<?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>0001074992</rptOwnerCik>
            <rptOwnerName>VIGLIOTTI STEVEN R</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>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>Chief Financial Officer</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Employee stock option (right to buy)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>4.6</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2007-10-02</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>118631</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F1"/>
            </exerciseDate>
            <expirationDate>
                <value>2017-10-02</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>118631</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>118631</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
        <derivativeTransaction>
            <securityTitle>
                <value>Employee stock option (right to buy)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>4.6</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2007-12-11</value>
                <footnoteId id="F2"/>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>181369</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F3"/>
            </exerciseDate>
            <expirationDate>
                <value>2017-10-02</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>181369</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>300000</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">The option vests and becomes exercisable as to 25% of the shares on March 10, 2008, and as to 2.0833% of the shares on the tenth day of each month thereafter through and including March 10, 2011.  This grant was previously reported.</footnote>
        <footnote id="F2">The option grant was approved by a committee of the Board of Directors of NYFIX, Inc. on October 2, 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="F3">The option vests and becomes exercisable as to 25% of the shares on March 10, 2008, and as to 2.0833% of the shares on the tenth day of each month thereafter through and including March 10, 2011.</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
0001074992
VIGLIOTTI STEVEN R
C/O NYFIX, INC.
100 WALL STREET
NEW YORK
NY
10005
0
1
0
0
Chief Financial Officer
Employee stock option (right to buy)
4.6
2007-10-02
4
A
0
118631
0
A
2017-10-02
Common stock
118631
118631
D
Employee stock option (right to buy)
4.6
2007-12-11
4
A
0
181369
0
A
2017-10-02
Common stock
181369
300000
D
The option vests and becomes exercisable as to 25% of the shares on March 10, 2008, and as to 2.0833% of the shares on the tenth day of each month thereafter through and including March 10, 2011.  This grant was previously reported.
The option grant was approved by a committee of the Board of Directors of NYFIX, Inc. on October 2, 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.
The option vests and becomes exercisable as to 25% of the shares on March 10, 2008, and as to 2.0833% of the shares on the tenth day of each month thereafter through and including March 10, 2011.
Brian Bellardo, by Power of Attorney
2007-12-13