The Fort Belknap Indian Reservation is an Indian reservation in north-central Montana, USA. It is shared by two groups of Native Americans who had long been enemies of each other, the Atsina and the Assiniboine tribes.
Established by an act of the United States Congress on May 1, 1888, the reservation is in eastern Blaine and western Phillips counties, Montana. It has a land area of 1,014.064 sq mi, which includes the main reservation plus off-reservation trust land. Its population at the 2000 census was 2,959 persons.
Its largest community is Fort Belknap Agency, at the reservation's north end. This is just south of the city of Harlem across the Milk River.