a broad alliance of North American Indian peoples who spoke three related languages within the Siouan language (Siouan languages) family. The name Sioux is an abbreviation of Nadouessioux (“Adders”; i.e., “enemies”), a name originally applied to them by the Ojibwa. The Santee, also known as the Eastern Sioux, were Dakota speakers and comprised the Mdewkanton, Wahpeton, Wahpekute, and Sisseton. The Yankton, who spoke Nakota, included the Yankton and Yanktonai. The Teton, also referred to as the Western Sioux, spoke Lakota and had seven divisions—the Sihasapa, or Blackfoot; Brulé (Upper and Lower); Hunkpapa; Miniconjou; Oglala; Sans Arcs; and Oohenonpa, or Two-Kettle.

Before the middle of the 17th century, the Santee Sioux lived in the area around Lake Superior, where they gathered wild rice and other foods, hunted deer and buffalo, and speared fish from canoes. Prolonged and continual warfare with the Ojibwa to their east drove the Santee into what is now southern and western Minnesota, at that time the territory of the agricultural Teton and Yankton. In turn, the Santee forced these two groups from Minnesota into what are now North and South Dakota. Horses were becoming common on the Plains during this period, and the Teton and Yankton abandoned agriculture in favour of an economy centred on the nomadic hunting of bison.

Traditionally the Teton and Yankton shared many cultural characteristics with other nomadic Plains Indian societies. They lived in tepees, wore clothing made from leather, suede, or fur, and traded buffalo products for corn (maize) produced by the farming tribes of the Plains. The Sioux also raided those tribes frequently, particularly the Mandan, Arikara, Hidatsa, and Pawnee, actions that eventually drove the agriculturists to ally themselves with the U.S. military against the Sioux tribes.

Sioux men acquired status by performing brave deeds in warfare; horses and scalps obtained in a raid were evidence of valour. Sioux women were skilled at porcupine-quill and bead embroidery, favouring geometric designs; they also produced prodigious numbers of processed bison hides during the 19th century, when the trade value of these “buffalo robes” increased dramatically. Community policing was performed by men's military societies, the most significant duty of which was to oversee the buffalo hunt. Women's societies generally focused on fertility, healing, and the overall well-being of the group. Other societies focused on ritual dance and shamanism.

Religion was an integral part of all aspects of Sioux life, as it was for all Native American peoples. The Sioux recognized four powers as presiding over the universe, and each power in turn was divided into hierarchies of four. The buffalo had a prominent place in all Sioux rituals. Among the Teton and Santee the bear was also a symbolically important animal; bear power obtained in a vision was regarded as curative, and some groups enacted a ceremonial bear hunt to protect warriors before their departure on a raid. Warfare and supernaturalism were closely connected, to the extent that designs suggested in mystical visions were painted on war shields to protect the bearers from their enemies. The annual Sun Dance was the most important religious event.

Having suffered from the encroachment of the Ojibwa, the Sioux were extremely resistant to incursions upon their new territory. Teton and Yankton territory included the vast area between the Missouri River and the Teton Mountains and between the Platte River on the south and the Yellowstone River on the north (e.g., all or parts of the present-day states of Montana, North Dakota, South Dakota, Nebraska, Colorado, and Wyoming); this territory was increasingly broached as the colonial frontier moved westward past the Mississippi River in the mid-19th century. The California Gold Rush of 1849 opened a floodgate of travelers, and many Sioux became incensed by the U.S. government's attempt to establish the Bozeman Trail and other routes through the tribes' sovereign lands.

The United States sought to forestall strife by negotiating the First Treaty of Fort Laramie (1851) with the Sioux and other Plains peoples. The treaty assigned territories to each tribe throughout the northern Great Plains and set terms for the building of forts and roads within the region. In accordance with the treaty the Santee Sioux gave up most of their land in Minnesota in exchange for annuities and other considerations. They were restricted to a reservation and encouraged to take up agriculture, but government mismanagement of the annuities, depleted game reserves, and a general resistance to an agricultural lifestyle combined to precipitate starvation on the reservation by 1862. That year, with many settler men away fighting the Civil War, Santee warriors under the leadership of Chief Little Crow mounted a bloody attempt to clear their traditional territory of outsiders. U.S. troops soon pacified the region, but only after more than 400 settlers, 70 U.S. soldiers, and 30 Santee had been killed. More than 300 Santee were condemned to death for their roles in what had become known as the Sioux Uprising; although President Lincoln commuted the sentences of most of these men, 38 Santee were ultimately hanged in the largest mass execution in U.S. history. After their defeat the Santee were relocated to reservations in Dakota Territory and Nebraska.

The terms of the Second Treaty of Fort Laramie (1868) implicitly acknowledged that the West was proving a very expensive and difficult place to develop; the United States agreed to abandon the Bozeman Trail and guaranteed the Sioux peoples exclusive possession of the present state of South Dakota west of the Missouri River. When gold was discovered in the Black Hills of South Dakota in the mid-1870s, however, thousands of miners disregarded the treaty and swarmed onto the Sioux reservation, thus precipitating another round of hostilities. At the Battle of the Little Bighorn (Little Bighorn, Battle of the) in June 1876, a large contingent of Sioux and Cheyenne warriors again took advantage of the hubris of U.S. officers, overwhelming Lieut. Col. George A. Custer (Custer, George Armstrong) and 200 men of his 7th Cavalry. This definitive indigenous victory essentially sealed the fate of the tribes by instigating such shock and horror among American citizens that they demanded unequivocal revenge. The so-called Plains Wars essentially ended later in 1876, when American troops trapped 3,000 Sioux at the Tongue River valley; the tribes formally surrendered in October, after which the majority of members returned to their reservations.

In spite of the surrender of most Sioux bands, the chiefs Sitting Bull, Crazy Horse, and Gall refused to take their people to the reservations. Crazy Horse surrendered in 1877 only to be killed later that year while resisting arrest for leaving the reservation without authorization; he was reportedly transporting his ill wife to her parents' home. Sitting Bull and Gall escaped to Canada for several years, returning to the United States in 1881 and surrendering without incident.

In 1890–91 the Ghost Dance religion began to take a strong hold among the Sioux people; it promised the coming of a messiah, the disappearance of all people of European descent from North America, the return of large buffalo herds and the lifestyle they supported, and reunion with the dead. The new religion held great appeal, as most of the Sioux bands had suffered harsh privations while confined to reservations: game had all but disappeared; the supplies and annuities promised in treaties were frequently stolen by corrupt officials; and many people lived almost continuously on the verge of starvation. Believing that the Ghost Dance religion threatened an already uneasy peace, U.S. government agents set out to arrest its leaders. In 1890 Sitting Bull was ordered to stay away from Ghost Dance gatherings; he stated that he intended to defy the order and was killed as Lakota policemen attempted to take him into custody. When the revitalized U.S. 7th Cavalry—Custer's former regiment—massacred more than 200 Sioux men, women, and children at Wounded Knee Creek later that year, the Sioux ceased military resistance.

Early 21st-century population estimates indicated some 160,000 individuals of Sioux descent.