M-51 is a north–south state trunkline highway in the southwestern portion of the US state of Michigan. The southern terminus is at a connection with State Road 933 across the Michigan–Indiana state line near South Bend, Indiana. From there the trunkline runs north through an interchange with US Highway 12 (US 12) into Niles along a route that was once part of Business US 12 (Bus. US 12). North of Niles, the highway runs parallel to a river and a rail line through rural areas. The northern terminus is on Interstate 94 (I-94) west of Paw Paw.