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    <submissionType>13F-HR/A</submissionType>
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      <periodOfReport>09-30-2020</periodOfReport>
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    <coverPage>
      <reportCalendarOrQuarter>09-30-2020</reportCalendarOrQuarter>
      <isAmendment>true</isAmendment>
      <amendmentNo>1</amendmentNo>
      <amendmentInfo>
        <amendmentType>RESTATEMENT</amendmentType>
      </amendmentInfo>
      <filingManager>
        <name>SEI INVESTMENTS CO</name>
        <address>
          <com:street1>1 FREEDOM VALLEY DRIVE</com:street1>
          <com:city>OAKS</com:city>
          <com:stateOrCountry>PA</com:stateOrCountry>
          <com:zipCode>19456-1100</com:zipCode>
        </address>
      </filingManager>
      <reportType>13F COMBINATION REPORT</reportType>
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          <name>Acadian Asset Management LLC</name>
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          <name>AllianceBernstein L.P.</name>
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          <form13FFileNumber>028-04895</form13FFileNumber>
          <name>Analytic Investors, LLC</name>
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          <name>AQR Capital Management, LLC</name>
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          <form13FFileNumber>028-13791</form13FFileNumber>
          <name>ArrowMark Partners</name>
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          <form13FFileNumber>028-05615</form13FFileNumber>
          <name>Axiom International Investors LLC</name>
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          <form13FFileNumber>028-18146</form13FFileNumber>
          <name>Benefit Street Partners LLC</name>
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          <name>Beutel, Goodman &amp; Company Ltd.</name>
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          <name>BlackRock Investment Management, LLC</name>
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          <name>Brandywine Global Investment Management, LLC</name>
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          <name>Brigade Capital Management, LP</name>
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          <name>Cardinal Capital Management, L.L.C.</name>
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          <name>Causeway Capital Management LLC</name>
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          <form13FFileNumber>028-18741</form13FFileNumber>
          <name>CenterSquare Investment Management LLC</name>
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          <name>Ceredex Value Advisors LLC</name>
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          <name>City National Rochdale, LLC</name>
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          <name>Coho Partners, Ltd.</name>
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          <name>Copeland Capital Management, LLC</name>
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          <name>EARNEST Partners, LLC</name>
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          <name>Falcon Point Capital, LLC</name>
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          <form13FFileNumber>028-14036</form13FFileNumber>
          <name>Fiera Capital Corporation</name>
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          <form13FFileNumber>028-18845</form13FFileNumber>
          <name>Fondsmaeglerselskabet Maj Invest A/S</name>
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          <form13FFileNumber>028-03935</form13FFileNumber>
          <name>Geneva Capital Management, LLC</name>
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          <form13FFileNumber>028-05229</form13FFileNumber>
          <name>Great Lakes Advisors, LLC</name>
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          <form13FFileNumber>028-04145</form13FFileNumber>
          <name>GW&amp;K Investment Management, LLC</name>
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          <form13FFileNumber>028-11957</form13FFileNumber>
          <name>Hillsdale Investment Management Inc.</name>
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          <form13FFileNumber>028-13710</form13FFileNumber>
          <name>IndexIQ Advisors LLC</name>
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          <form13FFileNumber>028-04211</form13FFileNumber>
          <name>INTECH Investment Management LLC</name>
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          <form13FFileNumber>028-11119</form13FFileNumber>
          <name>Integrity Asset Management</name>
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          <form13FFileNumber>028-11246</form13FFileNumber>
          <name>Invesco Advisers, Inc.</name>
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          <name>J.P. Morgan Investment Management Inc.</name>
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          <name>Jennison Associates, LLC</name>
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          <name>JO Hambro Capital Management Limited</name>
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          <form13FFileNumber>028-17689</form13FFileNumber>
          <name>Kettle Hill Capital Management, LLC</name>
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          <form13FFileNumber>028-06588</form13FFileNumber>
          <name>Lazard Asset Management LLC</name>
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          <form13FFileNumber>028-13608</form13FFileNumber>
          <name>LMCG Investments, LLC</name>
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          <form13FFileNumber>028-00398</form13FFileNumber>
          <name>Loomis, Sayles &amp; Company, L.P.</name>
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          <form13FFileNumber>028-06580</form13FFileNumber>
          <name>LSV Asset Management</name>
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          <form13FFileNumber>028-13124</form13FFileNumber>
          <name>Macquarie Investment Management</name>
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          <form13FFileNumber>028-03673</form13FFileNumber>
          <name>Manulife Investment Management</name>
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          <form13FFileNumber>028-17488</form13FFileNumber>
          <name>Maple-Brown Abbott Limited</name>
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          <name>Mar Vista Investment Partners, LLC</name>
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          <form13FFileNumber>028-03743</form13FFileNumber>
          <name>Marathon Asset Management, L.P.</name>
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          <form13FFileNumber>028-04632</form13FFileNumber>
          <name>Martingale Asset Management, L.P.</name>
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          <form13FFileNumber>028-13573</form13FFileNumber>
          <name>Neuberger Berman Investment Advisers LLC</name>
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          <form13FFileNumber>028-17435</form13FFileNumber>
          <name>NN Investment Partners</name>
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          <form13FFileNumber>028-01474</form13FFileNumber>
          <name>NWQ Investment Management Company, LLC</name>
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          <form13FFileNumber>028-12690</form13FFileNumber>
          <name>Pacific Investment Management Company LLC</name>
        </otherManager>
        <otherManager>
          <form13FFileNumber>028-03042</form13FFileNumber>
          <name>PanAgora Asset Management Inc.</name>
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        <otherManager>
          <form13FFileNumber>028-04558</form13FFileNumber>
          <name>Parametric Portfolio Associates LLC</name>
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        <otherManager>
          <form13FFileNumber>028-13350</form13FFileNumber>
          <name>Poplar Forest Capital, LLC</name>
        </otherManager>
        <otherManager>
          <form13FFileNumber>028-01549</form13FFileNumber>
          <name>Principal Global Investors</name>
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        <otherManager>
          <form13FFileNumber>028-14009</form13FFileNumber>
          <name>QS Investors, LLC</name>
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        <otherManager>
          <form13FFileNumber>028-19339</form13FFileNumber>
          <name>Qtron Investments, LLC</name>
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          <form13FFileNumber>028-11173</form13FFileNumber>
          <name>Quantitative Management Associates LLC</name>
        </otherManager>
        <otherManager>
          <form13FFileNumber>028-10674</form13FFileNumber>
          <name>Ramius Advisors, LLC</name>
        </otherManager>
        <otherManager>
          <form13FFileNumber>028-10467</form13FFileNumber>
          <name>Rice Hall James &amp; Associates, LLC</name>
        </otherManager>
        <otherManager>
          <form13FFileNumber>028-14481</form13FFileNumber>
          <name>RWC Asset Advisors</name>
        </otherManager>
        <otherManager>
          <form13FFileNumber>028-04691</form13FFileNumber>
          <name>Schafer Cullen Capital Management, Inc.</name>
        </otherManager>
        <otherManager>
          <form13FFileNumber>028-10613</form13FFileNumber>
          <name>Snow Capital Management L.P.</name>
        </otherManager>
        <otherManager>
          <form13FFileNumber>028-11300</form13FFileNumber>
          <name>Spectrum Asset Management, Inc.</name>
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          <form13FFileNumber>028-00733</form13FFileNumber>
          <name>SSGA Funds Management, Inc.</name>
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          <form13FFileNumber>028-10640</form13FFileNumber>
          <name>Towle &amp; Co</name>
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          <form13FFileNumber>028-07292</form13FFileNumber>
          <name>Voya Investment Management Co. LLC</name>
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          <form13FFileNumber>028-07104</form13FFileNumber>
          <name>WCM Investment Management</name>
        </otherManager>
      </otherManagersInfo>
      <provideInfoForInstruction5>Y</provideInfoForInstruction5>
      <additionalInformation>SEI Investments Company (SEI) is the parent holding company of certain institutional investment managers, as defined by Section 13(f) of the Securities Exchange Act of 1934, as amended (the "Act"). For purposes of the reporting requirements under Section 13(f) of the Act, and the rules promulgated thereunder, SEI does not exercise, and therefore disclaims, investment discretion with respect to any Section 13(f) securities positions over which its investment operating subsidiaries exercise such discretion. To the extent, however, that SEI's ownership interest in such subsidiaries may nevertheless give rise to a Form 13F obligation for SEI, the information required by Form 13F is reported herein on behalf of the following subsidiaries: SEI Investments Management Corporation ("SIMC"), SEI Trust Company and SEI Investments Canada Company (SEI Canada). Voting authority with respect to the Section 13(f) securities reported herein resides with and is exercised by SEI's operating subsidiaries, unless otherwise indicated. Although SEI has not been delegated voting authority over such Section 13(f) securities, and disclaims such voting authority, pursuant to instruction 12b.viii of Form 13F, and for that purpose only, such Section 13(f) securities are reported as "Sole Voting Authority." SIMC and SEI Canada each serve as an investment adviser to a number of pooled investment vehicles, which may include mutual funds, hedge funds, private equity funds, collective investment trusts and offshore investment funds (together, the "Pooled Investment Vehicles"). In addition, SIMC sponsor and may serve as adviser to, separately managed account programs (SMA Programs) . SIMC and SEI Canada may act as a "manager of managers" whereby each retain, subject to SIMC's and SEI Canada's oversight, respectively, sub-advisers to buy and sell securities on behalf of such Pooled Investment Vehicles and SMA Programs. To the extent that SIMC's and SEI Canada's role as a manager of managers could give rise to a Form 13F obligation, SIMC or SEI Canada has instructed its sub-advisers to file a Form 13F with respect to such securities and has included such sub-advisers in the "list of other managers reporting for this manager" below.</additionalInformation>
    </coverPage>
    <signatureBlock>
      <name>Michael N. Peterson</name>
      <title>Executive Vice President</title>
      <phone>610-676-1000</phone>
      <signature>Michael N. Peterson</signature>
      <city>Oaks</city>
      <stateOrCountry>PA</stateOrCountry>
      <signatureDate>12-04-2020</signatureDate>
    </signatureBlock>
    <summaryPage>
      <otherIncludedManagersCount>9</otherIncludedManagersCount>
      <tableEntryTotal>4513</tableEntryTotal>
      <tableValueTotal>34534035</tableValueTotal>
      <isConfidentialOmitted>false</isConfidentialOmitted>
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            <name>EAM Investors LLC</name>
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          <sequenceNumber>3</sequenceNumber>
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            <name>Global Credit Advisers, LLC</name>
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          <sequenceNumber>5</sequenceNumber>
          <otherManager>
            <name>Jackson Creek Investment Advisors</name>
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          <sequenceNumber>6</sequenceNumber>
          <otherManager>
            <name>Jupiter Asset Management Ltd</name>
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        <otherManager2>
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            <form13FFileNumber>028-16475</form13FFileNumber>
            <name>SEI Investments Management Corporation</name>
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            <form13FFileNumber>028-10297</form13FFileNumber>
            <name>SEI Trust Company</name>
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          <otherManager>
            <name>Sionna Investment Managers Inc.</name>
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          <otherManager>
            <name>Todd Asset Management LLC</name>
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            <form13FFileNumber>028-16449</form13FFileNumber>
            <name>SEI Investments Canada Company</name>
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