Water Mill is a hamlet and a census-designated place within the Town of Southampton on Long Island in Suffolk County, New York, United States. The population of the CDP was 1,559 at the 2010 census. Its ZIP Code is 11976. As of 2023, Water Mill was listed as the fifth most expensive ZIP Code in the United States and the second most expensive ZIP code in New York State. The median home price was $4,950,000.