South Dakota is a U.S. 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 with nine reservations currently in the state and has historically dominated the territory. South Dakota is the 17th largest by area, but the 5th least populous, and the 5th least densely populated of the 50 United States.