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's northern terminus is at Route 28 in Milton.

Milton is a town in Norfolk County, Massachusetts, United States and an affluent suburb of Boston. The population was 28,630 at the 2020 census. Milton is the birthplace of former U.S. President George H. W. Bush, and architect Buckminster Fuller. Milton was ranked by Money as the 2nd, 7th, 8th, and 17th best place to live in the United States in 2011, 2009, 2019, 2021, and 2022 respectively.