State Route 138 is an east–west state highway generally following the northern foothills of the San Gabriel Mountains of southern California, USA from its junction with Interstate 5 south of Gorman eastward to Mount Anderson Junction, its eastern junction with State Route 18 south of Crestline in the San Bernardino Mountains. Except for the western two miles (3 km) of the route between Interstate 5 and just east of Gorman Post Road and a segment shared with State Route 14 between Avenue D in Lancaster and Palmdale Boulevard in Palmdale, it is all surface road, mostly undivided two-lane road. The remaining section of the Ridge Route, California's first highway connecting the San Joaquin Valley to the Los Angeles Basin, ends at 138 in Gorman.