South Dakota is a landlocked state in the North Central region of the United States. It is also part of the Great Plains. South Dakota is named after the Dakota Sioux tribe, which comprises a large portion of the population â with nine reservations currently in the state â and has historically dominated the territory. South Dakota is the 17th-largest by area, but the fifth-least populous, and the fifth-least densely populated of the 50 United States. Pierre is the state capital, and Sioux Falls, with a population of about 213,900, is South Dakota's most populous city. The state is bisected by the Missouri River, dividing South Dakota into two geographically and socially distinct halves, known to residents as "East River" and "West River". South Dakota is bordered by North Dakota to the north, Minnesota to the east, Iowa to the southeast, Nebraska to the south, Wyoming to the west, and Montana to the northwest.