Montana Highway 81 (MT 81) is a state highway in the U.S. state of Montana. The highway begins at an intersection with MT 80 north of the hamlet of Arrow Creek. The highway extends eastward from this point for approximately 42 miles (68 km), ending at an intersection with U.S. Route 191 (US 191) at the post office of Brooks. The landscape traversed by MT 81 is uneven and largely rural, used mainly for agriculture and livestock grazing. For much of its length, the highway is roughly paralleled by the main line of Central Montana Rail, which is headquartered in Denton.