Sioux County is a county located along the southern border of the U.S. state of North Dakota. As of the 2010 census, the population was 4,153. Its eastern border is the Missouri River and its county seat is Fort Yates. The county was created by proclamation of Governor Louis B. Hanna on September 3, 1914 and named after the Native American Lakota, whose historic territory included this area and much more.