<SUBMISSION>
<ACCESSION-NUMBER>0001048695-05-000075
<TYPE>4
<PUBLIC-DOCUMENT-COUNT>1
<PERIOD>20051101
<FILING-DATE>20051103
<DATE-OF-FILING-DATE-CHANGE>20051103
<ISSUER>
<COMPANY-DATA>
<CONFORMED-NAME>F5 NETWORKS INC
<CIK>0001048695
<ASSIGNED-SIC>3576
<IRS-NUMBER>911714307
<STATE-OF-INCORPORATION>WA
<FISCAL-YEAR-END>0930
</COMPANY-DATA>
<BUSINESS-ADDRESS>
<STREET1>401 ELLIOT AVE WEST
<STREET2>STE 500
<CITY>SEATTLE
<STATE>WA
<ZIP>98119
<PHONE>2062725555
</BUSINESS-ADDRESS>
<MAIL-ADDRESS>
<STREET1>401 ELLIOT AVE WEST
<STREET2>STE 500
<CITY>SEATTLE
<STATE>WA
<ZIP>98119
</MAIL-ADDRESS>
<FORMER-COMPANY>
<FORMER-CONFORMED-NAME>F5 LABS INC
<DATE-CHANGED>19990305
</FORMER-COMPANY>
</ISSUER>
<REPORTING-OWNER>
<OWNER-DATA>
<CONFORMED-NAME>Reinland Andrew
<CIK>0001342165
</OWNER-DATA>
<FILING-VALUES>
<FORM-TYPE>4
<ACT>34
<FILE-NUMBER>000-26041
<FILM-NUMBER>051177537
</FILING-VALUES>
<BUSINESS-ADDRESS>
<PHONE>206-272-5555
</BUSINESS-ADDRESS>
<MAIL-ADDRESS>
<STREET1>C/O F5 NETWORKS, INC.
<STREET2>401 ELLIOTT AVE. WEST
<CITY>SEATTLE
<STATE>WA
<ZIP>98119
</MAIL-ADDRESS>
</REPORTING-OWNER>
<DOCUMENT>
<TYPE>4
<SEQUENCE>1
<FILENAME>primary_doc.xml
<DESCRIPTION>PRIMARY DOCUMENT
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0202</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2005-11-01</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001048695</issuerCik>
        <issuerName>F5 NETWORKS INC</issuerName>
        <issuerTradingSymbol>FFIV</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001342165</rptOwnerCik>
            <rptOwnerName>Reinland Andrew</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O F5 NETWORKS, INC.</rptOwnerStreet1>
            <rptOwnerStreet2>401 ELLIOTT AVE. WEST</rptOwnerStreet2>
            <rptOwnerCity>SEATTLE</rptOwnerCity>
            <rptOwnerState>WA</rptOwnerState>
            <rptOwnerZipCode>98119</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>Sr. VP, Chief Finance Officer</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Restricted Stock Units</value>
                <footnoteId id="F1"/>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>0</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2005-11-01</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>5000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <value>2006-01-01</value>
                <footnoteId id="F2"/>
            </exerciseDate>
            <expirationDate>
                <value>2015-11-01</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>5000</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>5000</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">Right to receive shares of F5 Networks, Inc. common stock on the vest date.</footnote>
        <footnote id="F2">1/8 of these RSUs will vest on January 1, 2006, with the remainder vesting 1/8 on the first day of the subsequent seven quarters.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Andrew Reinland</signatureName>
        <signatureDate>2005-11-03</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SUBMISSION>
