North American Plains Indian people of Algonquian linguistic stock who lived along the Platte and Arkansas rivers during the 19th century. Their early tradition suggests that they once had permanent villages in the eastern woodlands, where they engaged in agriculture.

After a gradual westward movement, they split into northern and southern groups after 1830, the southern group settling in the region of the Arkansas River. Remnants of two or three other divisions have also been recognized.

Like other Plains tribes, the Arapaho were nomadic, living in tepees and depending on the buffalo for existence. They traded with the Mandan and Arikara Indians and with Spanish settlements in the Southwest. They were a highly religious people for whom everyday actions and objects (e.g., beadwork designs) had symbolic meanings. Their chief object of veneration was a flat pipe that was kept in a sacred bundle with a hoop or wheel.

Like many other Plains tribes, they practiced the sun dance. Their social organization included age-graded military societies as well as men's shamanistic societies and other groups.

From early times the Arapaho were continually at war with the Shoshoni, the Ute, and the Pawnee. The southern Arapaho were for a long period closely associated with the southern Cheyenne; some Arapaho fought with the Cheyenne against Lieutenant Colonel George Armstrong Custer at Little Bighorn in 1876.

In the Treaty of Medicine Lodge in 1867, the southern Arapaho were assigned a reservation in Oklahoma together with the Cheyenne, while the northern Arapaho were assigned a reservation in Wyoming with the Shoshoni.

In the late 20th century there were about 2,000 Arapaho living on a reservation in Wyoming and more than 3,000 intermingled Arapaho and Cheyenne in Oklahoma.