<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0202</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2007-12-11</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000099047</issuerCik>
        <issuerName>NYFIX INC</issuerName>
        <issuerTradingSymbol>nyfx</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001186007</rptOwnerCik>
            <rptOwnerName>GORMAN LON</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O NYFIX, INC.</rptOwnerStreet1>
            <rptOwnerStreet2>100 WALL STREET</rptOwnerStreet2>
            <rptOwnerCity>NEW YORK</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10005</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Stock option (right to buy)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>4.3</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2007-12-11</value>
                <footnoteId id="F1"/>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>350000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F2"/>
            </exerciseDate>
            <expirationDate>
                <value>2017-10-23</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>350000</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>350000</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">The option grant was approved by the Board of Directors of NYFIX, Inc. on October 23, 2007 subject to stockholder approval of the equity compensation plan under which the option was granted.  NYFIX stockholders approved the plan on December 11, 2007.</footnote>
        <footnote id="F2">Subject to the condition noted in footnote 1, which was satisfied on December 11, 2007, the option vests and becomes exercisable as to 27.0834% of the shares on October 23, 2007, and as to 2.0833% of the shares on the fourth day of each month thereafter through and including September 4, 2010.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>Brian Bellardo, by Power of Attorney</signatureName>
        <signatureDate>2007-12-13</signatureDate>
    </ownerSignature>
</ownershipDocument>
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X0202
4
2007-12-11
0
0000099047
NYFIX INC
nyfx
0001186007
GORMAN LON
C/O NYFIX, INC.
100 WALL STREET
NEW YORK
NY
10005
1
0
0
0
Stock option (right to buy)
4.3
2007-12-11
4
A
0
350000
0
A
2017-10-23
Common stock
350000
350000
D
The option grant was approved by the Board of Directors of NYFIX, Inc. on October 23, 2007 subject to stockholder approval of the equity compensation plan under which the option was granted.  NYFIX stockholders approved the plan on December 11, 2007.
Subject to the condition noted in footnote 1, which was satisfied on December 11, 2007, the option vests and becomes exercisable as to 27.0834% of the shares on October 23, 2007, and as to 2.0833% of the shares on the fourth day of each month thereafter through and including September 4, 2010.
Brian Bellardo, by Power of Attorney
2007-12-13