3-1172d4b45e1a" xlink:to="loc_us-gaap_NetIncomeLoss_019f28d8-3cb2-7699-9b35-2a4e3bd97a72" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.archcapgroup.com/role/OtherComprehensiveIncomeLossTaxeffectsoncomponentsofothercomprehensiveincomeDetails" xlink:type="simple" xlink:href="acgl-20260630.xsd#OtherComprehensiveIncomeLossTaxeffectsoncomponentsofothercomprehensiveincomeDetails"/>
  <link:definitionLink xlink:role="http://www.archcapgroup.com/role/OtherComprehensiveIncomeLossTaxeffectsoncomponentsofothercomprehensiveincomeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_019f28d8-3cb2-77e6-9da6-689e8183ef3a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_019f28d8-3cb2-72ac-84c0-9c65d2003319" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_019f28d8-3cb2-77e6-9da6-689e8183ef3a" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_019f28d8-3cb2-72ac-84c0-9c65d2003319" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_019f28d8-3cb2-7b5c-b087-a056253bb37d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_019f28d8-3cb2-72ac-84c0-9c65d2003319" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_019f28d8-3cb2-7b5c-b087-a056253bb37d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019f28d8-3cb2-7b5c-b087-a056253bb37d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019f28d8-3cb2-7b5c-b087-a056253bb37d" xlink:to="loc_us-gaap_EquityComponentDomain_019f28d8-3cb2-7b5c-b087-a056253bb37d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019f28d8-3cb2-7e08-8cc5-456f6885d6db" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019f28d8-3cb2-7b5c-b087-a056253bb37d" xlink:to="loc_us-gaap_EquityComponentDomain_019f28d8-3cb2-7e08-8cc5-456f6885d6db" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember_019f28d8-3cb2-7a74-a419-c160535b8dfb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019f28d8-3cb2-7e08-8cc5-456f6885d6db" xlink:to="loc_us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember_019f28d8-3cb2-7a74-a419-c160535b8dfb" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedTranslationAdjustmentMember_019f28d8-3cb2-796a-9e96-39d691a5bb2a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedTranslationAdjustmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019f28d8-3cb2-7e08-8cc5-456f6885d6db" xlink:to="loc_us-gaap_AccumulatedTranslationAdjustmentMember_019f28d8-3cb2-796a-9e96-39d691a5bb2a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossBeforeTaxPeriodIncreaseDecreaseAbstract_019f28d8-3cb2-77b8-a801-0b8cf4dff48e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeLossBeforeTaxPeriodIncreaseDecreaseAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_019f28d8-3cb2-77e6-9da6-689e8183ef3a" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossBeforeTaxPeriodIncreaseDecreaseAbstract_019f28d8-3cb2-77b8-a801-0b8cf4dff48e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OciBeforeReclassificationsBeforeTaxAttributableToParent_019f28d8-3cb2-7f87-9827-98e288b2dd72" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OciBeforeReclassificationsBeforeTaxAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherComprehensiveIncomeLossBeforeTaxPeriodIncreaseDecreaseAbstract_019f28d8-3cb2-77b8-a801-0b8cf4dff48e" xlink:to="loc_us-gaap_OciBeforeReclassificationsBeforeTaxAttributableToParent_019f28d8-3cb2-7f87-9827-98e288b2dd72" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationFromAociCurrentPeriodBeforeTaxAttributableToParent_019f28d8-3cb2-77f6-8832-b1b7d0206add" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ReclassificationFromAociCurrentPeriodBeforeTaxAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherComprehensiveIncomeLossBeforeTaxPeriodIncreaseDecreaseAbstract_019f28d8-3cb2-77b8-a801-0b8cf4dff48e" xlink:to="loc_us-gaap_ReclassificationFromAociCurrentPeriodBeforeTaxAttributableToParent_019f28d8-3cb2-77f6-8832-b1b7d0206add" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossBeforeTaxPortionAttributableToParent_019f28d8-3cb2-77e7-8cbc-5f412d02182c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeLossBeforeTaxPortionAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherComprehensiveIncomeLossBeforeTaxPeriodIncreaseDecreaseAbstract_019f28d8-3cb2-77b8-a801-0b8cf4dff48e" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossBeforeTaxPortionAttributableToParent_019f28d8-3cb2-77e7-8cbc-5f412d02182c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossTaxAbstract_019f28d8-3cb2-7bad-a27c-c6683f9892b0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeLossTaxAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_019f28d8-3cb2-77e6-9da6-689e8183ef3a" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossTaxAbstract_019f28d8-3cb2-7bad-a27c-c6683f9892b0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsTax_019f28d8-3cb2-7fef-9542-1e3d13348b77" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherComprehensiveIncomeLossTaxAbstract_019f28d8-3cb2-7bad-a27c-c6683f9892b0" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsTax_019f28d8-3cb2-7fef-9542-1e3d13348b77" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationFromAociCurrentPeriodTax_019f28d8-3cb2-7d3b-817e-85b25bdb2159" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ReclassificationFromAociCurrentPeriodTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherComprehensiveIncomeLossTaxAbstract_019f28d8-3cb2-7bad-a27c-c6683f9892b0" xlink:to="loc_us-gaap_ReclassificationFromAociCurrentPeriodTax_019f28d8-3cb2-7d3b-817e-85b25bdb2159" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossTaxPortionAttributableToParent1_019f28d8-3cb2-72e9-abb3-3f689c5b20bd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeLossTaxPortionAttributableToParent1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherComprehensiveIncomeLossTaxAbstract_019f28d8-3cb2-7bad-a27c-c6683f9892b0" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossTaxPortionAttributableToParent1_019f28d8-3cb2-72e9-abb3-3f689c5b20bd" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPeriodIncreaseDecreaseAbstract_019f28d8-3cb2-7d6e-9494-8f52f14c1cf8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTaxPeriodIncreaseDecreaseAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_019f28d8-3cb2-77e6-9da6-689e8183ef3a" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPeriodIncreaseDecreaseAbstract_019f28d8-3cb2-7d6e-9494-8f52f14c1cf8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OciBeforeReclassificationsNetOfTaxAttributableToParent_019f28d8-3cb2-7995-ba79-235e332dc7ee" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OciBeforeReclassificationsNetOfTaxAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPeriodIncreaseDecreaseAbstract_019f28d8-3cb2-7d6e-9494-8f52f14c1cf8" xlink:to="loc_us-gaap_OciBeforeReclassificationsNetOfTaxAttributableToParent_019f28d8-3cb2-7995-ba79-235e332dc7ee" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationFromAociCurrentPeriodNetOfTaxAttributableToParent_019f28d8-3cb2-793f-9f97-e5e60d5402e4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ReclassificationFromAociCurrentPeriodNetOfTaxAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPeriodIncreaseDecreaseAbstract_019f28d8-3cb2-7d6e-9494-8f52f14c1cf8" xlink:to="loc_us-gaap_ReclassificationFromAociCurrentPeriodNetOfTaxAttributableToParent_019f28d8-3cb2-793f-9f97-e5e60d5402e4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_019f28d8-3cb2-7c8f-8fc0-ab94107f9f47" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPeriodIncreaseDecreaseAbstract_019f28d8-3cb2-7d6e-9494-8f52f14c1cf8" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_019f28d8-3cb2-7c8f-8fc0-ab94107f9f47" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.archcapgroup.com/role/TransactionsWithRelatedPartiesDetails" xlink:type="simple" xlink:href="acgl-20260630.xsd#TransactionsWithRelatedPartiesDetails"/>
  <link:definitionLink xlink:role="http://www.archcapgroup.com/role/TransactionsWithRelatedPartiesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionLineItems_019f28d8-3cb2-7e1b-b691-e2587437d775" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyTransactionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_019f28d8-3cb2-7b79-9df3-2b53c3a842b1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019f28d8-3cb2-7e1b-b691-e2587437d775" xlink:to="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_019f28d8-3cb2-7b79-9df3-2b53c3a842b1" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019f28d8-3cb2-7466-af15-b6eaf4d7ab67" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_019f28d8-3cb2-7b79-9df3-2b53c3a842b1" xlink:to="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019f28d8-3cb2-7466-af15-b6eaf4d7ab67" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_019f28d8-3cb2-7466-af15-b6eaf4d7ab67_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019f28d8-3cb2-7466-af15-b6eaf4d7ab67" xlink:to="loc_us-gaap_RelatedPartyDomain_019f28d8-3cb2-7466-af15-b6eaf4d7ab67_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_019f28d8-3cb2-7092-862b-ccc99226e458" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019f28d8-3cb2-7466-af15-b6eaf4d7ab67" xlink:to="loc_us-gaap_RelatedPartyDomain_019f28d8-3cb2-7092-862b-ccc99226e458" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvesteeMember_019f28d8-3cb2-70c1-8144-993d90c42fce" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InvesteeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_019f28d8-3cb2-7092-862b-ccc99226e458" xlink:to="loc_us-gaap_InvesteeMember_019f28d8-3cb2-70c1-8144-993d90c42fce" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityMethodInvesteeMember_019f28d8-3cb2-763c-b6dc-cdac126c25c5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityMethodInvesteeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_019f28d8-3cb2-7092-862b-ccc99226e458" xlink:to="loc_us-gaap_EquityMethodInvesteeMember_019f28d8-3cb2-763c-b6dc-cdac126c25c5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CounterpartyNameAxis_019f28d8-3cb2-7827-a35a-294c47a7e6fc" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_CounterpartyNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_019f28d8-3cb2-7b79-9df3-2b53c3a842b1" xlink:to="loc_srt_CounterpartyNameAxis_019f28d8-3cb2-7827-a35a-294c47a7e6fc" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019f28d8-3cb2-7827-a35a-294c47a7e6fc_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_CounterpartyNameAxis_019f28d8-3cb2-7827-a35a-294c47a7e6fc" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019f28d8-3cb2-7827-a35a-294c47a7e6fc_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019f28d8-3cb2-7ab0-a414-788f6b1568a6" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_CounterpartyNameAxis_019f28d8-3cb2-7827-a35a-294c47a7e6fc" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019f28d8-3cb2-7ab0-a414-788f6b1568a6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_PremiaHoldingsLtdMember_019f28d8-3cb2-7d99-9c29-7e04bb362e2b" xlink:href="acgl-20260630.xsd#acgl_PremiaHoldingsLtdMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019f28d8-3cb2-7ab0-a414-788f6b1568a6" xlink:to="loc_acgl_PremiaHoldingsLtdMember_019f28d8-3cb2-7d99-9c29-7e04bb362e2b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_SomersGroupHoldingsLtdMember_019f28d8-3cb2-74bc-85c5-cd1cd31012ee" xlink:href="acgl-20260630.xsd#acgl_SomersGroupHoldingsLtdMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019f28d8-3cb2-7ab0-a414-788f6b1568a6" xlink:to="loc_acgl_SomersGroupHoldingsLtdMember_019f28d8-3cb2-74bc-85c5-cd1cd31012ee" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_019f28d8-3cb2-7e52-97c4-e73768652fcc" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_019f28d8-3cb2-7b79-9df3-2b53c3a842b1" xlink:to="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_019f28d8-3cb2-7e52-97c4-e73768652fcc" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_EquityMethodInvesteeNameDomain_019f28d8-3cb2-7e52-97c4-e73768652fcc_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_EquityMethodInvesteeNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_019f28d8-3cb2-7e52-97c4-e73768652fcc" xlink:to="loc_srt_EquityMethodInvesteeNameDomain_019f28d8-3cb2-7e52-97c4-e73768652fcc_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_EquityMethodInvesteeNameDomain_019f28d8-3cb2-7294-85f0-5749e4c0b24b" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_EquityMethodInvesteeNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_019f28d8-3cb2-7e52-97c4-e73768652fcc" xlink:to="loc_srt_EquityMethodInvesteeNameDomain_019f28d8-3cb2-7294-85f0-5749e4c0b24b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_acgl_GreysbridgeHoldingsLtdMember_019f28d8-3cb2-7428-97da-0f0729670cf7" xlink:href="acgl-20260630.xsd#acgl_GreysbridgeHoldingsLtdMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_EquityMethodInvesteeNameDomain_019f28d8-3cb2-7294-85f0-5749e4c0b24b" xlink:to="loc_acgl_GreysbridgeHoldingsLtdMember_019f28d8-3cb2-7428-97da-0f0729670cf7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FundsHeldUnderReinsuranceAgreementsAsset_019f28d8-3cb2-7c71-8fc9-fc3696b844d8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FundsHeldUnderReinsuranceAgreementsAsset"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019f28d8-3cb2-7e1b-b691-e2587437d775" xlink:to="loc_us-gaap_FundsHeldUnderReinsuranceAgreementsAsset_019f28d8-3cb2-7c71-8fc9-fc3696b844d8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PremiumsWrittenNet_019f28d8-3cb2-7dd1-bf80-50f295ac256b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PremiumsWrittenNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019f28d8-3cb2-7e1b-b691-e2587437d775" xlink:to="loc_us-gaap_PremiumsWrittenNet_019f28d8-3cb2-7dd1-bf80-50f295ac256b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReinsuranceRecoverablesOnPaidAndUnpaidLosses_019f28d8-3cb2-73f1-9556-6a78f6641577" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ReinsuranceRecoverablesOnPaidAndUnpaidLosses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019f28d8-3cb2-7e1b-b691-e2587437d775" xlink:to="loc_us-gaap_ReinsuranceRecoverablesOnPaidAndUnpaidLosses_019f28d8-3cb2-73f1-9556-6a78f6641577" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReinsurancePayable_019f28d8-3cb2-74ef-9745-967299f3a795" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ReinsurancePayable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019f28d8-3cb2-7e1b-b691-e2587437d775" xlink:to="loc_us-gaap_ReinsurancePayable_019f28d8-3cb2-74ef-9745-967299f3a795" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherAssets_019f28d8-3cb2-73b6-a015-cb2859294183" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019f28d8-3cb2-7e1b-b691-e2587437d775" xlink:to="loc_us-gaap_OtherAssets_019f28d8-3cb2-73b6-a015-cb2859294183" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherLiabilities_019f28d8-3cb2-777c-9218-840beca408b9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019f28d8-3cb2-7e1b-b691-e2587437d775" xlink:to="loc_us-gaap_OtherLiabilities_019f28d8-3cb2-777c-9218-840beca408b9" xlink:type="arc" order="5"/>
  </link:definitionLink>
</link:linkbase>
