The Mad River (Shawnee: Hathennithiipi ) is a stream located in the west central part of the U.S. state of Ohio. It flows 66 miles (106 km) from Logan County to downtown Dayton, where it meets the Great Miami River. The stream flows southwest from its source near Campbell Hill through West Liberty, along U.S. Route 68 west of Urbana, past Springfield (the point of confluence with Buck Creek), then along Ohio State Route 4 into Dayton. The stream's confluence with the Great Miami River is in Deeds Park.