State Route 251 (SR 251) is a north–south state highway located in the southwestern portion of the U.S. state of Ohio. Its southern terminus is at U.S. Route 50 (US 50) about one mile (1.6 km) south of St. Martin in northern Brown County. The northern terminus of SR 251 is at US 68 nearly two and a half miles (4.0 km) northwest of St. Martin in southwestern Clinton County.