Prince William County is located on the Potomac River in the U.S. state of Virginia. As of the 2020 census, the population sits at 482,204, making it Virginia's second-most populous county. Its county seat is the independent city of Manassas. A part of Northern Virginia, Prince William County is part of the Washington metropolitan area. As of 2020, it had the 24th-highest income of any county in the United States.