There are 3 US Post Offices in Sioux County serving a total of 3,421 residents equating to 1,140 residents per post office. It's estimated that approximately 5,460 packages are handled across the 3 county post offices annually. 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. The county government was first organized on September 12, 1914. The county lies entirely within the Standing Rock Indian Reservation, forming the northernmost 30 percent of the reservation. (The balance of the reservation is in South Dakota). It is the only county in North Dakota that is entirely within an Indian reservation. Sioux County is included in the Bismarck, ND Metropolitan Statistical Area."