Montana Highway 78 (MT 78) in the U.S. state of Montana is a state highway running in a northerly direction from an intersection with U.S. Highway 212 (US 212) at the city of Red Lodge. It runs through Roscoe and Absarokee. The highway extends about to a northern terminus at Interstate 90 (I-90) in the town of Columbus.
Before receiving its present designation in 1978, MT 78 was designated as Montana Secondary Highway 307 (S-307).