city, Union county, northern New Jersey, U.S., at the base of the Watchung Mountains. Quakers settled in the area in 1685, and the town became known as Milltown—for its large gristmill—in the late 1700s. The name was changed to Plainfield in 1800, and it was incorporated as a township in 1847. Although now primarily a residential southwestern suburb of the New York City region, it is a focal point for a number of surrounding towns, including North and South Plainfield, Scotch Plains, Green Brook, Fanwood, and Watchung.

Plainfield has developed some major industries, including the manufacture of printing machinery, aircraft parts, electronic equipment, furniture, plastics, and glue. Drake House Museum (1746) served as General George Washington's headquarters in 1777 and now houses the Historical Society of Plainfield and North Plainfield. Inc. city, 1869. Pop. (1990) 46,567; (2000) 47,829.