Exhibit 5.1

February 9, 2001

Siebel Systems, Inc.
2207 Bridgepointe Parkway
San Mateo, California 94404

Re:  Form S-8

Ladies and Gentlemen:

You have requested our opinion with respect to certain matters in connection
with the filing by Siebel Systems, Inc. (the "Company") of a Registration
Statement on Form S-8 (the "Registration Statement") with the Securities and
Exchange Commission covering the offering of up to an aggregate of 49,895 shares
of the Company's Common Stock, $.001 par value (the "Shares"), issuable upon the
exercise of Sales.com, Inc. ("Sales.com") stock options originally granted under
the Sales.com, Inc. 1999 Equity Incentive Plan (the "Sales.com Plan"),
nonstatutory stock option of Sales.com stock granted outside the Sales.com Plan
and stock options of SalesRepsOnline, Inc. ("SRO") stock originally granted
under the SalesRepsOnline, Inc. 2000 Stock Option/Stock Issuance Plan (the "SRO
Plan") and assumed by the Company pursuant to the Agreement and Plan of Merger
and Reorganization, dated as of November 22, 2000, among the Company, SSI
Acquisition Corp. and Sales.com (the "Reorganization Agreement").

In connection with this opinion, we have examined the Reorganization Agreement,
the Registration Statement and related Prospectus, the Company's Amended and
Restated Certificate of Incorporation and Bylaws, each as amended to date, and
such other documents, records, certificates, memoranda and other instruments as
we deem necessary as a basis for this opinion. We have assumed the genuineness
and authenticity of all documents submitted to us as originals, the conformity
to originals of all documents submitted to us as copies thereof and the due
execution and delivery of all documents where due execution and delivery are a
prerequisite to the effectiveness thereof.

Our opinion is expressed only with respect to the federal laws of the United
States of America, the General Corporation Law of the State of Delaware and the
laws of the State of California. We express no opinion as to whether the laws of
any particular jurisdiction other than those identified above are applicable to
the subject matter hereof.

On the basis of the foregoing, and in reliance thereon, we are of the opinion
that the Shares, when sold and issued in accordance with the Sales.com Plan, the
SRO Plan and pursuant to the option granted outside the Sales.com Plan, the
Registration Statement and related Prospectus, will be validly issued, fully
paid, and nonassessable (except as to shares issued pursuant to certain deferred
payment arrangements, if any, which will be fully paid and nonassessable when
such deferred payments are made in full).

We consent to the filing of this opinion as an exhibit to the Registration
Statement.

Very truly yours,

Cooley Godward LLP

By: /s/ Vincent P. Pangrazio
    ------------------------
    Vincent P. Pangrazio