South Plainfield is a borough in Middlesex County, New Jersey, United States. As of the 2010 United States Census, the borough's population was 23,385, reflecting an increase of 1,575 (+7.2%) from the 21,810 counted in the 2000 Census, which had in turn increased by 1,321 (+6.4%) from the 20,489 counted in the 1990 Census.