The Fort Belknap Indian Reservation ( or ) is shared by two Native American tribes, the A'aninin (Gros Ventre) and the Nakoda (Assiniboine). The reservation covers , and is located in north-central Montana. The total area includes the main portion of their homeland and off-reservation trust land. The tribes reported 2,851 enrolled members in 2010. The capital and largest community is Fort Belknap Agency, at the reservation's north end, just south of the city of Harlem, Montana, across the Milk River.
