<?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>0001252878</rptOwnerCik>
            <rptOwnerName>EDELSTEIN P HOWARD</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>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>President and CEO</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common stock</value>
                <footnoteId id="F1"/>
            </securityTitle>
            <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>200000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
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                <sharesOwnedFollowingTransaction>
                    <value>200000</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <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>752570</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>752570</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>752570</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
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        </derivativeTransaction>
        <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>675069</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F4"/>
            </exerciseDate>
            <expirationDate>
                <value>2017-10-02</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>675069</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>1427639</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="F5"/>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>753786</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F6"/>
            </exerciseDate>
            <expirationDate>
                <value>2017-10-02</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>753786</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>2181425</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">Award is granted pursuant to a Restricted Stock Unit Agreement that provides for the issuance of 200,000 shares of Common Stock that vest on December 15, 2007.</footnote>
        <footnote id="F2">The restricted stock unit 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 restricted stock units were granted.  NYFIX stockholders approved the plan on December 11, 2007.</footnote>
        <footnote id="F3">The option is fully vested and exercisable as of October 2, 2007.  This grant was previously reported.</footnote>
        <footnote id="F4">The option grant vests and becomes exercisable as to 2.7778% of the shares on October 4, 2007 and on the fourth day of each month thereafter through and including September 4, 2010.  This grant was previously reported.</footnote>
        <footnote id="F5">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="F6">Subject to the condition noted in footnote 5, which was satisfied on December 11, 2007, the option grant vests and becomes exercisable as to 2.7778% of the shares on October 4, 2007, and 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
0001252878
EDELSTEIN P HOWARD
C/O NYFIX, INC.
100 WALL STREET
NEW YORK
NY
10005
1
1
0
0
President and CEO
Common stock
2007-12-11
4
A
0
200000
0
A
200000
D
Employee stock option (right to buy)
4.6
2007-10-02
4
A
0
752570
0
A
2017-10-02
Common stock
752570
752570
D
Employee stock option (right to buy)
4.6
2007-10-02
4
A
0
675069
0
A
2017-10-02
Common stock
675069
1427639
D
Employee stock option (right to buy)
4.6
2007-12-11
4
A
0
753786
0
A
2017-10-02
Common stock
753786
2181425
D
Award is granted pursuant to a Restricted Stock Unit Agreement that provides for the issuance of 200,000 shares of Common Stock that vest on December 15, 2007.
The restricted stock unit 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 restricted stock units were granted.  NYFIX stockholders approved the plan on December 11, 2007.
The option is fully vested and exercisable as of October 2, 2007.  This grant was previously reported.
The option grant vests and becomes exercisable as to 2.7778% of the shares on October 4, 2007 and on the fourth day of each month thereafter through and including September 4, 2010.  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.
Subject to the condition noted in footnote 5, which was satisfied on December 11, 2007, the option grant vests and becomes exercisable as to 2.7778% of the shares on October 4, 2007, and on the fourth day of each month thereafter through and including September 4, 2010.
Brian Bellardo, by Power of Attorney
2007-12-13