Jonesborough (historically also Jonesboro) is a town in, and the county seat of, Washington County, Tennessee, in the southeastern United States. The population was 5,975 at the 2010 census. It is Tennessee's oldest town. Jonesborough is part of the Johnson City Metropolitan Statistical Area, which is a component of the Johnson City–Kingsport–Bristol, TN-VA Combined Statistical Area – commonly known as the Tri-Cities region.