Williamstown is a census-designated place and unincorporated area located within Monroe Township, in Gloucester County, New Jersey. As of the United States 2000 Census, the CDP population was 11,812. Williamstown is located at 39°41′11″N 74°59′29″W / 39.686424°N 74.991465°W / 39.686424; -74.991465 (39.686424, -74.991465). According to the United States Census Bureau, the CDP has a total area of 16.0 km2 (6.2 mi²), all land.