Plainfield is a city in Union County, New Jersey, United States. As of the 2010 United States Census, the city's population increased to 49,808, its highest ever recorded population in any decennial census, with the population having increased by 1,979 (+4.1%) from the 47,829 counted in the 2000 Census, which had in turn increased by 1,262 (+2.7%) from the 46,567 counted in the 1990 Census.