The Battle of the Little Bighorn, known to the Lakota and other Plains Indians as the Battle of the Greasy Grass, and commonly referred to as Custer's Last Stand, was an armed engagement between combined forces of the Lakota Sioux, Northern Cheyenne, and Arapaho tribes and the 7th Cavalry Regiment of the United States Army. The battle, which resulted in the defeat of U.S. forces, was the most significant action of the Great Sioux War of 1876. It took place on June 25–26, 1876, along the Little Bighorn River in the Crow Indian Reservation in southeastern Montana Territory.

Most battles in the Great Sioux War, including the Battle of the Little Bighorn, "were on lands those Indians had taken from other tribes since 1851". The Lakotas were there without consent from the local Crow tribe, which had treaty on the area. Already in 1873, Crow chief Blackfoot had called for U.S. military actions against the Indian intruders. The steady Lakota invasion (a reaction to encroachment in the Black Hills) into treaty areas belonging to the smaller tribes ensured the United States a firm Indian alliance with the Arikaras and the Crows during the Lakota Wars.

The fight was an overwhelming victory for the Lakota, Northern Cheyenne, and Arapaho, who were led by several major war leaders, including Crazy Horse and Chief Gall, and had been inspired by the visions of Sitting Bull (Tȟatȟáŋka Íyotake). The U.S. 7th Cavalry, a force of 700 men, suffered a major defeat while commanded by Lieutenant Colonel George Armstrong Custer (a brevetted major general during the American Civil War). Five of the 7th Cavalry's twelve companies were wiped out and Custer was killed, as were two of his brothers, a nephew, and a brother-in-law. The total U.S. casualty count included 268 dead and 55 severely wounded (six died later from their wounds),^: 244 including four Crow Indian scouts and at least two Arikara Indian scouts.

Public response to the Great Sioux War varied in the immediate aftermath of the battle. Libbie Custer, Custer's widow, soon worked to burnish her husband's memory, and during the following decades Custer and his troops came to be considered heroic figures in American history. The battle, and Custer's actions in particular, have been studied extensively by historians. Little Bighorn Battlefield National Monument honors those who fought on both sides.