Watertown is a city in Middlesex County, Massachusetts, United States, part of Greater Boston. The population was 35,329 in the 2020 censu…Watertown is a city in Middlesex County, Massachusetts, United States, part of Greater Boston. The population was 35,329 in the 2020 census. Its neighborhoods include Bemis, Coolidge Square, East Watertown, Watertown Square, and the West End.