State Route 138 (SR 138) is an east–west state highway in the U.S. state of California that generally follows the northern foothills of the San Gabriel Mountains and the western Mojave Desert. The scenic highway begins in the west at its junction with Interstate 5 located south of Gorman in the Sierra Pelona Mountains, continues eastward through the Antelope Valley and Cajon Pass, to its junction with State Route 18 in the east, located in the San Bernardino Mountains south of Crestline. Except for the western two miles (3.2 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 a mostly undivided two-lane surface road. The remaining section of the Ridge Route, California's first highway connecting the San Joaquin Valley to the Los Angeles Basin, ends at Route 138 near Gorman.
