Exhibit 5.1

Steven M. Przesmicki

(858) 550-6070

przes@cooley.com

March 8, 2010

Arena Pharmaceuticals, Inc.

6166 Nancy Ridge Drive

San Diego, CA 92121

Ladies and Gentlemen:

You have requested our opinion with respect to certain matters in connection with the offering by Arena Pharmaceuticals, Inc., a Delaware corporation (the “Company”), of 8,278,432 shares of the Company’s common stock, par value $0.0001 (the “Shares”), pursuant to a Registration Statement on Form S-3 (No. 333-155660) (the “Initial Registration Statement”), filed with the Securities and Exchange Commission (the “Commission”) under the Securities Act of 1933, as amended (the “Act”), a Registration Statement on Form S-3 (No. 333-165348) filed with the Commission pursuant to Rule 462(b) of the Act (the “462(b) Registration Statement” and collectively with the Initial Registration Statement, the “Registration Statements”), the prospectus dated March 23, 2009 (the “Base Prospectus”), and the prospectus supplement dated March 8, 2010 relating to the Shares, filed or to be filed with the Commission pursuant to Rule 424(b) of the Rules and Regulations of the Act (the “Prospectus Supplement”). (The Base Prospectus and Prospectus Supplement are collectively referred to as the “Prospectus.”) All of the Shares are to be sold by the Company as described in the Registration Statements and Prospectus.

In connection with this opinion, we have examined and relied upon the Registration Statements, the Prospectus, the Company’s Fifth Amended and Restated Certificate of Incorporation, as amended, its Amended and Restated Bylaws, and the originals or copies certified to our satisfaction of such records, documents, certificates, memoranda and other instruments as in our judgment are necessary or appropriate to enable us to render the opinion expressed below. We have assumed the genuineness and authenticity of all documents submitted to us as originals, and the conformity to originals of all documents submitted to us as copies thereof and the due execution and delivery of all documents where due execution and delivery are a prerequisite to the effectiveness thereof.

On the basis of the foregoing, and in reliance thereon, we are of the opinion that the Shares, when sold in accordance with the Registration Statements and Prospectus, will be validly issued, fully paid and nonassessable.

4401 EASTGATE MALL, SAN DIEGO, CA 92121    T: (858) 550-6000    F: (858) 550-6420    WWW.COOLEY.COM

Arena Pharmaceuticals, Inc.

March 8, 2010

Page Two

We consent to the reference to our firm under the caption “Legal Matters” in the Prospectus and to the filing of this opinion as an exhibit to the Registration Statements.

Very truly yours,

Cooley Godward Kronish LLP

4401 EASTGATE MALL, SAN DIEGO, CA 92121    T: (858) 550-6000    F: (858) 550-6420    WWW.COOLEY.COM