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. The highway was first signed in 1938 on much the same alignment as today. SR 108 replaced the SR 33 and SR 188 designation of the highway which dated back to 1923. SR 188 ran between Ottawa and Holgate and SR 33 ran between Holgate and the Michigan state line. Some of the highway was paved in 1923, with the rest of the route being paved in 1929.