Exhibit 5.1

Steven M. Przesmicki

(858) 550-6070

przes@cooley.com

August 3, 2009

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 filing by ARENA PHARMACEUTICALS, INC., a Delaware corporation (the “Company”), of a Registration Statement on Form S-3 (the “Registration Statement”) with the Securities and Exchange Commission (the “Commission”), including a related prospectus filed with the Registration Statement (the “Prospectus”), pursuant to which the Company is registering for resale under the Securities Act of 1933, as amended, on behalf of certain selling stockholders, up to 28,000,000 shares of Common Stock (the “Shares”) issuable upon the exercise of warrants held by such selling stockholders (the “Warrants”).

In connection with this opinion, we have examined and relied upon the Registration Statement, the Prospectus, the Warrants, 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 issued and paid for in accordance with the terms of the Warrants, 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.

August 3, 2009

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 Statement.

Sincerely,
Cooley Godward Kronish LLP
By:	/s/    STEVEN M. PRZESMICKI
	Steven M. Przesmicki

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