EXHIBIT 5.1

April 26, 2005

TIBCO Software Inc.

3303 Hillview Avenue

Palo Alto, CA 94304

Re: Registration Statement on Form S-8

Ladies and Gentlemen:

We have examined the Registration Statement on Form S-8 to be filed by you with the Securities and Exchange Commission on or about April 26, 2005 (the “Registration Statement”), in connection with the registration under the Securities Act of 1933, as amended, of 10,691,808 shares of Common Stock, par value $0.001 (the “Shares”), which are reserved for issuance pursuant to your 1996 Stock Option Plan (the “Plan”). As your legal counsel, we have examined the proceedings taken and are familiar with the proceedings proposed to be taken by you in connection with the issuance and sale of the Shares under the Plan.

It is our opinion that the Shares will be, when issued and sold in the manner referred to in the Plan, legally issued, fully paid and nonassessable.

We consent to the use of this opinion as an exhibit to the Registration Statement and further consent to the use of our name wherever appearing in the Registration Statement and any subsequent amendment thereto.

Very truly yours,

WILSON SONSINI GOODRICH & ROSATI

Professional Corporation

/S/ WILSON SONSINI GOODRICH & ROSATI, P.C.