State Route 138 (SR 138) is part of Maine's system of numbered state highways, located entirely in Sagadahoc County.

It runs from U.S. Route 201 (US 201) in Bowdoin, to Interstate 295 (I-295) while running concurrent with SR 125 for one mile (1.6 km).

SR 138 begins at an intersection with US 201 in Bowdoin.

It heads east towards I-295, and SR 125 in Bowdoinham, where the two state routes run concurrent for one mile (1.6 km).

The entire route is in Sagadahoc County.