<?xml version="1.0"?>
<ownershipDocument>
    <schemaVersion>X0201</schemaVersion>

    <documentType>4</documentType>
<periodOfReport>2003-10-14</periodOfReport>
<notSubjectToSection16>0</notSubjectToSection16>
    <issuer>
        <issuerCik>0000703559</issuerCik>
        <issuerName>HUDSON UNITED BANCORP</issuerName>
        <issuerTradingSymbol>HU</issuerTradingSymbol>
</issuer>
<reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001185286</rptOwnerCik>
            <rptOwnerName>THOMAS NELSON</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1></rptOwnerStreet1>
            <rptOwnerStreet2>1000 MACARTHUR BOULEVARD</rptOwnerStreet2>
            <rptOwnerCity>MAHWAH</rptOwnerCity>
            <rptOwnerState>NJ</rptOwnerState>
            <rptOwnerZipCode>07430</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
            <reportingOwnerRelationship>
<isDirector>0</isDirector>
<isOfficer>1</isOfficer>
<isTenPercentOwner>0</isTenPercentOwner>
<isOther>0</isOther>
<officerTitle>Executive Vice President</officerTitle>
<otherText></otherText>
</reportingOwnerRelationship>
</reportingOwner>
<nonDerivativeTable>
<nonDerivativeTransaction>
<securityTitle>
<value>Hudson United Bancorp Common Stock</value>
</securityTitle>
<transactionDate>
<value>2003-10-14</value>
</transactionDate>
<deemedExecutionDate>
<value>2003-10-14</value>
</deemedExecutionDate>
<transactionCoding>
<transactionFormType>4</transactionFormType>
<transactionCode>J</transactionCode>
<equitySwapInvolved>0</equitySwapInvolved>
</transactionCoding>
<transactionTimeliness>
<value></value>
</transactionTimeliness>
<transactionAmounts>
<transactionShares>
<value>2.2297</value>
</transactionShares>
<transactionPricePerShare>
<value> 35.88</value>
</transactionPricePerShare>
<transactionAcquiredDisposedCode>
<value>A</value>
</transactionAcquiredDisposedCode>
</transactionAmounts>
<postTransactionAmounts>
<sharesOwnedFollowingTransaction>
<value>10258.0199</value>
</sharesOwnedFollowingTransaction>
</postTransactionAmounts>
<ownershipNature>
<directOrIndirectOwnership>
<value>I</value>
</directOrIndirectOwnership>
<natureOfOwnership>
<value>401 K</value>
</natureOfOwnership>
</ownershipNature>
</nonDerivativeTransaction>
<nonDerivativeTransaction>
<securityTitle>
<value>Hudson United Bancorp Common Stock</value>
</securityTitle>
<transactionDate>
<value>2003-10-17</value>
</transactionDate>
<deemedExecutionDate>
<value>2003-10-17</value>
</deemedExecutionDate>
<transactionCoding>
<transactionFormType>4</transactionFormType>
<transactionCode>J</transactionCode>
<equitySwapInvolved>0</equitySwapInvolved>
</transactionCoding>
<transactionTimeliness>
<value></value>
</transactionTimeliness>
<transactionAmounts>
<transactionShares>
<value>88.9846</value>
<footnoteId id="F1"/>
</transactionShares>
<transactionPricePerShare>
<value>34.98</value>
</transactionPricePerShare>
<transactionAcquiredDisposedCode>
<value>A</value>
</transactionAcquiredDisposedCode>
</transactionAmounts>
<postTransactionAmounts>
<sharesOwnedFollowingTransaction>
<value>10347.0045</value>
</sharesOwnedFollowingTransaction>
</postTransactionAmounts>
<ownershipNature>
<directOrIndirectOwnership>
<value>I</value>
</directOrIndirectOwnership>
<natureOfOwnership>
<value>401 K</value>
</natureOfOwnership>
</ownershipNature>
</nonDerivativeTransaction>
<nonDerivativeTransaction>
<securityTitle>
<value>Hudson United Bancorp Common Stock</value>
</securityTitle>
<transactionDate>
<value>2003-10-20</value>
</transactionDate>
<deemedExecutionDate>
<value>2003-10-20</value>
</deemedExecutionDate>
<transactionCoding>
<transactionFormType>4</transactionFormType>
<transactionCode>J</transactionCode>
<equitySwapInvolved>0</equitySwapInvolved>
</transactionCoding>
<transactionTimeliness>
<value></value>
</transactionTimeliness>
<transactionAmounts>
<transactionShares>
<value>.0691</value>
<footnoteId id="F1"/>
</transactionShares>
<transactionPricePerShare>
<value> 34.28</value>
</transactionPricePerShare>
<transactionAcquiredDisposedCode>
<value>A</value>
</transactionAcquiredDisposedCode>
</transactionAmounts>
<postTransactionAmounts>
<sharesOwnedFollowingTransaction>
<value>10347.0736</value>
</sharesOwnedFollowingTransaction>
</postTransactionAmounts>
<ownershipNature>
<directOrIndirectOwnership>
<value>I</value>
</directOrIndirectOwnership>
<natureOfOwnership>
<value>401 K</value>
</natureOfOwnership>
</ownershipNature>
</nonDerivativeTransaction>
<nonDerivativeTransaction>
<securityTitle>
<value>Hudson United Bancorp Common Stock</value>
</securityTitle>
<transactionDate>
<value>2003-10-23</value>
</transactionDate>
<deemedExecutionDate>
<value>2003-10-23</value>
</deemedExecutionDate>
<transactionCoding>
<transactionFormType>4</transactionFormType>
<transactionCode>J</transactionCode>
<equitySwapInvolved>0</equitySwapInvolved>
</transactionCoding>
<transactionTimeliness>
<value></value>
</transactionTimeliness>
<transactionAmounts>
<transactionShares>
<value>2.3048</value>
</transactionShares>
<transactionPricePerShare>
<value> 34.71</value>
</transactionPricePerShare>
<transactionAcquiredDisposedCode>
<value>A</value>
</transactionAcquiredDisposedCode>
</transactionAmounts>
<postTransactionAmounts>
<sharesOwnedFollowingTransaction>
<value>10349.3784</value>
</sharesOwnedFollowingTransaction>
</postTransactionAmounts>
<ownershipNature>
<directOrIndirectOwnership>
<value>I</value>
</directOrIndirectOwnership>
<natureOfOwnership>
<value>401 K</value>
</natureOfOwnership>
</ownershipNature>
</nonDerivativeTransaction>
</nonDerivativeTable>
<footnotes>
<footnote id="F1">Dividends</footnote>
</footnotes>
<remarks></remarks>
<ownerSignature>
<signatureName>Thomas R. Nelson</signatureName>
<signatureDate>2003-11-06</signatureDate>
</ownerSignature>
</ownershipDocument>

