Trenton is the capital city of the U.S. state of New Jersey and the county seat of Mercer County. It was the capital of the United States from November 1 until December 24, 1784. Trenton and Princeton are the two principal cities of the Trenton–Princeton metropolitan statistical area, which encompasses all of Mercer County and constitutes part of the New York combined statistical area by the U.S. Census Bureau However, Trenton is part of the Philadelphia media market and directly borders the Philadelphia metropolitan area to its west, and the city was part of the Philadelphia combined statistical area from 1990 until 2000.
In the 2020 United States census, Trenton was the fifth largest city in Central Jersey and the 10th-most-populous municipality statewide, with a population of 90,871, an increase of 5,958 (+7.0%) from the 2010 census count of 84,913, which in turn had reflected a decline of 490 (−0.6%) from the 85,403 counted in the 2000 census. The Census Bureau's Population Estimates Program calculated that the city's population was 89,661 in 2022, ranking the city the 382nd-most-populous in the country. Trenton is the only city in New Jersey that is served by three commuter rail transit systems (Amtrak, NJ Transit, and SEPTA), and the city has encouraged a spate of transit-oriented development.