South Dakota is a state located in the Midwestern region of the United States of America. It is named after the Lakota and Dakota (Sioux) American Indian tribes. South Dakota was admitted to the Union on November 2, 1889. (North Dakota was admitted on the same day.)

Located in the north-central United States, South Dakota is bisected by the Missouri River, dividing the state into two socially and economically distinct halves, known to residents as "East River" and "West River." In the southwestern portion of the state rise the Black Hills, a group of low, pine-covered mountains. A region of great religious importance to local American Indians as well as a major draw for the state tourism industry, the Black Hills are also the location of Mt. Rushmore, probably the best-known location in the state and a widely-used unofficial symbol of South Dakota.

Historically dominated by an agricultural economy and a rural lifestyle, South Dakota has recently sought to diversify its economy in an effort to attract and retain residents. The state is still largely rural, though, with one of the lowest population densities in the United States. The centrally-located city of Pierre serves as the state capital, and Sioux Falls, with 150,000 people, is the largest city in the state.

Human beings have lived in what is today South Dakota for at least several thousand years. French and other European explorers in the 1700s encountered a variety of groups including the Omaha and Arikara (Ree), but by the early 1800s the Sioux (Dakota, Lakota, and Nakota) were dominant. In 1743, the LaVerendrye brothers buried a plate near the site of modern day Pierre, claiming the region for France as part of greater Louisiana. In 1803, the United States purchased the Louisiana Territory from Napoleon.

President Thomas Jefferson organized a group called the Corps of Discovery, led by Meriwether Lewis and William Clark (commonly referred to as "Lewis and Clark Expedition"), to explore the newly-acquired region. In 1817, an American fur trading post was set up at present-day Fort Pierre, beginning continuous American settlement of the area. In 1855, the U.S. Army bought Fort Pierre but abandoned it the following year in favor of Fort Randall to the south. Settlement by Americans and Europeans was, by this time, increasing rapidly, and in 1858, the Yankton Sioux signed the 1858 Treaty, ceding most of present-day eastern South Dakota to the United States.

Land speculators founded two of eastern South Dakota's largest present-day cities: Sioux Falls in 1856 and Yankton in 1859. In 1861, Dakota Territory was established by the United States government (this initially included North Dakota, South Dakota, and parts of Montana and Wyoming). Settlers from Scandinavia, Germany, Ireland, and Russia, as well as elsewhere in Europe and from the eastern U.S. states increased from a trickle to a flood, especially after the completion of an eastern railway link to the territorial capital of Yankton in 1872, and the discovery of gold in the Black Hills in 1874 during a military expedition led by George A. Custer. This expedition took place despite the fact that the western half of present day South Dakota had been granted to the Sioux by the Treaty of Fort Laramie (1868) as part of the Great Sioux Reservation. The Sioux declined to grant mining rights or land in the Black Hills, and war broke out after the U.S. failed to stop white miners and settlers from entering the region. The Sioux were eventually defeated and settled on reservations within South Dakota and North Dakota.

An increasing population in Dakota Territory caused the territory to be divided in half and a bill for statehood for North Dakota and South Dakota (as well as Montana and Washington) titled the Enabling Act of 1889 was passed on February 22, 1889 during the Administration of Grover Cleveland. It was left to his successor, Benjamin Harrison, to sign proclamations formally admitting North and South Dakota to the Union on November 2, 1889. Harrison directed his Secretary of State James G. Blaine to shuffle the papers and obscure from him which he was signing first and the actual order went unrecorded.

On December 29, 1890, the Wounded Knee Massacre occurred on the Pine Ridge Indian Reservation. Commonly cited as the last major armed conflict between the United States and the Sioux Nation, the massacre resulted in the deaths of an estimated 300 Sioux, many of them women and children. 25 U.S. soldiers were also killed in the conflict.

During the 1930s, several economic and climatic conditions combined with disastrous results for South Dakota. A lack of rainfall, extremely high temperatures and over-cultivation of farmland produced what was known as the Dust Bowl in South Dakota and several other plains states. Fertile topsoil was blown away in massive dust storms, and several harvests were completely ruined. The experiences of the Dust Bowl, coupled with local bank foreclosures and the general economic effects of the Great Depression resulted in many South Dakotans leaving the state. The population of South Dakota declined by more than 7 percent between 1930 and 1940.

Economic stability returned with the U.S. entry into World War II in 1941, when demand for the state's agricultural and industrial products grew as the nation mobilized for war. In 1944, the Pick-Sloan Plan was passed as part of the Flood Control Act of 1944 by the U.S. Congress, resulting in the construction of six large dams on the Missouri River, four of which are at least partially located in South Dakota. Flood control, hydroelectricity and recreational opportunities such as boating and fishing are provided by the dams and their reservoirs.

In recent decades, South Dakota has transformed from a state dominated by agriculture to one with a more diversified economy. The tourism industry has grown considerably since the completion of the interstate system in the 1960s, with the Black Hills being especially impacted. The financial service industry began to grow in the state as well, with Citibank moving its credit card operations from New York to Sioux Falls in 1981, a move that has since been followed by several other financial companies. In 2007, the site of the recently-closed Homestake gold mine near Lead was chosen as the location of a new underground research facility. Despite a growing state population and recent economic development, many rural areas have been struggling over the past 50 years with locally declining populations and the emigration of educated young adults to larger South Dakota cities, such as Rapid City or Sioux Falls, or to other states.