Exhibit 5.1

OPINION OF WILSON SONSINI GOODRICH & ROSATI, PROFESSIONAL CORPORATION

November 25, 2009

Fortinet, Inc.

1090 Kifer Road

Sunnyvale, California 94086

Re: Registration Statement on Form S-8

Ladies and Gentlemen:

We have examined the Registration Statement on Form S-8 (the “Registration Statement”) to be filed by Fortinet, Inc., a Delaware corporation, with the Securities and Exchange Commission on or about the date hereof, in connection with the registration under the Securities Act of 1933, as amended, of an aggregate of 26,564,163 shares of your common stock, par value $0.001 per share (the “Shares”), reserved for issuance pursuant to the 2009 Equity Incentive Plan, the 2008 Stock Plan and the Amended and Restated 2000 Stock Plan (the “Plans”). As your legal counsel, we have reviewed the actions proposed to be taken by you in connection with the issuance and sale of the Shares to be issued under the Plan.

It is our opinion that the Shares, when issued and sold in the manner referred to in the Plans and pursuant to the agreements that accompany the Plans, will be 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 amendments thereto.

Very truly yours,

WILSON SONSINI GOODRICH & ROSATI,

Professional Corporation

/s/ Wilson Sonsini Goodrich & Rosati, P.C.