State Route 251 (SR 251, OH 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 about 1 mile (1.6 km) south of St. Martin in northern Brown County. The northern terminus of State Route 251 is at U.S. Route 68 nearly 2.50 miles (4.02 km) northwest of St. Martin in southwestern Clinton County.