-----BEGIN PRIVACY-ENHANCED MESSAGE-----
Proc-Type: 2001,MIC-CLEAR
Originator-Name: webmaster@www.sec.gov
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 TWSM7vrzLADbmYQaionwg5sDW3P6oaM5D3tdezXMm7z1T+B+twIDAQAB
MIC-Info: RSA-MD5,RSA,
 S9vpKehwlrM7RyWc5IxFfytZixDRZINVNW19HfCFF/HFNe7jR3SF8q+T2r7IwbG4
 FQHat5YIunEVZ/FkyFE2mQ==

<SEC-DOCUMENT>0001246151-05-000025.txt : 20050202
<SEC-HEADER>0001246151-05-000025.hdr.sgml : 20050202
<ACCEPTANCE-DATETIME>20050202205753
ACCESSION NUMBER:		0001246151-05-000025
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20050131
FILED AS OF DATE:		20050202
DATE AS OF CHANGE:		20050202

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			WESTON NEIL M
		CENTRAL INDEX KEY:			0001275829

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	000-20725
		FILM NUMBER:		05571172

	BUSINESS ADDRESS:	
		STREET 1:		SIEBEL SYSTEMS INC
		STREET 2:		THE GLANTY, SIEBEL CENTRE
		CITY:			EGHAM, SURREY, ENGLAND
		STATE:			X0
		ZIP:			TW209DW
		BUSINESS PHONE:		011441784494826

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
</SEC-HEADER>
<DOCUMENT>
<TYPE>4
<SEQUENCE>1
<FILENAME>c0001275829d20050202f4f.xml
<DESCRIPTION>PRIMARY DOCUMENT
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0202</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2005-01-31</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001275829</rptOwnerCik>
            <rptOwnerName>WESTON NEIL M</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>TANYARD BARNS, WOODHILL LANE</rptOwnerStreet1>
            <rptOwnerStreet2>SHAMLEY GREEN</rptOwnerStreet2>
            <rptOwnerCity>SURREY</rptOwnerCity>
            <rptOwnerState></rptOwnerState>
            <rptOwnerZipCode>GU50SP</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>President, EMEA Sales</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable></nonDerivativeTable>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Restricted Stock Unit</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>0</value>
                <footnoteId id="F1"/>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2005-01-31</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>30000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <value>2009-01-31</value>
                <footnoteId id="F2"/>
            </exerciseDate>
            <expirationDate>
                <value>2009-01-31</value>
                <footnoteId id="F2"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>30000</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>30000</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">Shares will be issued in consideration of past services rendered.</footnote>
        <footnote id="F2">100% of the Restricted Stock Unit will vest on January 31, 2009; provided, however, that if certain corporate performance targets are achieved, a portion of such units will accelerate and vest earlier.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>Kimberley Henningsen, Power of Attorney</signatureName>
        <signatureDate>2005-02-02</signatureDate>
    </ownerSignature>
</ownershipDocument>

</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
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