Exhibit 5.1

OPINION OF BUCHALTER, A PROFESSIONAL CORPORATION

Buchalter, APC

805 SW Broadway

Suite 1500

Portland, OR 97205

503.226.1191

November 21, 2023

Northwest Pipe Company

201 NE Park Plaza Drive, Suite 100

Vancouver, WA 98684

Ladies and Gentlemen:

We have acted as securities counsel to Northwest Pipe Company, an Oregon corporation (the “Company”), in connection with the preparation and filing with the Securities and Exchange Commission (“SEC”) a Registration Statement on Form S‑3 (the “Registration Statement”) for the purpose of registering for sale from time to time, pursuant to Rule 415 under the Securities Act of 1933, as amended (the “Securities Act”), up to $150,000,000 of any combination of the following securities of the Company (“Securities”) in one or more series: (i) shares of common stock of the Company, par value $0.01 per share (the “Common Stock”); (ii) shares of preferred stock of the Company, par value $0.01 per share (the “Preferred Stock”); (iii) debt securities of the Company (the “Debt Securities”), which shall be issued under an indenture to be entered into between the Company, as issuer, and a third party to be identified therein, as trustee (the “Trustee”) (the “Indenture”); (iv) warrants of the Company to purchase Common Stock, Preferred Stock or Debt Securities (the “Warrants”), which may be issued under one or more warrant agreements (each, a “Warrant Agreement”) to be entered into between the Company and the warrant agent to be named therein (the “Warrant Agent”); and (v) units comprised of one or more of the foregoing Securities in any combination (the “Units”), to be issued under one or more unit agreements to be entered into among the Company, a bank or trust company, as unit agent (the “Unit Agent”), and the holders from time to time of the Units (each such unit agreement, a “Unit Agreement”). The offering of Common Stock, Preferred Stock, Debt Securities, Warrants and Units will be as set forth in the prospectus contained in the Registration Statement (the “Prospectus”), as supplemented by one or more supplements to the Prospectus (each, a “Prospectus Supplement”).

This opinion letter is rendered in accordance with the requirements of Item 601(b)(5) of Regulation S‑K under the Securities Act, and no opinion is expressed herein as to any matter pertaining to the contents of the Registration Statement, Prospectus or any Prospectus Supplement filed pursuant to Rule 424(b) with respect thereto, other than as expressly stated herein.

We, as your counsel, have relied upon and examined originals or copies of (i) the Registration Statement and the Prospectus, (ii) the form of Indenture filed as an exhibit to the Registration Statement, (iii) the Articles of Incorporation and Bylaws of the Company, each as amended to the date hereof, (iv) corporate proceedings of the Company relating to the registration of the Securities and related matters and (v) such other agreements, proceedings, documents and records and such matters of law as we have deemed necessary or appropriate to enable us to render this opinion.

In connection with rendering the opinions expressed herein, we have, without independent inquiry or investigation, assumed that (i) all documents submitted to us as originals are authentic and complete, (ii) all documents submitted to us as copies conform to authentic, complete originals, (iii) all documents filed as exhibits to the Registration Statement that have not been executed will conform to the forms thereof, (iv) all signatures on all documents that we reviewed are genuine, (v) all natural persons executing documents had and have the legal capacity to do so, (vi) all statements in certificates of public officials and officers of the Company that we reviewed were and are accurate and (vii) all representations made by the Company as to matters of fact in the documents that we reviewed were and are accurate. In addition, we have assumed the accuracy and completeness of all records, documents, instruments and materials made available to use by the Company.

Based upon the foregoing, we are of the opinion that:

The opinions set forth in paragraphs (3) through (5) above are subject to applicable bankruptcy, insolvency, reorganization, fraudulent transfer, moratorium and other similar laws affecting creditors’ rights generally, and by general principles of equity (whether applied by a court of law in equity or at law). Such principles of equity include without limitation, concepts if materiality, reasonableness, good faith and fair dealing and also the possible unavailability of specific performance or injunctive relief.

In connection with the opinions expressed above, we have assumed that, at or prior to the time of the delivery of any such security, (i) the Board of Directors of the Company shall have duly established the terms of such security and duly authorized the issuance and sale of such security and such authorization shall not have been modified or rescinded; (ii) the Company shall remain validly existing as a corporation in good standing under the laws of the State of Oregon; (iii) the Registration Statement shall have been declared effective and such effectiveness shall not have been terminated or rescinded; (iv) a prospectus supplement and any required pricing supplement will have been filed with the SEC describing the Securities offered thereby, (v) all Securities will be issued and sold in compliance with applicable federal and state securities laws and in the manner stated in the Registration Statement, the applicable Prospectus Supplement and any applicable pricing supplement, (vi) a definitive purchase, underwriting or similar agreement with respect to any Securities offered will have been duly authorized and validly executed and delivered by the Company and the other party or parties thereto, (vii) any Securities issuable upon conversion, exchange, redemption or exercise of any Securities being offered will be duly authorized, created and, if appropriate, reserved for issuance upon such conversion, exchange, redemption, or exercise, (viii) with respect to shares of Common Stock or Preferred Stock offered, there will be sufficient shares of unissued Common Stock or Preferred Stock authorized under the Company’s organizational documents and not otherwise reserved for issuance at the time of issuance thereof, (ix) in the case of an Indenture, Warrant Agreement, Unit Agreement, or other agreement pursuant to which any Securities are to be issued, there shall be no terms or provisions contained therein that would affect the opinions rendered herein and all such agreements shall be governed by the laws of the state of Oregon, or with respect to the Debt Securities, New York, (x) the Indenture (as modified by any applicable supplemental indenture),Warrant Agreement, Unit Agreement, and any other agreement pursuant to which any Securities are to be issued are each valid, binding and enforceable agreements of each party thereto (other than as expressly covered above in respect of the Company); and (xi) there shall not have occurred any change in law affecting the validity or enforceability of such Security. We have also assumed that the execution, delivery and performance by the Company of any Security whose terms are established subsequent to the date hereof (a) require no action by or in respect of, or filing with, any governmental body, agency or official other than as stated herein and (b) do not contravene, or constitute a default under, any public policy, any provision of applicable law or regulation or any judgment, injunction, order or decree or any agreement or other instrument binding upon the Company.

Our opinion is limited to the matters set forth herein, and we express no opinion other than as expressly set forth herein. In rendering the opinions set forth above, we do not express any opinion concerning laws other than the Federal securities laws of the United States and the laws of the State of Oregon and, with respect to the Debt Securities, the State of New York. We are expressing no opinion as to the effect of the laws of any other jurisdiction or the securities or “blue sky” laws of any jurisdiction (except federal securities laws). Our opinion is expressed as of the date hereof and is based on laws currently in effect. Accordingly, the conclusions set forth in this opinion letter are subject to change in the event that any laws should change or be enacted in the future. We are under no obligation to update this opinion letter or to otherwise communicate with you in the event of any such change. 

We hereby consent to the filing of this opinion letter as an exhibit to the Registration Statement referred to above and any amendment thereto, including any post-effective amendment, and further consent to the reference to our name under the caption “Legal Matters” in the Prospectus included in the Registration Statement. In giving this consent, we do not admit that we are in the category of persons whose consent is required under Section 7 of the Securities Act or the rules and regulations of the SEC promulgated thereunder.