Cerus (CERS) S-8Registration of securities for employees

Exhibit 5.1

Chadwick L. Mills

T: +1 650 843 5654

cmills@cooley.com

August 1, 2024

Cerus Corporation

1220 Concord Ave., Suite 600

Concord, California 94520

Ladies and Gentlemen:

We have acted as counsel to Cerus Corporation, a Delaware corporation (the “Company”), in connection with the filing of a registration statement on Form S-8 (the “Registration Statement”) with the Securities and Exchange Commission covering the offering of up to (i) 37,256,220 shares of the Company’s Common Stock, $0.001 par value per share (the “Common Stock”), to be issued pursuant to the Company’s 2024 Equity Incentive Plan (the “2024 Plan”) and (ii) 2,000,000 shares of the Company’s Common Stock to be issued pursuant to the Company’s Amended and Restated Employee Stock Purchase Plan (together with the 2024 Plan, the “Plans”).

In connection with this opinion, we have examined and relied upon (a) the Registration Statement and related prospectus, (b) the Plans, (c) the Company’s certificate of incorporation and bylaws, as currently in effect, and (d) such other 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 of all signatures, the authenticity of all documents submitted to us as originals and the conformity to originals of all documents submitted to us as copies, the accuracy, completeness and authenticity of certificates of public officials, and the due authorization, execution and delivery, by all persons other than by the Company, of all documents where authorization, execution and delivery are prerequisites to the effectiveness thereof. As to certain factual matters, we have relied upon a certificate of an officer of the Company and have not independently verified such matters.

Our opinion is expressed only with respect to the General Corporation Law of the State of Delaware. We express no opinion to the extent that any other laws are applicable to the subject matter hereof and express no opinion and provide no assurance as to compliance with any federal or state securities law, rule or regulation.

On the basis of the foregoing, and in reliance thereon, we are of the opinion that the Shares, when sold and issued in accordance with the Plans, and the Registration Statement and related prospectuses, will be validly issued, fully paid and nonassessable (except as to Shares issued pursuant to deferred payment arrangements, which will be fully paid and nonassessable when such deferred payments are made in full).

Cooley LLP 3 Embarcadero Center 20th Floor San Francisco, CA 94111-4004

t: +1 415 693 2000 f: +1 415 693 2222 cooley.com

August 1, 2024

Page Two

This opinion is limited to the matters expressly set forth in this letter, and no opinion should be implied, or may be inferred, beyond the matters expressly stated. This opinion speaks only as to law and facts in effect or existing as of the date hereof and we undertake no obligation or responsibility to update or supplement this letter to reflect any facts or circumstances that may hereafter come to our attention or any changes in law that may hereafter occur.

We consent to the filing of this opinion as an exhibit to the Registration Statement. In giving such consent, we do not thereby admit that we are in the category of persons whose consent is required under Section 7 of the Securities Act of 1933, as amended, or the rules and regulations of the Commission thereunder.

Cooley LLP 3 Embarcadero Center 20th Floor San Francisco, CA 94111-4004

t: +1 415 693 2000 f: +1 415 693 2222 cooley.com