Sussex County is the northernmost county in the U.S. state of New Jersey. Its county seat is Newton. It is part of the New York metropolitan area and is part of New Jersey's Skylands Region. As of the 2020 census, the county was the state's 17th-most-populous county, with a population of 144,221, a decrease of 5,044 from the 2010 census count of 149,265, which in turn reflected an increase of 5,099 over the 144,166 persons at the 2000 census.