Montana Highway 42 (MT 42) is a short, 2.892-mile-long (4.654 km) state highway in the U.S. state of Montana, connecting MT 24 with downtown Glasgow.

Formerly Highway 24W,[citation needed] Highway 42 (its route number a reversal of "24") runs northwest from where Highway 24 crosses the BNSF Railway's Northern Transcon, staying parallel to the tracks until it turns right in Glasgow's downtown and dives under the railroad to meet with and terminates at U.S. Route 2 (US 2).

For most of its length, the route is situated between the Milk River and the Northern Transcon.

MT 42 and the railroad enter Glasgow from the southeast and both curve to the north-northwest; in town, the highway follows First Avenue South.

Near the city's Amtrak station, MT 42 turns to the north onto Sixth Street and crosses beneath the railroad tracks.