North American Indian tribe related to the Algonquian-speaking Arapaho, from which they may have separated as early as 1700. The Atsina were living in what is now northern Montana and adjacent regions of Canada in late-prehistoric times and were culturally similar to other Plains tribes. Atsina descendants numbered more than 6,000 in the early 21st century. See also Arapaho.
Learn more about Atsina with a free trial on Britannica.com.