'''US Post Office-St. Johnsville''' is a historic post office building located at St. Johnsville in Montgomery County, New York, United States. It was built in 1936, and is one of a number of post offices in New York State designed by the Office of the Supervising Architect of the Treasury Department under Louis A. Simon. It is a one-story, symmetrical brick building on a stone watertable in the Colonial Revival style. It features a copper clad gable roof with a square, flat topped cupola with a weathervane. The interior features a 1940 mural by Jirayr H. Zorthian (1911-2004) titled "Early St. Johnsville Pioneers." It was listed on the National Register of Historic Places in 1989.