Between Port Huron and Port Austin it is the north–south highway that used to be US 25 before the designation was removed from Michigan. Between Port Austin and Bay City it is an east–west route that appeared on some maps as US 25 and on some maps as a M-25. Since the 1970s, when all of US 25 was deleted north of Cincinnati, Ohio it is now entirely M-25.
North to Port Austin
The starting point of M-25 at a junction with Business Loop I-69/Business Loop I-94 (BL I-69/BL I-94) in Port Huron. M-25 is part of the Lake Huron Circle Tour for its entire length. From here M-25 heads north on Pine Grove Avenue until meeting M-136. At this intersection, M-25 turns north on 24th Avenue to Lakeshore Road then runs parallel to the Lake Huron shoreline.