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

    <documentType>4</documentType><periodOfReport>2003-10-01</periodOfReport>
    <issuer>
        <issuerCik>0000754673</issuerCik>
        <issuerName>SUFFOLK BANCORP</issuerName>
        <issuerTradingSymbol>SUBK</issuerTradingSymbol></issuer><reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001235041</rptOwnerCik>
            <rptOwnerName>DICK ROBERT C</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O SUFFOLK BANCORP</rptOwnerStreet1>
            <rptOwnerStreet2>4 WEST SECOND STREET, PO BOX 9000</rptOwnerStreet2>
            <rptOwnerCity>RIVERHEAD</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>11901</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
            <reportingOwnerRelationship>
<isDirector>0</isDirector><isOfficer>1</isOfficer><isTenPercentOwner>0</isTenPercentOwner><isOther>0</isOther><officerTitle>Executive Vice President</officerTitle></reportingOwnerRelationship></reportingOwner><nonDerivativeTable><nonDerivativeTransaction><securityTitle><value>Suffolk Bancorp Common Stock</value></securityTitle><transactionDate><value>2003-10-01</value></transactionDate><transactionCoding><transactionFormType>5</transactionFormType><transactionCode>P</transactionCode><equitySwapInvolved>0</equitySwapInvolved></transactionCoding><transactionAmounts><transactionShares><value>17.732</value></transactionShares><transactionPricePerShare><value>33.89</value></transactionPricePerShare><transactionAcquiredDisposedCode><value>A</value></transactionAcquiredDisposedCode></transactionAmounts><postTransactionAmounts><sharesOwnedFollowingTransaction><value>3180.589</value></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>D</value></directOrIndirectOwnership><natureOfOwnership><value/></natureOfOwnership></ownershipNature></nonDerivativeTransaction><nonDerivativeTransaction><securityTitle><value>Suffolk Bancorp Common Stock</value></securityTitle><transactionDate><value>2003-10-01</value></transactionDate><transactionCoding><transactionFormType>5</transactionFormType><transactionCode>P</transactionCode><equitySwapInvolved>0</equitySwapInvolved></transactionCoding><transactionAmounts><transactionShares><value>5.307</value></transactionShares><transactionPricePerShare><value>33.89</value></transactionPricePerShare><transactionAcquiredDisposedCode><value>A</value></transactionAcquiredDisposedCode></transactionAmounts><postTransactionAmounts><sharesOwnedFollowingTransaction><value>951.02</value></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>I</value></directOrIndirectOwnership><natureOfOwnership><value>Son</value></natureOfOwnership></ownershipNature></nonDerivativeTransaction><nonDerivativeHolding><securityTitle><value>Suffolk Bancorp Common Stock</value></securityTitle><postTransactionAmounts><sharesOwnedFollowingTransaction><value>50.27</value></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>I</value></directOrIndirectOwnership><natureOfOwnership><value>Spouse</value></natureOfOwnership></ownershipNature></nonDerivativeHolding></nonDerivativeTable><derivativeTable><derivativeHolding><securityTitle><value>Suffolk Bancorp Option</value></securityTitle><conversionOrExercisePrice><value>15.50</value></conversionOrExercisePrice><exerciseDate><value>2002-01-19</value></exerciseDate><expirationDate><value>2011-01-19</value></expirationDate><underlyingSecurity><underlyingSecurityTitle><value>Suffolk Bancorp Common Stock</value></underlyingSecurityTitle><underlyingSecurityShares><value>5000</value></underlyingSecurityShares></underlyingSecurity><postTransactionAmounts><sharesOwnedFollowingTransaction><value>5000</value></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>D</value></directOrIndirectOwnership><natureOfOwnership><value/></natureOfOwnership></ownershipNature></derivativeHolding><derivativeHolding><securityTitle><value>Suffolk Bancorp Option</value></securityTitle><conversionOrExercisePrice><value>31.83</value></conversionOrExercisePrice><exerciseDate><value>2004-02-24</value></exerciseDate><expirationDate><value>2013-02-24</value></expirationDate><underlyingSecurity><underlyingSecurityTitle><value>Suffolk Bancorp Common Stock</value></underlyingSecurityTitle><underlyingSecurityShares><value>2500</value></underlyingSecurityShares></underlyingSecurity><postTransactionAmounts><sharesOwnedFollowingTransaction><value>2500</value></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>D</value></directOrIndirectOwnership><natureOfOwnership><value/></natureOfOwnership></ownershipNature></derivativeHolding></derivativeTable><footnotes/><remarks/><ownerSignature><signatureName>Robert C. Dick</signatureName><signatureDate>2003-10-02</signatureDate></ownerSignature></ownershipDocument>

