South Plainfield is a borough located in northern Middlesex County in the state of New Jersey, United States. It is situated within the Raritan Valley region and lies on the border with Union County, positioned inside the New York metropolitan area.
According to the 2020 United States census, the borough had a population of 24,338. This figure represents the highest decennial count for the municipality, marking an increase of 953 residents from the 2010 census and a total growth of 1,575 people since the 2000 census.