M-31 was a state trunkline highway in the Lower Peninsula in the US state of Michigan. It generally ran north from Port Huron along the Lake Huron shoreline through The Thumb region before turning inland. The highway crossed The Thumb and then ran along the Saginaw Bay shoreline before running inland again, terminating at Saginaw. It was one of the original state highways signposted in 1919, but it was renumbered as other highways in 1926, decommissioning the designation in the process. Parts of its route are now M-25, M-81 and M-142.