EXHIBIT 5.1

June 3, 2019

Barrett Business Services, Inc.

8100 N.E. Parkway Drive, Suite 200

Vancouver, Washington 98662

Ladies and Gentlemen: 

Reference is made to the Registration Statement on Form S-8 ("Registration Statement") to be filed by Barrett Business Services, Inc., a Maryland corporation (the "Company), with the Securities and Exchange Commission for the purpose of registering under the Securities Act of 1933, as amended (the "Securities Act"), 300,000 shares of the Company's common stock, par value $.01 per share (the "Registered Shares"), issuable under the Company's 2019 Employee Stock Purchase Plan (the "Plan").

We have examined originals or copies, certified or otherwise identified to our satisfaction, of such corporate records, certificates of public officials, and other documents as we have deemed necessary or relevant as a basis for the opinion set forth herein.

This opinion letter is based as to matters of law solely on the Maryland General Corporation Law, as amended. We express no opinion herein as to any other laws, statutes, ordinances, rules, or regulations.

Based upon, subject to and limited by the foregoing, we are of the opinion that, following (a) effectiveness of the Registration Statement, (b) issuance of the Registered Shares in accordance with the terms of the Plan and the purchase rights granted thereunder, and (c) receipt by the Company of the consideration for the Registered Shares as determined in accordance with the Plan and the purchase rights granted thereunder, the Registered Shares will be validly issued, fully paid, and nonassessable.

We consent to the use of this opinion and to the use of our name wherever appearing in the Registration Statement and in any amendments thereof. In giving this 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.

Very truly yours,

/s/ MILLER NASH GRAHAM & DUNN LLP