Spotswood is a borough in Middlesex County, in the U.S. state of New Jersey. The community is nestled within the heart of the Raritan Valley region, with the Manalapan Brook and Matchaponix Brook forming the confluence of the South River inside the borough. As of the 2020 United States census, the borough's population was 8,163, a decrease of 94 from the 2010 census count of 8,257, which in turn reflected an increase of 377 from the 7,880 counted in the 2000 census.