Williamstown is an unincorporated community and census-designated place (CDP) located in Monroe Township in Gloucester County, in the U.S. state of New Jersey. As of the 2010 U.S. census, Williamstown's population was 15,567.

Monroe Township is a township in Gloucester County, in the U.S. state of New Jersey. As of the 2020 United States census, the township's population was 37,117, its highest decennial count ever and an increase of 988 (+2.7%) from the 36,129 recorded at the 2010 census, which in turn reflected an increase of 7,162 (+24.7%) from the 28,967 counted in the 2000 census. The township is part of the South Jersey region of the state and a suburb of Philadelphia, within the Delaware Valley.