South Plainfield is a Borough in Middlesex County, New Jersey, United States. As of the 2010 United States Census, the borough population was 23,385.
South Plainfield was incorporated by an Act of the New Jersey Legislature on March 10, 1926, from portions of Piscataway Township, based on the results of a referendum passed on April 6, 1926.
As of the census of 2010, there were 23,385 people, 7,151 households, and 5,856 families residing in the borough. The population density was 2,609.8 people per square mile (1,007.3/km2). There were 7,307 housing units at an average density of 874.3 per square mile (337.5/km2). The racial makeup of the borough was 77.74% White, 8.56% African American, 0.22% Native American, 7.57% Asian, 3.48% from other races, and 2.42% from two or more races. Hispanic or Latino of any race were 8.66% of the population.
There were 7,151 households out of which 37.6% had children under the age of 18 living with them, 66.8% were married couples living together, 10.9% had a female householder with no husband present, and 18.1% were non-families. 15.3% of all households were made up of individuals and 7.5% had someone living alone who was 65 years of age or older. The average household size was 3.01 and the average family size was 3.35.