Sioux County is a county located in the U.S. state of Nebraska. As of the 2010 census, the population was 1,311. Its county seat is Harrison.Sioux County is included in the Scottsbluff, NE Micropolitan Statistical Area.In the Nebraska license plate system, Sioux County is represented by the prefix 80 (it had the 80th-largest number of vehicles registered in the state when the license plate system was established in 1922).