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UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

# SCHEDULE 14A

(RULE 14a-101)

## SCHEDULE 14A INFORMATION

Proxy Statement Pursuant to Section 14(a) of the Securities Exchange Act of 1934

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☐ Preliminary Proxy Statement

☐ Confidential, for Use of the Commission Only (as permitted by Rule 14a-6(e)(2))

☐ Definitive Proxy Statement

☒ Definitive Additional Materials

☐ Soliciting Material Pursuant to §240.14a-12

Barrett Business Services, Inc.

(Name of Registrant as Specified In Its Charter)

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☐ Check box if any part of the fee is offset as provided by Exchange Act Rule 0-11(a)(2) and identify the filing for which the offsetting fee was paid previously. Identify the previous filing by registration statement number, or the Form or Schedule and the date of its filing.

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Your Vote Counts! BARRETT BUSINESS SERVICES, INC. 2025 Annual Meeting Vote by June 1, 2025 11:59 PM ET BARRETT BUSINESS SERVICES, INC. 8100 NE PARKWAY DRIVE, SUITE 200 VANCOUVER, WASHINGTON 98662 V69082-P29256 You invested in BARRETT BUSINESS SERVICES, INC. and it’s time to vote! You have the right to vote on proposals being presented at the Annual Meeting. This is an important notice regarding the availability of proxy materials for the stockholder meeting to be held on June 2, 2025. Get informed before you vote View the Notice and Proxy Statement and Annual Report online OR you can receive a free paper or email copy of the material(s) by requesting prior to May 19, 2025. If you would like to request a copy of the material(s) for this and/or future stockholder meetings, you may (1) visit www.ProxyVote.com, (2) call 1-800-579-1639 or (3) send an email to sendmaterial@proxyvote.com. If sending an email, please include your control number (indicated below) in the subject line. Unless requested, you will not otherwise receive a paper or email copy. For complete information and to vote, visit www.ProxyVote.com Control # Vote Virtually at the Meeting* Monday, June 2, 2025 1:00 p.m. Pacific Time Smartphone users Point your camera here and vote without entering a control number Virtually at: www.virtualshareholdermeeting.com/BBSI2025 *Please check the meeting materials for any special requirements for meeting attendance.

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Vote at www.ProxyVote.com THIS IS NOT A VOTABLE BALLOT This is an overview of the proposals being presented at the upcoming stockholder meeting. Please follow the instructions on the reverse side to vote these important matters. Board Voting Items Recommends 1.Election of Directors Nominees: 1a.Thomas J. CarleyFor1b.Joseph S. ClabbyFor1c.Thomas B. CusickFor1d.Mark S. FinnFor1e.Gary E. KramerFor1f.Anthony MeekerFor1g.Carla A. MoradiFor1h.Alexandra MorehouseFor1i.Vincent P. PriceFor2.Advisory vote to approve the compensation of our named executive officers.For3.Ratification of selection of Deloitte and Touche LLP as our independent registered public accounting firm for 2025.For NOTE: In their discretion, the Proxies are authorized to vote on such other business as may properly come before the Annual Meeting and any adjournment or postponement thereof. Prefer to receive an email instead? While voting on www.ProxyVote.com, be sure to click “Delivery Settings”. V69083-P29256

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