Michigan 25, or M-25, is a highway of an arc-like shape closely following the "Thumb" of the eastern Lower Peninsula of Michigan along Lake Huron between Port Huron, Michigan (junction Interstate 94, Interstate 69, and Business Loop 94) and Bay City, Michigan (junction Interstate 75, U.S. Highway 23, and U.S. Highway 10), the latter into which it feeds. It serves the lakeshore resorts along Lake Huron and Saginaw Bay and generally lies within sight of the lake and the bay. All is surface road, generally scenic (by Michigan standards, that is; don't expect it to resemble California State Highway 1) except for the freeway segment near the junction with Interstate 75 and feeds into the US-10 freeway.

Between Port Huron and Port Austin it is the north-south highway that used to be U.S. Highway 25 before it was decommissioned. 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 (Michigan) state 25. Since the 1970s, when all of US-25 was decommissioned north of Cincinnati, Ohio it is now entirely M-25.

Most of it is part of the Lake Huron Circle Tour.

The highway largely serves such lakeside communities, aside from those already mentioned, as Lexington, Port Sanilac, Harbor Beach, and Sebewaing