M-125 is a 19+1⁄2-mile-long (31.4 km) state trunkline highway in the US state of Michigan. The highway is entirely within Monroe County with the southern terminus on the Ohio state border near Toledo and a northern terminus at US Highway 24 (US 24) in Frenchtown Township, 5 miles (8.0 km) north of Monroe. M-125 runs through rural farmlands and connects a couple of smaller towns with Monroe. It has an unsigned connector highway that links the main highway with Interstate 75 (I-75).

Monroe County is a county in the U.S. state of Michigan. As of the 2020 Census, the population was 154,809. The largest city and county seat is Monroe. The county was established as the second county in the Michigan Territory in 1817 and was named for then-President James Monroe. Monroe County is coterminous with the Monroe metropolitan statistical area.