State Route 18 (SR 18) in the U.S. state of California runs from State Route 210 in San Bernardino to State Route 138 near Adelanto. It is the primary route into the San Bernardino Mountains and has two discontinunities: one in Big Bear Lake, the other in Victorville.

SR 18 is part of the California Freeway and Expressway System and is eligible for the State Scenic Highway System. However, it has not been designated as a scenic highway by Caltrans.

The constructed portion of Route 18 begins at State Route 210 and quickly enters the San Bernardino Mountains as a four-lane expressway. SR 18 is known as Waterman Avenue in the city of San Bernardino before turning into SR 18. The route climbs north at a gentle grade until it turns east, where it begins to offer panoramic views of San Bernardino. Hence, this portion of Route 18 is known as the Rim of the World Highway. After meeting State Route 138 at Crestline, the expressway reverts to a two-lane mountain road. It meets two other highways, State Route 189 just east of Arrowhead Highlands and State Route 173 in Lake Arrowhead, before running over State Route 330 in Running Springs. SR 18 continues through several valleys and near ski resorts before reaching a summit. At that point, Route 18 again runs along a lengthy mountain ridge, offering more spectacular scenery at an even higher elevation. At its west junction with State Route 38 near the west corner of Big Bear Lake, Route 18 jogs across a dam and travels along the south shore of Big Bear Lake. After going through the City of Big Bear Lake, it overlaps with SR 38 (considered the first discontinuity since Big Bear Boulevard has no clear signage of where SR 18 turns off) and moves from the south to the north side of Bear Valley east of both Big Bear Lake and the Big Bear City airport before it descends from the mountains to the desert on the northerly side of the mountains, first heading northeast, briefly north, and then northwest.

SR 18 at this point follows the cities on the north face of the San Bernardino Mountains in the Mojave Desert: In Lucerne Valley, it intersects State Route 247 and then turns west for 21 miles (junctioning with major cutoff Bear Valley Road). It becomes an expressway (Happy Trails Highway, given for the High Desert's residency and final resting place of the legendary singing cowboy, Roy Rogers) through Apple Valley and Victorville. It joins with Interstate 15 inside of Victorville (this is considered a discontinuity, since the freeway entries refer to I-15 only and not State Route 18, which has partial route markings along the freeway, mainly on southbound I-15) for a few miles to the southeast and south before taking off west across the desert, reducing into a two-lane highway again outside of Victorville and meeting U.S. Route 395 in Adelanto, crosses into Los Angeles County as the beginning of county-named Pearblossom Highway and ending at State Route 138 east of Llano.

SR 18 thus takes an unusual path in the shape of a question mark. On most of the signs of the highway, cardinal descriptions are not posted below the route markers, since it does not clearly go either west/east or north/south.

Much of SR 18 has been signed since 1917, with the portion north of Big Bear Lake dating from 1933. Since 1963, State Route 18 has generally been legislatively defined as from Interstate 10 to State Route 138 going through San Bernardino through Waterman Canyon and near Big Bear Lake and Victorville. However, the section from Interstate 10 to present-day State Route 210 was never built, although it was designated as Legislative Route 26 in 1916 and Legislative Route 275 in 1959. In 1964, SR 18 was rerouted from the northern side of Big Bear Lake to the southern side along the former routing of SR 30 and lost its Legislative Route 43 designation. In 1984, the portion of SR 18 running concurrently with Interstate 15 in Victorville was removed from the legislative definition. In 1991, SR 18 was rerouted to Lakeview Drive and out of the downtown area of Big Bear Lake city proper, and this is now a business route of SR 18.

Before 1956, SR 18 continued south of San Bernardino to Lakewood, routed on what is now Interstate 215 and State Route 91 as well as Lakewood Boulevard. SR 18 then ran concurrently with State Route 19 to the western terminus of U.S. Route 6 in Long Beach.

Except where prefixed with a letter, postmiles were measured on the road as it was in 1964, and do not necessarily reflect current mileage. The numbers reset at county lines; the start and end postmiles in each county are given in the county column.