State Route 231 (SR 231) is a 35.77-mile (57.57 km) long north–south state highway in the northwestern quadrant of the U.S. state of Ohio. SR 231 has its southern terminus at a CSX railroad crossing in the village of Morral. Its northern terminus is in downtown Tiffin at a signalized intersection with SR 18 and SR 101, following a 0.35 miles (0.56 km) long concurrency with SR 100, which continues north as a solo route north of this point.