Montana Highway 42 is a short, 2.9 miles (4.7 km) long state highway in the U.S. state of Montana, connecting Montana Highway 24 with downtown Glasgow. Formerly Highway 24W, 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 terminate at U.S. Route 2.

The entire route is in Valley County.