Exhibit 5.1

425 MARKET STREET SAN FRANCISCO CALIFORNIA 94105-2482   TELEPHONE: 415.268.7000 FACSIMILE: 415.268.7522   WWW.MOFO.COM MORRISON & FOERSTER LLP   NEW YORK, SAN FRANCISCO, LOS ANGELES, PALO ALTO, SAN DIEGO, WASHINGTON, D.C.   DENVER, NORTHERN VIRGINIA, ORANGE COUNTY, SACRAMENTO, WALNUT CREEK, CENTURY CITY   TOKYO, LONDON, BEIJING, SHANGHAI, HONG KONG, SINGAPORE, BRUSSELS

March 1, 2007

Altera Corporation

101 Innovation Drive

San Jose, California 95134

Re: 2005 Equity Incentive Plan and 1987 Equity Stock Purchase Plan

Ladies and Gentlemen:

At your request, we have examined the Registration Statement on Form S-8 to be filed with the Securities and Exchange Commission (the “Commission”) in connection with the registration under the Securities Act of 1933, as amended, of an aggregate of 11,000,000 shares of your common stock, $0.001 par value (the “Common Shares”) which will be issuable from time to time under Altera Corporation’s 2005 Equity Incentive Plan and 1987 Employee Stock Purchase Plan (collectively, the “Plans”).

As your counsel in connection with the Registration Statement, we have examined the proceedings taken by you in connection with the adoption of the Plans and the authorization of the issuance of the Common Shares under the Plans (the “Plan Shares”) and such documents as we have deemed necessary to render this opinion. For the purpose of the opinion rendered below, we have assumed that in connection with the issuance of the Plan Shares under the Plans, Altera Corporation will receive consideration in an amount not less than the aggregate par value of the Plan Shares covered by each such issuance.

Based upon the foregoing, it is our opinion that the Plan Shares, when issued and outstanding pursuant to the terms of the Plans, will be validly issued, fully paid and nonassessable Common Shares.

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

Very truly yours,

/s/ Morrison & Foerster LLP

Morrison & Foerster LLP