Exhibit 5.1

Steven M. Przesmicki

T: +1 858 550 6070

przes@cooley.com

March 8, 2012

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 up to 14,414,370 shares of the Company’s common stock, par value $0.0001 (the “Shares”), and the preferred stock purchase rights (the “Rights”) associated with the Shares to be issued pursuant to that certain Rights Agreement (the “Rights Agreement”) dated October 30, 2002 between the Company and Computershare Trust Company, Inc., as Rights Agent (the “Rights Agent”), as amended on December 30, 2003 and November 16, 2006, pursuant to a Registration Statement on Form S-3 (No. 333-166481) (the “Registration Statement”), filed with the Securities and Exchange Commission (the “Commission”) under the Securities Act of 1933, as amended (the “Act”), the prospectus dated November 8, 2011 (the “Base Prospectus”), and the prospectus supplement dated March 8, 2012 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 Statement and Prospectus.

In connection with this opinion, we have examined and relied upon the Registration Statement, 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.

We have also assumed that the Rights Agreement has been duly authorized, executed and delivered by the Rights Agent and that the members of the Board of Directors of the Company have acted in a manner consistent with their fiduciary duties as required under applicable law in adopting the Rights Agreement. As to certain factual matters, we have relied upon a certificate of an officer of the Company and have not independently sought to verify such matters. Our opinion is expressed only with respect to the federal laws of the United States of America and the General Corporation Law of the State of Delaware.

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

Arena Pharmaceuticals, Inc.

March 8, 2012

Page Two

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

We consent to the reference to our firm under the caption “Legal Matters” in the Prospectus included in the Registration Statement and to the filing of this opinion as an exhibit to a current report of the Company on Form 8-K.

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