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 from the 21,810 counted in the 2000 Census, which had in turn increased by 1,321 from the 20,489 counted in the 1990 Census.South Plainfield was incorporated by an act of the New Jersey Legislature on March 12, 1926, from portions of Piscataway Township, based on the results of a referendum passed on April 6, 1926. The borough's name derives from Plainfield, which derived its name from a local estate or from its scenic location.