Route 138 is a 43.58-mile-long (70.14 km) north–south state highway in Massachusetts.

From the state line in Tiverton, Rhode Island to Milton, Route 138 runs as an extension of Rhode Island Route 138, which is itself an extension of Connecticut Route 138.

Route 138 meets the western terminus of Route 139 at the beginning of an extremely short concurrency with Route 27 in downtown Stoughton, before crossing into Canton and meeting Interstate 93/U.S.

The route finally enters Milton, travels by Curry College and the Neponset Valley Parkway, and terminates at Route 28, directly before that route crosses the Neponset River and enters Boston.

During the Blizzard of 1978, Route 138's intersection with Interstate 93 (which at the time still had Route 128 concurrent with it) gained some notoriety as the location shot of photos taken featuring cars trapped in the snow along the underpassing freeway.