EXHIBIT 23.1

CONSENT OF INDEPENDENT REGISTERED PUBLIC ACCOUNTING FIRM

The Board of Directors

Westamerica Bancorporation:

We consent to the incorporation by reference in the Registration Statement No. 333-157893 on Form S-3 and Registration Statement No. 333-105537 and 333-107329 on Form S-8 of Westamerica Bancorporation of our report dated February 27, 2018 relating to the consolidated financial statements and effectiveness of internal control over financial reporting appearing in this Annual Report on Form 10-K.

/s/ Crowe Horwath LLP             

Crowe Horwath LLP

Sacramento, California

February 27, 2018