EXHIBIT 5.1

[Cooley Godward LLP Letterhead]

July 10, 2003

Rigel Pharmaceuticals, Inc.
1180 Veterans Blvd.
South San Francisco, CA 94080

Ladies and Gentlemen:

You have requested our opinion with respect to certain matters in connection with the filing by Rigel Pharmaceuticals, Inc. (the "Company") of a Registration Statement on Form S-3 (the "Registration Statement") with the Securities and Exchange Commission covering the offering for resale of up to 7,986,110 shares (the "Shares") of the Company's common stock, $0.001 par value (the "Common Stock") and 1,597,221 shares of Common Stock (the "Warrant Shares") issuable upon exercise of warrants (the "Warrants") issued pursuant to the Common Stock and Warrant Purchase Agreement, dated as of April 29, 2003 (the "Purchase Agreement").

In connection with this opinion, we have examined and relied upon the Registration Statement and related Prospectus included therein, your Amended and Restated Certificate of Incorporation and Bylaws, a certificate executed by an officer of the Company, to the effect that the consideration for the Shares and the Warrants was received by the Company in accordance with the terms of the Purchase Agreement, and such other documents, records, certificates, memoranda and other instruments as we deem necessary as a basis for this opinion. We have assumed the genuineness and authenticity of all documents submitted to us as originals, 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 have been validly issued and are fully paid and nonassessable, and that the Warrant Shares, when issued by the Company upon exercise of the Warrants in accordance with the terms of the Warrants, will be validly issued, fully paid and nonassessable.

We consent to the filing of this opinion as an exhibit to the Registration Statement and to the reference to our firm under the caption "Legal Matters" in the Prospectus included in the Registration Statement.

Very truly yours,

QuickLinks

EXHIBIT 5.1