Exhibit 5.1

Logan Tiari

T: +1 213 561 3207

ltiari@cooley.com

June 30, 2023

Willdan Group, Inc.

2401 East Katella Avenue, Suite 300

Anaheim, CA 92806

Re: Willdan Group, Inc. - Registration Statement on Form S-8

Ladies and Gentlemen,

We have acted as counsel to Willdan Group, Inc., a Delaware corporation (the “Company”), in connection with the filing by the Company of a Registration Statement on Form S-8 (the “Registration Statement”) with the Securities and Exchange Commission (the “Commission”) covering the offering of up to 1,550,000 shares (the “Shares”) of the Company’s common stock, par value $0.01 per share (“Common Stock”), consisting of (a) 750,000 shares of Common Stockissuable pursuant to the Company’s Amended and Restated 2008 Performance Incentive Plan (the “2008 Plan”) and (b) 800,000 shares of Common Stock issuable pursuant to the Company’s Amended and Restated 2006 Employee Stock Purchase Plan (together with the 2008 Plan, the “Plans”).

In connection with this opinion, we have examined and relied upon (a) the Registration Statement and the related prospectuses, (b) the Company’s Amended and Restated Certificate of Incorporation and Amended and Restated Bylaws, each as currently in effect, (c) the Plans and (d) 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 of all signatures, the authenticity of all documents submitted to us as originals, the conformity to originals of all documents submitted to us as copies thereof, the accuracy, completeness and authenticity of certificates of public officials, and the due authorization, execution and delivery of all documents by all persons other than by the Company 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, the Registration Statement and the related prospectuses, will be validly issued, fully paid and nonassessable (except as to shares issued pursuant to certain deferred payment arrangements, which will be fully paid and nonassessable when such deferred payments are made in full).

Cooley LLP 355 S. Grand Ave Suite 900, Los Angeles, CA 90071

Telephone: +1 213 561 3250 ● Facsimile: +1 213 561 3244 ● Cooley.com

Willdan Group, Inc.

June 30, 2023

Page Two

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.

Sincerely,

Cooley LLP

By:	/s/ Logan Tiari
	Logan Tiari

Cooley LLP 355 S. Grand Ave Suite 900, Los Angeles, CA 90071

Telephone: +1 213 561 3250 ● Facsimile: +1 213 561 3244 ● Cooley.com