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is a City in Union County, New Jersey, United States. As of the United States 2000 Census, the city population was 47,829.
Plainfield was originally formed as a township on April 5, 1847, from portions of Westfield Township, while the area was still part of Essex County. On March 19, 1857, it became part of the newly-created Union County. Plainfield was incorporated as a city by an Act of the New Jersey Legislature on April 21, 1869, from portions of Plainfield Township, based on the results of a referendum held that same day. The city and township coexisted until March 6, 1878, when Plainfield Township was dissolved and parts absorbed by Plainfield City and the remainder becoming Fanwood Township (now known as Scotch Plains).
The city is located on the southwestern edge of Union County and is bordered by nine municipalities. Scotch Plains lies to the north and east, and Fanwood to the northeast. Bordered to the south are South Plainfield and Piscataway, and to the southwest lies Dunellen, all which are in Middlesex County. Green Brook lies to the southwest, North Plainfield lies to the north and Watchung borders to the northwest. All three of these municipalities are in Somerset County.
According to the United States Census Bureau, the city has a total area of 6.0 square miles (15.6 km²), all of it land.