State Highway 51 is a 92 mile (148 km) road in southwestern Idaho from Mountain Home south to the Nevada border, where it continues as State Route 225 to Elko. At its northern terminus in Mountain Home, SH-51 connects at the junction of U.S. Highway 20 and Interstate 84. It is the major north–south road in Owyhee County.

SH-51's northern terminus is at the east edge of the city of Mountain Home in Elmore County. The road crosses the Snake River into Owyhee County and passes the towns of Bruneau, Grasmere, and Riddle; about 11 miles (18 km) from the state border it enters the Duck Valley Indian Reservation.

As the road crosses into Nevada it becomes State Route 225; the town of Owyhee is about four miles (7 km) south of the state border, still within the reservation. After another 95 miles (153 km) through Elko County, SR-225 connects with Interstate 80 in Elko.

Elevation on the highway ranges from a low of 2461 feet (750 m) above sea level at the Snake River crossing, to a high of 5400 feet (1646 m) at the Nevada border.