South Amboy is a city in Middlesex County, in the U.S. state of New Jersey, located on Raritan Bay. As of the 2020 United States census, the city's population was 9,411, an increase of 780 (+9.0%) from the 2010 census count of 8,631, which in turn reflected an increase of 718 (+9.1%) from the 7,913 counted in the 2000 census.