U.S. Route 66 (US 66) is a former US Highway in the U.S. state of California. It was truncated during the 1964 renumbering and its signage removed in 1974.

U.S. Route 66 was assigned by the American Association of State Highway Officials in late 1926 and signed in 1928 by the Automobile Club of Southern California; however historic US 66 shields and even present day US 66 shields remain.

US 66 continued to be signed east of Pasadena until 1974, when it was removed, and the remaining separate section became State Route 66.

In 1977, Hotel California (song) alluded to Route 66 in its opening lines, "On a dark desert highway, cool wind in my hair, Warm smell of colitas rising up through the air, Up ahead in the distance, I saw a shimmering light, My head grew heavy and my sight grew dim, I had to stop for the night". According to Eagles guitarist Don Felder, "Everybody had driven into Los Angeles on what used to be Route 66. And as you drive in through the desert at night, you can see the glow of Los Angeles from a hundred miles away. The closer and closer you get, you start seeing all of these images, and these things pounded into our heads: the stars on Hollywood Boulevard, movie stars, palm trees, beaches and girls in bikinis."

Nationally, Route 66 has been a decommissioned highway since 1985, with the last section through Williams, Arizona bypassed by Interstate 40 in 1984. The first efforts to return the route to maps as "Historic Route 66" date to 1987 and Angel Delgadillo's Arizona Historic Route 66 Association. This initiative was soon followed in all eight US Route 66 states, including California.

The California Historic Route 66 Association, established in December 1990 to advocate the preservation, restoration and promotion of historic Route 66 in California, is the youngest of the eight state-level Route 66 Associations.

From San Bernardino to the Arizona state line, US 66 followed the old National Old Trails Highway. The old highway veers significantly away from I-15 between Victorville and Barstow passing through Oro Grande, Helendale, and Lenwood. Through Barstow, it is Main Street. East of Barstow, the National Old Trails Highway passes through a Marine Corps base, limiting public access and forcing traffic onto I-40. From Daggett, Historic 66 leaves I-40, crossing it three times before winding away through Bagdad, Amboy, and Essex.

This area is desert; towns such as Amboy originated as Atlantic and Pacific Railroad stops, were sustained by Route 66 traffic during the Mother Road's heyday, then became ghost towns when I-40 bypassed them using a more northerly path. From Essex, the original alignment followed Goffs Road through Goffs, joining I-40 at the U.S. Route 95 exit. A later alignment is now I-40 from Goffs. The original highway winds around I-40 in the Needles area, before crossing the Colorado River into Arizona.

At the time of the 1964 renumbering, US 66 used the following route (in today's terms):

U.S. Route 66 originally exited San Bernardino on Mount Vernon Avenue and Cajon Boulevard, which meandered its way up to Cajon Pass. U.S. Route 66 originally followed 7th Street and D Street through Victorville then the National Old Trails Highway through Barstow, California (where it took on the name "Main Street") and across the Mojave Desert to Needles. US 66 followed Broadway Street through Needles, then crossed into Arizona on the Trails Arch Bridge (1916-1946) or the Red Rock Bridge (converted from railroad bridge in 1946, abandoned 1966, dismantled 1976).

By 1964, most or all of this route had been replaced by Interstate highway:

Roy's Motel and Café was once the largest roadside business in what is now the desert ghost town of Amboy, California. The motel has been closed for years as the town died when Interstate 40 in California diverted traffic ten miles further north. Amboy, as a ghost town, had been used as a filming location for various movies. More recently, the Roy's filling station and café have operated at least sporadically in an attempt to preserve this landmark for tourism.

The Summit Inn, a diner and filling station originally located in 1928 at the summit of the Cajon Pass, moved to Oak Hills, California when the road was re-routed in 1952. Its visitors include Elvis Presley, reported to have kicked the jukebox and left without dining after finding none of his own records among the available selections. This oversight has since been corrected; the diner remains open, complete with memorabilia of the roadside Texaco which is no longer in operation at the site.

Route 66 has attracted campers since the Great Depression era, where The Grapes of Wrath describes a large but primitive riverside campground as one of the first sights when arriving in Needles, California from Arizona. Needles later became the site of the Carty's Camp cabins seen briefly in The Grapes of Wrath (film) (now a deteriorating ghost tourist court) and the later adjacent 66 Motel (Needles) (which currently offers long-term rental only).

The 1924 Aztec Hotel in Monrovia, a National Historic Landmark in the San Gabriel Valley, is notable for its Mayan Revival architecture.

One of three restored Wigwam Motels accommodates motorists in San Bernardino near Rialto. Based on a once-patented novelty architecture, these are tourist courts in which each cabin is a free-standing concrete wigwam. This group of motels served as an inspiration for the Cozy Cone Motel in Cars (film).

California devotes a pair of museums to the history of old Route 66, the California Route 66 Museum in the former Red Rooster Café in Victorville and the Barstow Route 66 "Mother Road" Museum in the town's former Harvey House Railroad Depot.

The Figueroa Street Tunnels carry the Arroyo Seco Parkway through Elysian Park in Los Angeles; a Four Level Interchange connecting the Hollywood Freeway (Route 101) to Route 66 was the first stack interchange in the world.

The historic 1916 Trails Arch Bridge spanning the Colorado River from Topock, Arizona to Needles, California still stands but is no longer open to traffic.