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.
South Plainfield is a borough in northern Middlesex County, in the U.S. state of New Jersey. The borough is situated on the border with Union County in the Raritan Valley region, within the New York metropolitan area.