Exhibit 99.1

Certification Pursuant to
Section 906 of the Sarbanes-Oxley Act of 2002

Pursuant to Section 906 of the Sarbanes-Oxley Act of 2002 (18 U.S.C. § 1350, as adopted), Thomas M. Siebel, Chairman and CEO of Siebel Systems, Inc. (the "Company"), and Kenneth A. Goldman, Senior Vice President, Finance and Administration and Chief Financial Officer of the Company, each hereby certify that, to the best of their knowledge:

1. The Company's Quarterly Report on Form 10-Q for the period ended March 31, 2003, to which this Certification is attached as Exhibit 99.1 (the "Periodic Report"), fully complies with the requirements of section 13(a) or section 15(d) of the Securities Exchange Act of 1934, and

2. The information contained in the Periodic Report fairly presents, in all material respects, the financial condition and results of operations of the Company.

In Witness Whereof, the undersigned have set their hands hereto as of the 15^th day of May, 2003.

/s/ Thomas M. Siebel
Thomas M. Siebel
Chairman and CEO
(Principal Executive Officer)

/s/ Kenneth A. Goldman
Kenneth A. Goldman
Senior Vice President, Finance and
Administration and Chief Financial Officer
(Principal Financial Officer)

This certification "accompanies" the Periodic Report to which it relates, is not deemed filed with the Securities and Exchange Commission and is not to be incorporated by reference into any filing of the Company under the Securities Act of 1933, as amended, or the Securities Exchange Act of 1934, as amended (whether made before or after the date of the Period Report), irrespective of any general incorporation language contained in such filing.

A signed original of this written statement required by Section 906 has been provided to the Company and will be retained by the Company and furnished to the Securities and Exchange Commission or its staff upon request.