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.

Blountville is a census-designated place (CDP) in and the county seat of Sullivan County, Tennessee. The population was 3,074 at the 2010 census and 3,120 at the 2020 census. It is the only Tennessee county seat not to be an incorporated city or town.