M-103 is a state trunkline highway entirely within St. Joseph County in the southwestern region of the U.S. state of Michigan. The trunkline runs between U.S. Highway 12 (US 12) in Mottville and the Indiana state line along undivided highway near the St. Joseph River. With the connection to State Road 15 (SR 15), it provides access to the Indiana Toll Road. The current highway is the second to carry the number; the first version of M-103 has been incorporated into the routing of US 131.