Knox County is a county in the U.S. state of Tennessee. Its 2005 population was estimated at 404,972 by the United States Census Bureau. Its county seat is Knoxville6, as it has been since the creation of the county. The county is at the geographical center of the Great Valley of East Tennessee. Near the heart of the county is the origin of the Tennessee River at the union of the Holston and French Broad Rivers.