1
                                                                     EXHIBIT 5.1

   [Letterhead of Wilson Sonsini Goodrich & Rosati, Professional Corporation]


                                  May 15, 2001


Simplex Solutions, Inc.
521 Almanor Avenue
Sunnyvale, California 94085

     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 May 15, 2001 (the
"Registration Statement"), in connection with the registration under the
Securities Act of 1933, as amended (the "Securities Act"), of an aggregate of
500,000 shares of common stock, par value $0.001, of Simplex Solutions, Inc.
(the "Shares"), reserved for issuance under the 2001 Employee Stock Purchase
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 sale and issuance 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 and validly 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. In giving such
consent, we do not consider that we are "experts" within the meaning of such
term as used in the Securities Act, or the rules and regulations of the
Securities and Exchange Commission issued thereunder, with respect to any part
of the Registration Statement, including this opinion as an exhibit or
otherwise.

                                   Very truly yours,

                                   WILSON SONSINI GOODRICH & ROSATI
                                   Professional Corporation

                                   /s/ Wilson Sonsini Goodrich & Rosati