State highway in Bay and Tuscola counties in Michigan, United States
M-138 is a 20-mile-long (32 km) east–west state trunkline highway in The Thumb region of the US state of Michigan. The highway runs from a junction with M-15 west of the unincorporated community of Munger in Bay County to the intersection with M-24 east of Akron in Tuscola County. The trunkline runs through rural agricultural areas outside of the villages and communities it serves. It was designated in the 1930s and extended in the 1940s. M-138 was truncated in the 1990s as well.