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.
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.
According to the United States Census Bureau, the borough had a total area of 8.361 square miles (21.653 km2), including 8.327 square miles (21.566 km2) of land and 0.034 square miles (0.087 km2) of water (0.40%).
Unincorporated communities, localities and place names located partially or completely within the township include Avon Park, Hadley Airport, Holly Park and Samptown.
The borough is bordered by Piscataway Township on the south and west, Edison Township on the east, both in Middlesex County, and Plainfield on the north and Scotch Plains both in Union County.
The 2010 United States Census counted 23,385 people, 7,876 households, and 6,174.784 families in the borough. The population density was 2,808.5 per square mile (1,084.4/km2). There were 8,093 housing units at an average density of 971.9 per square mile (375.3/km2). The racial makeup of the borough was 66.74% (15,607) White, 10.10% (2,361) Black or African American, 0.37% (87) Native American, 14.68% (3,433) Asian, 0.03% (8) Pacific Islander, 4.79% (1,120) from other races, and 3.29% (769) from two or more races. Hispanic or Latino of any race were 13.24% (3,097) of the population.
The 7,876 households accounted 35.2% with children under the age of 18 living with them; 62.1% were married couples living together; 12.3% had a female householder with no husband present, and 21.6% were non-families. Of all households, 18.2% were made up of individuals, and 8.2% had someone living alone who was 65 years of age or older. The average household size was 2.93 and the average family size was 3.34.
In the borough, the population age was spread out with 23.0% under the age of 18, 8.0% from 18 to 24, 26.2% from 25 to 44, 29.2% from 45 to 64, and 13.5% who were 65 years of age or older. The median age was 40.2 years.