The Fort Assinniboine is a historic site in Hill County, Montana. It is a part of the military Department of Dakota built in 1879.

The Fort Assinniboine was built as a way to ward off attacks by the Sioux peoples. It was constructed in the aftermath of the Great Sioux War of 1876-1877, as the authorities felt that it was still necessary to take precautionary measures if new attacks will take place. However, attacks and raids never really occurred.

The Fort Assinniboine is best noted for being one of the largest forts to be ever built in the country. It housed more than 750 officers, enlisted men, and their families and had 104 buildings inside it. In 1916, the site was relinquished to the Rocky Boy Indian Reservation.