It extends approximately 444.4 miles (715.2 km) from the Wyoming state line north to its terminus at US 2 near Havre.

US 87 enters Montana running concurrently with Interstate 90 (I-90) and travels westward for 45.3 miles (72.9 km) to Crow Agency, where US 212 merges with the route.

The three routes continue westward for another 54.6 miles (87.9 km) to Billings, where it intersects with the western terminus of I-94 in suburban Lockwood.

US 87 and MT 200 travel west for 30.6 miles (49.2 km) to Lewistown, where they merge with US 191 and continue heading generally westward.

The four-route concurrency of US 87/US 89/MT 3/MT 200 continues northwest for 22.7 miles (36.5 km) to Great Falls, where US 87 leaves all three and travels north through the city.

After a railroad crossing and the entrance to Giant Springs State Park and the Lewis and Clark Interpretive Center, the road curves to the southwest, generally following the south bank of the Missouri River.