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

    <documentType>4</documentType><periodOfReport>2003-10-16</periodOfReport>
    <issuer>
        <issuerCik>0000754673</issuerCik>
        <issuerName>SUFFOLK BANCORP</issuerName>
        <issuerTradingSymbol>SUBK</issuerTradingSymbol></issuer><reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001235034</rptOwnerCik>
            <rptOwnerName>KOHLMANN THOMAS S</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>2003-10-16</value></transactionDate><transactionCoding><transactionFormType>5</transactionFormType><transactionCode>F</transactionCode><equitySwapInvolved>0</equitySwapInvolved></transactionCoding><transactionAmounts><transactionShares><value>2569</value></transactionShares><transactionPricePerShare><value>37.25</value></transactionPricePerShare><transactionAcquiredDisposedCode><value>D</value></transactionAcquiredDisposedCode></transactionAmounts><postTransactionAmounts><sharesOwnedFollowingTransaction><value>10204.023</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>2003-10-16</value></transactionDate><transactionCoding><transactionFormType>5</transactionFormType><transactionCode>M</transactionCode><equitySwapInvolved>0</equitySwapInvolved></transactionCoding><transactionAmounts><transactionShares><value>5200</value></transactionShares><transactionPricePerShare><value>9.75</value></transactionPricePerShare><transactionAcquiredDisposedCode><value>A</value></transactionAcquiredDisposedCode></transactionAmounts><postTransactionAmounts><sharesOwnedFollowingTransaction><value>15404.023</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>2003-10-16</value></transactionDate><transactionCoding><transactionFormType>5</transactionFormType><transactionCode>M</transactionCode><equitySwapInvolved>0</equitySwapInvolved></transactionCoding><transactionAmounts><transactionShares><value>3000</value></transactionShares><transactionPricePerShare><value>15.00</value></transactionPricePerShare><transactionAcquiredDisposedCode><value>A</value></transactionAcquiredDisposedCode></transactionAmounts><postTransactionAmounts><sharesOwnedFollowingTransaction><value>18404.023</value></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>D</value></directOrIndirectOwnership><natureOfOwnership><value/></natureOfOwnership></ownershipNature></nonDerivativeTransaction></nonDerivativeTable><derivativeTable><derivativeTransaction><securityTitle><value>Suffolk Bancorp Option</value></securityTitle><conversionOrExercisePrice><value>9.75</value></conversionOrExercisePrice><transactionDate><value>2003-10-16</value></transactionDate><transactionCoding><transactionFormType>5</transactionFormType><transactionCode>M</transactionCode><equitySwapInvolved>0</equitySwapInvolved></transactionCoding><transactionAmounts><transactionShares><value>5200</value></transactionShares><transactionPricePerShare><footnoteId id="F1"/></transactionPricePerShare><transactionAcquiredDisposedCode><value>D</value></transactionAcquiredDisposedCode></transactionAmounts><exerciseDate><value>1997-12-23</value></exerciseDate><expirationDate><value>2006-12-23</value></expirationDate><underlyingSecurity><underlyingSecurityTitle><value>Suffolk Bancorp Common Stock</value></underlyingSecurityTitle><underlyingSecurityShares><value>5200</value></underlyingSecurityShares></underlyingSecurity><postTransactionAmounts><sharesOwnedFollowingTransaction><value>5200</value></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>D</value></directOrIndirectOwnership><natureOfOwnership><value/></natureOfOwnership></ownershipNature></derivativeTransaction><derivativeTransaction><securityTitle><value>Suffolk Bancorp Option</value></securityTitle><conversionOrExercisePrice><value>15.00</value></conversionOrExercisePrice><transactionDate><value>2003-10-16</value></transactionDate><transactionCoding><transactionFormType>5</transactionFormType><transactionCode>M</transactionCode><equitySwapInvolved>0</equitySwapInvolved></transactionCoding><transactionAmounts><transactionShares><value>3000</value></transactionShares><transactionPricePerShare><footnoteId id="F2"/></transactionPricePerShare><transactionAcquiredDisposedCode><value>D</value></transactionAcquiredDisposedCode></transactionAmounts><exerciseDate><value>1998-12-22</value></exerciseDate><expirationDate><value>2007-12-22</value></expirationDate><underlyingSecurity><underlyingSecurityTitle><value>Suffolk Bancorp Common Stock</value></underlyingSecurityTitle><underlyingSecurityShares><value>3000</value></underlyingSecurityShares></underlyingSecurity><postTransactionAmounts><sharesOwnedFollowingTransaction><value>3000</value></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>D</value></directOrIndirectOwnership><natureOfOwnership><value/></natureOfOwnership></ownershipNature></derivativeTransaction><derivativeHolding><securityTitle><value>Suffolk Bancorp Option</value></securityTitle><conversionOrExercisePrice><value>13.13</value></conversionOrExercisePrice><exerciseDate><value>2000-01-22</value></exerciseDate><expirationDate><value>2009-01-22</value></expirationDate><underlyingSecurity><underlyingSecurityTitle><value>Suffolk Bancorp Common Stock</value></underlyingSecurityTitle><underlyingSecurityShares><value>3000</value></underlyingSecurityShares></underlyingSecurity><postTransactionAmounts><sharesOwnedFollowingTransaction><value>3000</value></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>D</value></directOrIndirectOwnership><natureOfOwnership><value/></natureOfOwnership></ownershipNature></derivativeHolding><derivativeHolding><securityTitle><value>Suffolk Bancorp Option</value></securityTitle><conversionOrExercisePrice><value>13.13</value></conversionOrExercisePrice><exerciseDate><value>2001-01-18</value></exerciseDate><expirationDate><value>2010-01-18</value></expirationDate><underlyingSecurity><underlyingSecurityTitle><value>Suffolk Bancorp Common Stock</value></underlyingSecurityTitle><underlyingSecurityShares><value>10000</value></underlyingSecurityShares></underlyingSecurity><postTransactionAmounts><sharesOwnedFollowingTransaction><value>10000</value></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>D</value></directOrIndirectOwnership><natureOfOwnership><value/></natureOfOwnership></ownershipNature></derivativeHolding><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>12000</value></underlyingSecurityShares></underlyingSecurity><postTransactionAmounts><sharesOwnedFollowingTransaction><value>12000</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>5000</value></underlyingSecurityShares></underlyingSecurity><postTransactionAmounts><sharesOwnedFollowingTransaction><value>5000</value></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>D</value></directOrIndirectOwnership><natureOfOwnership><value/></natureOfOwnership></ownershipNature></derivativeHolding></derivativeTable><footnotes><footnote id="F1">Payment is 1,361 shares of Suffolk Bancorp Common Stock based on the market price of $37.25</footnote><footnote id="F2">Payment is 1,208 shares of Suffolk Bancorp Common Stock based on the market price of $37.25</footnote></footnotes><remarks/><ownerSignature><signatureName>Thomas S. Kohlmann</signatureName><signatureDate>2003-10-17</signatureDate></ownerSignature></ownershipDocument>

