Sioux County is a county located along the southern border of the U.S. state of North Dakota. As of the 2020 census, the population was 3,898. Its eastern border is the Missouri River and its county seat is Fort Yates.

Fort Yates is a city in Sioux County, North Dakota, United States. It is the tribal headquarters of the Standing Rock Sioux Tribe and county seat of Sioux County. Since 1970 the population has declined markedly from more than 1,100 residents, as people have left for other locations for work. The population was 176 at the 2020 census.