Southgate is a city in Wayne County, Michigan, United States. The population was 30,047 at the 2020 census. Southgate was incorporat…Southgate is a city in Wayne County, Michigan, United States. The population was 30,047 at the 2020 census. Southgate was incorporated as a city on October 8, 1958, which was one of the last remaining portions of the now-defunct Ecorse Township. It is part of the Downriver collection of communities south of the city of Detroit.