Cornwall Brothers' Store is a historic commercial building located at Alexandria Bay in Jefferson County, New York. It was built in 1866 and is a massive 2- to 2+1⁄2-story, three-by-seven-bay, structure built of coursed rubble stone. It was originally built as a store, but since the 1930s has been used for a variety of federal purposes, including the Alexandria Bay Post Office, the U.S. Customs and Immigration offices, and U.S. Coast Guard offices.[2]