State Route 173 (SR 173) is a state highway in the U.S. state of California that runs entirely in San Bernardino County, mostly in the San Bernardino National Forest. Its west end is at State Route 138 near the west end of Silverwood Lake in the Summit Valley south of Hesperia. Its east end is at State Route 18 south of Lake Arrowhead. The route starts at the Mojave River Forks, skims the easterly and southerly sides of Lake Arrowhead and meets State Route 189, Lake's Edge Road, at the south entrance to the Lake Arrowhead Village.