State Route 137 (SR 137) is a route that runs for 56.25 miles (90.53 km) west to east that begins at the intersection of U.S. Route 2 (US 2) in Mercer and covers a large expanse of land, ending in the city of Belfast at an interchange with US 1 and SR 3.

SR 137 is the main road in the communities of Smithfield and Oakland, near the western terminus of the route.

[2] East of Waterville and Winslow, it meets US 202 and SR 9 in the town of China where they form a concurrency along one road until an intersection in the town of Albion.

[2] State Route 137 Business (SR 137 Bus.)

is a business route through the towns of Waterville and Winslow.