State Route 108 (SR 108) is a 49.30-mile (79.34 km) state route that runs between SR 15 and the Michigan state line in the US state of Ohio. Most of the route is a rural two-lane highway and passes through both farmland and residential properties.

State Route 15 is a north–south and east–west route in northwestern Ohio. Its southern (eastern) terminus is at its interchange with U.S. Route 23/State Route 103 (US 23/SR 103) near Carey, and its northern (western) terminus is at the Michigan state line north of Pioneer, where the route continues in Michigan as M-99. The route is signed east–west from Carey to Bryan, and it is signed north–south from there to the Michigan state line.