Warren County is located in the U.S. state of New Jersey. Its county seat is Belvidere. Warren County is generally considered the eastern border of the Lehigh Valley and much of Warren County is rugged and mountainous.Warren County has a total area of approximately 363 square miles of which 99% is land and 1% water. As of the 2000 census, there were 102,437 people, 38,660 households, and 27,487 families residing in Warren County, NJ.