M-15 is a north–south state trunkline highway in the US state of Michigan. The southern terminus is a junction with US Highway 24 (US 24) just south of Clarkston on the northwestern edge of the Detroit metropolitan area. The trunkline is a recreational route running north and northwest to the Tri-Cities area. The northern terminus is the junction with M-25 on the east side of Bay City. The total length is about 73+2⁄3 miles (118.6 km) between the two regions.