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. There were two other highways that bore the M-51 designation. The first connected Holland and Grand Rapids with the birth of the highway system in 1919. After the creation of the United States Numbered Highway System in 1926, the number was moved to a different highway in The Thumb area. That second highway was scaled back and later decommissioned in the 1960s. The current highway dates back to 1971 when the southern end of M-40 was rerouted, and the previous alignment was given the M-51 moniker. It was extended to the state line in 1998 to complete the current highway. Route description M-51 starts at the Indiana state line as a continuation of SR 933 into Michigan. The trunkline runs north through residential areas along 11th Street into Niles. On the south side of town, it meets US 12 at an interchange before continuing northward through commercial areas. Between the intersections with Maple and Main streets, M-51 splits into a one-way pair of streets to follow 12th Street northbound and 11th Street southbound. Oak Street marks the western end of Bus. M-60, and M-51 turns westward on Main Street into downtown Niles. At 5th Street, the highway turns north again to exit downtown. The highway crosses a rail line owned by Amtrak before curving northeasterly near the Plym Park Golf Course. M-51 angles parallel to the Dowagiac River and the Amtrak line as the highway crosses from Berrien County into Cass County. The landscape transitions to farmland along the river, and the highway turns to the east between Sumnerville and Pokagon. Past Pokagon, M-51 turns back northeasterly toward Dowagiac. Once the highway reaches that city, it merges with M-62. The two highways run concurrently eastward along Spruce, Main and Division streets through downtown. M-51 turns to the north along Front Street, separating from