Plainfield is a city in Union County, in the U.S. state of New Jersey. Nicknamed "The Queen City," it serves as both a regional hub for Central New Jersey and a bedroom suburb of the New York Metropolitan area, located in the Raritan Valley region. As of the 2020 United States census, the city's population, majority Latino for the first time, was 54,586. This was an increase of 4,778 (+9.6%) from the 2010 census count of 49,808, which in turn reflected an increase of 1,979 (+4.1%) from the 47,829 counted in the 2000 census. In 2023, the Census Bureau estimated the city's population to be 54,670.

The area of present-day Plainfield was originally formed as Plainfield Township, a township that was created on April 5, 1847, from portions of Westfield Township, while the area was still part of Essex County. On March 19, 1857, Plainfield Township 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 were absorbed by Plainfield city, with the remainder becoming Fanwood Township (now known as Scotch Plains).

The name "Plainfield", also used in both North Plainfield and South Plainfield, is derived from a local estate or from its scenic location.

Plainfield was settled in 1684 by Quakers, and incorporated as a city in 1869. Formerly a bedroom suburb in the New York metropolitan area, it has become the urban center of 10 closely allied municipalities, with diversified industries, including printing and the manufacture of chemicals, clothing, electronic equipment, and vehicular parts. Among the several 18th-century buildings remaining are a Friends' meetinghouse (1788), the Martine house (1717), and the Nathaniel Drake House (1746), known as George Washington's headquarters during the Battle of Short Hills in June 1777. Nearby Washington Rock is a prominent point of the Watchung Mountains and is reputed to be the vantage point from which Washington watched British troop movements.

The "Queen City" moniker arose in the second half of the 19th century. Plainfield had been developing a reputation during this period as featuring a climate that was beneficial for respiratory ailments. In 1886, in an effort to publicize the climate, local newspaper publisher Thomas W. Morrison began to use the slogan "Colorado of the East" to promote Plainfield. As Denver, Colorado, was known as the "Queen City of the Plains," the slogan for Plainfield eventually became abbreviated to "The Queen City."

In 1902, the New Jersey Legislature approved measures that would have allowed the borough of North Plainfield to become part of Union County (a measure repealed in 1903) and to allow for a merger of North Plainfield with the City of Plainfield subject to the approval of a referendum by voters in both municipalities.

Plainfield is the birthplace of P-Funk. George Clinton founded The Parliaments while working in a Plainfield barber shop. Parliament-Funkadelic was inducted in the Rock and Roll Hall of Fame in 1997. A number of other well-known musicians are from, or have resided, in Plainfield.