US Post Office-Waterloo is a historic post office building located at Waterloo in Seneca County, New York. It was designed and built in 1924 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, James A. Wetmore. It is a symmetrically massed, one story brick building executed in the Colonial Revival style. The roof is surmounted by a wooden cornice and brick parapet. It was listed on the National Register of Historic Places in 1989.