Horsethief Canyon in San Bernardino County, California, forms the upper part of Summit Valley near Cajon Pass. Beginning just east of the pass summit at about 3,800 feet and descending to a mouth near 3,123 feet, it lies within an important mountain passage in the region. California State Routes 138 and 173 run through the canyon, underscoring its role in regional travel. It is distinct from another canyon of the same name in San Diego County near Pine Valley and the Mexico–United States border.