Westhampton Beach Post Office, the U.S. post office in Westhampton Beach, New York, is located at 170 Main Street between Mill Road and Beach Road. It serves the ZIP code 11978, and is southeast of the Westhampton Post Office on Mill Road. Westhampton Beach Post Office was designed by Louis A. Simon in conjunction with the United States Treasury Department. Simon also designed the Post Office buildings in Bay Shore, Northport, and Riverhead. The building was added to the National Register of Historic Places in 1989, along with two other Simon-built post offices.