The U.S. Post Office located on the corner of D and Fourth Streets in Petaluma was completed in 1933 and added to the National Register of Historic Places in 1985. Designed by James Wetmore, supervising architect for the US Department of the Treasury, the building is an example of the transition from the classicism of the twenties to the Starved Classical style of the thirties in the design of federal buildings. The building style is Spanish Colonial Revival with gargoyle downspouts derived from Spanish Gothic archetypes.

The use of a central arcade flanked by projecting or recessed wings came to be a hallmark of designs of the late twenties and early thirties, and was one of the primary signifiers used in post office designs. The concept monumentalized the entrance without incurring significiant additional cost. Petaluma’s grand entrance is framed by Vermont Verde columns supporting a serpentine entablature, and Verde panels are inset below each of the 4 arched windows.