State Route 251 (SR 251, now Northport–Boundary Road) is a former 10.86-mile (17.48 km) long state highway in Stevens County, Washington. The highway began at SR 25 in Northport and continued northeast parallel to the Columbia River to Boundary, an unincorporated community, where it crossed the Canada – United States border into British Columbia as British Columbia Highway 22A. SR 251 was originally a county road until 1913, when it was added to the state highway system, but was later removed.