-----BEGIN PRIVACY-ENHANCED MESSAGE-----
Proc-Type: 2001,MIC-CLEAR
Originator-Name: webmaster@www.sec.gov
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MIC-Info: RSA-MD5,RSA,
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<SEC-DOCUMENT>0001246151-05-000051.txt : 20050802
<SEC-HEADER>0001246151-05-000051.hdr.sgml : 20050802
<ACCEPTANCE-DATETIME>20050802215014
ACCESSION NUMBER:		0001246151-05-000051
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20050715
FILED AS OF DATE:		20050802
DATE AS OF CHANGE:		20050802

ISSUER:		

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			SIEBEL SYSTEMS INC
		CENTRAL INDEX KEY:			0001006835
		STANDARD INDUSTRIAL CLASSIFICATION:	SERVICES-PREPACKAGED SOFTWARE [7372]
		IRS NUMBER:				943187233
		STATE OF INCORPORATION:			DE
		FISCAL YEAR END:			1231

	BUSINESS ADDRESS:	
		STREET 1:		2207 BRIDGEPOINTE PARKWAY
		CITY:			SAN MATEO
		STATE:			CA
		ZIP:			94404
		BUSINESS PHONE:		6504775000

	MAIL ADDRESS:	
		STREET 1:		2207 BRIDGEPOINTE PARKWAY
		CITY:			SAN MATEO
		STATE:			CA
		ZIP:			94404

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			RECHAN LESLIE J
		CENTRAL INDEX KEY:			0001333062

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	000-20725
		FILM NUMBER:		05993684

	BUSINESS ADDRESS:	
		BUSINESS PHONE:		(650) 477-5000

	MAIL ADDRESS:	
		STREET 1:		2207 BRIDGEPOINTE PARKWAY
		CITY:			SAN MATEO
		STATE:			CA
		ZIP:			94404
</SEC-HEADER>
<DOCUMENT>
<TYPE>4
<SEQUENCE>1
<FILENAME>c0001333062d20050802f4f.xml
<DESCRIPTION>PRIMARY DOCUMENT
<TEXT>
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<ownershipDocument>

    <schemaVersion>X0202</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2005-07-15</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001006835</issuerCik>
        <issuerName>SIEBEL SYSTEMS INC</issuerName>
        <issuerTradingSymbol>SEBL</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001333062</rptOwnerCik>
            <rptOwnerName>RECHAN LESLIE J</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O SIEBEL SYSTEMS, INC.</rptOwnerStreet1>
            <rptOwnerStreet2>2207 BRIDGEPOINTE PARKWAY</rptOwnerStreet2>
            <rptOwnerCity>SAN MATEO</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>94404</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>SVP &amp; GM, Americas Sales</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
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                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2005-07-15</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>5</transactionFormType>
                <transactionCode>P</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>3.4126</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>8.7</value>
                    <footnoteId id="F2"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
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            <postTransactionAmounts>
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            <transactionTimeliness>
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            <transactionAmounts>
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                <transactionPricePerShare>
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                </transactionPricePerShare>
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        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
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            <transactionDate>
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            <transactionTimeliness>
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            <transactionAmounts>
                <transactionShares>
                    <value>4290</value>
                    <footnoteId id="F4"/>
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                <transactionPricePerShare>
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                <transactionAcquiredDisposedCode>
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            <securityTitle>
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            <transactionDate>
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                <transactionFormType>4</transactionFormType>
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            <transactionTimeliness>
                <value></value>
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            <transactionAmounts>
                <transactionShares>
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                <transactionPricePerShare>
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                <transactionAcquiredDisposedCode>
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            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
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        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
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            <conversionOrExercisePrice>
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                </transactionShares>
                <transactionPricePerShare>
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                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <value>2006-07-29</value>
                <footnoteId id="F5"/>
            </exerciseDate>
            <expirationDate>
                <value>2011-07-29</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>200000</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>200000</value>
                </sharesOwnedFollowingTransaction>
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            <ownershipNature>
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                    <value>D</value>
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        </derivativeTransaction>
        <derivativeTransaction>
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            <transactionDate>
                <value>2005-07-29</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
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                <transactionCode>J</transactionCode>
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            <transactionAmounts>
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                    <value>10000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <value>2005-07-29</value>
            </exerciseDate>
            <expirationDate>
                <value>2005-11-29</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>10000</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
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                <sharesOwnedFollowingTransaction>
                    <value>10000</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
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        </derivativeTransaction>
        <derivativeTransaction>
            <securityTitle>
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            <transactionDate>
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            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
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                <transactionFormType>4</transactionFormType>
                <transactionCode>J</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
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            <transactionAmounts>
                <transactionShares>
                    <value>12000</value>
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                <transactionPricePerShare>
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                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
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            <exerciseDate>
                <value>2005-07-29</value>
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            <expirationDate>
                <value>2009-07-29</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
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                <underlyingSecurityShares>
                    <value>12000</value>
                </underlyingSecurityShares>
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                <sharesOwnedFollowingTransaction>
                    <value>48000</value>
                </sharesOwnedFollowingTransaction>
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            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
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        </derivativeTransaction>
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    <footnotes>
        <footnote id="F1">On June 8, 2005, the Issuer declared a cash dividend of $0.025 per share, payable on July 15, 2005 to all holders of record at the close of business on June 30, 2005.  The dividend paid on shares held by the Reporting Person in an employee stock purchase plan brokerage account was reinvested through an open market purchase.</footnote>
        <footnote id="F2">Reflects the open market purchase price on July 15, 2005, the payment date for the dividend.</footnote>
        <footnote id="F3">Ending balance of securities beneficially owned in lines 2 through 5 of Table I includes 1,242 shares purchased under the Siebel Systems, Inc. Employee Stock Purchase Plan on July 29, 2005.</footnote>
        <footnote id="F4">Shares were withheld from issuance to satisfy tax obligation.</footnote>
        <footnote id="F5">This Restricted Stock Unit vests 20% per year beginning on the first anniversary of the date of grant.  The Restricted Stock Unit does not have an expiration date.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>Leslie J Rechan</signatureName>
        <signatureDate>2005-08-02</signatureDate>
    </ownerSignature>
</ownershipDocument>
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</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
-----END PRIVACY-ENHANCED MESSAGE-----