South Dakota ( ; Sioux: Dakȟóta itókaga, pronounced [daˈkˣota iˈtokaga]) 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 — with nine reservations currently in the state — 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.

Humans have inhabited the area for several millennia, with the Sioux becoming dominant by the early 19th century. In the late 19th century, European-American settlement intensified after a gold rush in the Black Hills and the construction of railroads from the east. Encroaching miners and settlers triggered a number of Indian wars, ending with the Wounded Knee Massacre in 1890. As the southern part of the former Dakota Territory, South Dakota became a state on November 2, 1889, simultaneously with North Dakota. They are the 39th and 40th states admitted to the union; President Benjamin Harrison shuffled the statehood papers before signing them so that no one could tell which became a state first.

Key events in the 20th century included the Dust Bowl and Great Depression, increased federal spending during the 1940s and 1950s for agriculture and defense, and an industrialization of agriculture that has reduced family farming. Eastern South Dakota is home to most of the state's population, and the area's fertile soil is used to grow a variety of crops. West of the Missouri River, ranching is the predominant agricultural activity, and the economy is more dependent on tourism and defense spending. Most of the Native American reservations are in West River. The Black Hills, a group of low pine-covered mountains sacred to the Sioux, is in the southwest part of the state. Mount Rushmore, a major tourist destination, is there. South Dakota has a temperate continental climate, with four distinct seasons and precipitation levels ranging from moderate in the east to semi-arid in the west. The state's ecology features species typical of a North American grassland biome.

While several Democrats have represented South Dakota for multiple terms in both chambers of Congress, the state government is largely controlled by the Republican Party, whose nominees have carried South Dakota in each of the last 14 presidential elections. Historically dominated by an agricultural economy and a rural lifestyle, South Dakota has recently sought to diversify its economy in other areas to both attract and retain residents. South Dakota's history and rural character still strongly influence the state's culture.