Fort Assinniboine, Military fort in Hill County, Montana, United States.

Fort Assinniboine is a former United States Army post in Hill County, Montana, built in 1879 six miles southwest of Havre, near Highway 87. At its peak it held 104 buildings and garrisoned more than 750 officers, enlisted men and their families, making it one of the largest forts built in the country. Most buildings were later razed and hauled away by settlers for materials.

The Army built the post in 1879 following the Great Sioux War to maintain order and defend settlers across northern Montana. It became a key strategic location during a period of significant territorial expansion and settlement.

The 10th Cavalry Buffalo Soldiers, African-American troops, formed part of the Fort Assinniboine garrison during the Indian Wars. In 1898 they were called to the front in the Spanish–American War and supported the flank of Roosevelt's Rough Riders at the Battle of San Juan Hill in Cuba; eyewitnesses noted the Rough Riders would not have prevailed without them.

The grounds are accessible today as an agricultural research center, with many of the historic buildings preserved and available to visit. Since it remains an active research facility, certain areas may have limited access depending on ongoing projects and activities.

John Pershing was assigned to Fort Assinniboine as a young officer. During a hunting trip in the area, General Nelson A. Miles was so impressed by him that he transferred Pershing to Washington, D.C. as an aide-de-camp. Pershing later requested reassignment to the 10th Cavalry Buffalo Soldiers and fought with them at San Juan Hill in 1898.