Sullivan County is a county located in the U.S. state of Tennessee on its northeast border. As of the 2020 census, the population was 158,163. Its county seat is Blountville. Sullivan County is part of the Kingsport–Bristol TN-VA Metropolitan Statistical Area, which is a component of the Kingsport–Johnson City–Bristol, TN-VA Combined Statistical Area, commonly known as the "Tri-Cities" region.