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

    <documentType>4</documentType><periodOfReport>2003-12-01</periodOfReport>
    <issuer>
        <issuerCik>0000754673</issuerCik>
        <issuerName>SUFFOLK BANCORP</issuerName>
        <issuerTradingSymbol>SUBK</issuerTradingSymbol></issuer><reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001235039</rptOwnerCik>
            <rptOwnerName>STARK J DOUGLAS</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>1</isDirector><isOfficer>0</isOfficer><isTenPercentOwner>0</isTenPercentOwner><isOther>0</isOther></reportingOwnerRelationship></reportingOwner><nonDerivativeTable><nonDerivativeTransaction><securityTitle><value>Suffolk Bancorp Common Stock</value></securityTitle><transactionDate><value>2003-12-01</value></transactionDate><transactionCoding><transactionFormType>5</transactionFormType><transactionCode>G</transactionCode><equitySwapInvolved>0</equitySwapInvolved></transactionCoding><transactionAmounts><transactionShares><value>600</value></transactionShares><transactionPricePerShare><footnoteId id="F1"/></transactionPricePerShare><transactionAcquiredDisposedCode><value>D</value></transactionAcquiredDisposedCode></transactionAmounts><postTransactionAmounts><sharesOwnedFollowingTransaction><value>60948.995</value></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>D</value></directOrIndirectOwnership><natureOfOwnership><value/></natureOfOwnership></ownershipNature></nonDerivativeTransaction><nonDerivativeHolding><securityTitle><value>Suffolk Bancorp Common Stock</value></securityTitle><postTransactionAmounts><sharesOwnedFollowingTransaction><value>25400</value></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>I</value></directOrIndirectOwnership><natureOfOwnership><value>Spouse</value></natureOfOwnership></ownershipNature></nonDerivativeHolding></nonDerivativeTable><footnotes><footnote id="F1">Gifted 600 shares to 4 grandchildren (150 shares each) for Christmas</footnote></footnotes><remarks/><ownerSignature><signatureName>Douglas Ian Shaw Power of Attorney for J. Douglas Stark</signatureName><signatureDate>2003-12-02</signatureDate></ownerSignature></ownershipDocument>

