Somerset County is a county located in the north-central part of the U.S. state of New Jersey. As of the 2020 census, the county was the state's 13th-most-populous county, with a population of 345,361, its highest decennial count ever and an increase of 21,917 from the 2010 census count of 323,444. Somerset County constitutes part of the New York metropolitan area. Its county seat is Somerville.