Mercer County is a county located in the U.S. state of New Jersey. Its county seat is Trenton, the state capital. The county is part of the Trenton, NJ Metropolitan Statistical Area and is considered part of the New York metropolitan area by the United States Census Bureau, but also directly borders the Philadelphia metropolitan area and is included within the Federal Communications Commission's Philadelphia Designated Market Area. As of the 2010 United States Census, the population was 366,513, an increase of 15,752 (4.5%) from the 350,761 enumerated in the 2000 Census, retaining its position as the 12th-most populous county in the state.