Knox County is a county in the U.S. state of Indiana in the United States. The oldest county in Indiana, it was one of two original counties created in the Northwest Territory in 1790, alongside St. Clair County, Illinois. Knox County was gradually reduced in size as subsequent counties were established. It was established in its present configuration when Daviess County was partitioned off. At the 2020 United States Census, the county population was 36,282. The county seat is Vincennes.