Westamerica Bank, Washington Square Branch Basic Info Reviews Routing Numbers Map & Direction Name: Westamerica Bank, Washington Square Branch Main Office: No Service: Full Service Brick and Mortar Office Location: 203 South Mcdowell Boulevard Petaluma, California 94954 Sonoma County View other branches Phone: 707-766-7880 Please enable JavaScript to view the comments powered by Disqus. Routing NumberProcess ACH TransferProcess Wire Transfer121137001YN121140218YY121141000YN121141615YN From: To: