In the U.S. state of California, U.S. Route 101 (US 101) is one of the last remaining and longest U.S. Routes still active in the state, and the longest highway of any kind in California. US 101 was also one of the original U.S. Routes established in 1926.

Although the highway has been superseded in overall importance for transport through the state by Interstate 5, US 101 continues to be the major coastal north–south route that links the Greater Los Angeles Area, the Central Coast, the San Francisco Bay Area, the North Coast(Redwood Empire), and the southern border of coastal Oregon. Significant portions of US 101 between the Los Angeles area and the San Francisco Bay Area follow El Camino Real, the historic road connecting the former Alta California's 21 missions. US 101 also has portions designated as the Santa Ana Freeway, Hollywood Freeway, Ventura Freeway, South Valley Freeway, and the Bayshore Freeway. The Redwood Highway, the 350-mile-long (560 km) northernmost segment of the highway, begins at the Golden Gate and passes through the world's tallest and only extensive preserves of virgin, old-growth coast redwood trees.

This route is part of the California Freeway and Expressway System.

The south terminus of U.S. Route 101 is in Los Angeles, about one mile (1.6 km) east of downtown Los Angeles at the East Los Angeles Interchange, also known as the "Commuters' Complex." This southernmost portion is named the Santa Ana Freeway, inheriting that title as the northerly extension of the roadway now known as Interstate 5.

After merging with westbound traffic from the San Bernardino Freeway (I-10), U.S. Route 101 then proceeds northwest via the Downtown Slot under the northern edge of Los Angeles' Civic Center to State Route 110 at the Four Level Interchange. From here, US 101 becomes the Hollywood Freeway. It then heads to Hollywood and up through the Cahuenga Pass before reaching the San Fernando Valley.

Highway 101 then intersects with State Route 134 and State Route 170 at the interchange known as the Hollywood Split. Here, the alignment of US 101 'shifts' to the alignment of SR 134 (i.e. heading northbound, the road's alignment turns left, or westbound) and thereafter is referred to as the Ventura Freeway until it reaches Ventura. Confusingly, the "Hollywood Freeway" name continues northward from this interchange on SR 170, and the "Ventura Freeway" name continues eastward to SR 134.

From the Hollywood Split, US 101 is an east–west highway until it reaches Gaviota State Park in Santa Barbara County where it shifts back to a north–south alignment. Along the way, it meets with Interstate 405. This interchange was named the worst interchange for traffic in the United States. The east–west geographical alignment of the Ventura Freeway and the north–south designation on freeway signs can be confusing to visitors; the same freeway entrance can often be signed as "101 North" and "101 West"; this is most common in the San Fernando Valley where the City of Los Angeles' local E/W signing does not match the Caltrans' proper statewide N/S designation.

Upon reaching Ventura, there is an interchange with SR 126, aka the Santa Paula Freeway.

North of Ventura, US 101 switches intermittently between freeway and expressway status (i.e. there is occasional cross-traffic), but there are no traffic signals until San Francisco. The last traffic signals along this stretch of the route were removed in the early 1990s when the section through downtown Santa Barbara was constructed to freeway standards after years of resistance from the local community.

From Ventura and through Santa Barbara, Highway 101 closely follows the Pacific coastline (generally no more than one to two miles (3 km) from the shore) until Gaviota State Park, about 23 miles (37 km) west of Goleta. At Gaviota State Park, the highway shifts back from an east–west highway to a north–south alignment. About 1-mile (1.6 km) north of this point, US 101 passes through the Gaviota Tunnel.

A few miles north of the Gaviota Tunnel, State Route 1 splits from US 101 and heads northwest, running along the Pacific coastline parallel and to the west of US 101.

Farther north, State Route 1 rejoins 101 for a section between Pismo Beach and San Luis Obispo. Then US 101 takes an inland route through the Salinas Valley, while Highway 1 heads northwest, running along the Pacific coastline in California, parallel and to the west of US 101.

A steep segment (7% grade) between San Luis Obispo and Atascadero is known as the Cuesta Grade. North of Atascadero, the highway joins State Route 46 (SR 46) for about three miles (5 km) through Paso Robles.

From Paso Robles to Salinas, US 101 is an expressway known as the Salinas River Valley Highway, since the Salinas River Valley extends from Santa Margarita to the State Route 156 junction in Prunedale. The highway passes Camp Roberts south of Bradley as well as the San Ardo Oil Field about five miles (8 km) south of the town of San Ardo. Near this point, the wide agricultural bottomlands of the Salinas Valley begins. Upon leaving Salinas, after going past the southern terminus of State Route 156, US 101 remains mostly a four-lane highway until it reaches Gilroy.

When US 101 reaches Gilroy, it becomes the South Valley Freeway, and at the same time, it expands to three lanes in each direction. It then heads through the suburb of Morgan Hill, but shortly afterwards expands to four lanes in each direction before reaching San Jose. From San Jose to San Francisco, Highway 101 is known as the Bayshore Freeway as it passes through Palo Alto and the other major communities along the San Francisco Peninsula. This segment was originally Bayshore Boulevard, later redesignated the 101A bypass and upgraded to freeway by the late 1950s. This section of 101 from San Jose north to San Francisco is very heavily travelled. The stretch between Redwood City and South San Francisco was the Bay Area's first freeway when it opened in 1947; over the next 20 years the stretches from San Jose to Redwood City and South San Francisco to San Francisco were upgraded to freeway.

Highway 101 is called the James Lick Freeway, named for James Lick, a philanthropist, from the San Francisco county/city line, through the interchange with I-280 at the Alemany Maze, until the junction with the San Francisco Skyway (Interstate 80) and the Central Freeway near the city's Civic Center. Highway 101 continues in a northwestern direction on the Central Freeway, and then leaves the freeway to run north on Van Ness Avenue. At the intersection of Van Ness Avenue and Lombard Street, Highway 101 heads west on Lombard Street, and then on Richardson Avenue, entering The Presidio, where it becomes a divided highway again (the partially built Presidio Parkway). It is then joined by Route 1 before approaching and crossing the Golden Gate Bridge.

From San Francisco north the highway is heavily traveled by commuters through to Windsor, just north of Santa Rosa. North of the Golden Gate Bridge, US 101 enters Marin County and is known as the Redwood Highway. After crossing the bridge, US 101 climbs the Waldo Grade and passes through the Waldo Tunnel, the first of its kind in 300 miles (the other being the Gaviota Tunnel in Santa Barbara County), before passing through San Rafael and Novato. The section between Novato in Marin County and Petaluma in Sonoma County changes from its original six or eight lanes through Marin to two lanes, creating a bottleneck, and is thus called the "Novato Narrows" by locals. The Narrows continue up to North of Petaluma. The entire area between Novato and Petaluma is also the voter approved Novato–Petaluma Community Separator, which forbids most development.

Upon entering Sonoma County it continues as a four lane freeway until Santa Rosa, where it turns into a six-lane freeway to Windsor. (In Cotati and Rohnert Park, the freeway is under construction, being expanded from four lanes to six.) After leaving Healdsburg, it proceeds up the Alexander Valley with the Russian River before leaving Sonoma County and entering Mendocino County.

The freeway portion ends north of Cloverdale near the Mendocino County line, and it narrows to an expressway through the Russian River canyon and eventually a two-lane road south of Hopland, the first time since leaving San Francisco. In the community of Calpella, State Route 20 merges with US 101, where it will remain that way until reaching the city of Willits. South of Willits, US 101 ascends 1,953-foot (595 m) Ridgewood Summit, the highest elevation along the route's 807-mile (1,299 km) trek through California. SR 20 then splits from US 101 in downtown Willits.

US 101 leaves Willits as a two-lane road, but it soon widens to a four-lane expressway until the intersection with the western portion of State Route 162, where it reverts to a two-lane road. North of Laytonville, US 101 ascends Rattlesnake summit (1,796-foot (547 m)) before a descent to follow the South Fork Eel River and Eel River all the way to Fortuna near the mouth of the river. About 12 miles (19 km) northwest of Laytonville, US 101 becomes an undivided freeway near the community of Cummings. SR 271 is the old portion of US 101 through this area. At Leggett, US 101 meets SR 1 for the last time, and from this point until Piercy, US 101 runs along a section of highway with frequent landslides. Cal Trans bypassed the most difficult section in 2009 with unique construction of two bridges known collectively as the Confusion Hill Bridges. This project, funded by an emergency act from the State Legislature, moved the highway across the Eel River away from the troubled spots to prevent disruption in commerce and travel from infrequent, but costly, winter closures on the main transportation route to the far North Coast. North of Piercy, the freeway portion again ends and the road narrows down to two lanes, before another stretch of divided highway.

Arriving in Humboldt County, another narrow two lane portion of highway 101 bisects Richardson Grove State Park. Just after the park boundary, the highway becomes a divided freeway, which continuing north by northwest, passes through the 53,000-acre (210 km2) Humboldt Redwoods State Park, California's third largest State Park and the site of the largest remaining Redwood old growth forest in the world. A preserved portion of the original, bypassed highway route, known as the Avenue of the Giants for the huge, centuries-old redwood trees, parallels the highway for over 30 miles (48 km) in southern Humboldt County. There is also a section of freeway through Rio Dell.

After the junction for State Route 36 (also the western terminus for that route), US 101 becomes a freeway again between Fortuna and Eureka. North of Humboldt Hill, the road enters the City of Eureka (a potential bypass of the city was successfully blocked repeatedly). As the route traverses Eureka, the southern portion is known as "Broadway" and then as it bears east along Humboldt Bay, the Highway is aligned on a one-way couplet (4th and 5th Streets). Five miles later the highway leaves Eureka's northern city limit and continues north. The expressway style section between Eureka and Arcata, which is also a safety corridor, is named the "Michael J. Burns Freeway," in honor of the State Senator who was a proponent of California's Highways. The Highway becomes an unobstructed Freeway south of the center of Arcata. Proceeding north it passes the junction for State Route 299 (also the western terminus for that route), in the Valley West (northernmost) part of the college town.

The highway continues north as it skirts westerly around McKinleyville on a high bluff north of the Mad River. As the highway reaches Clam Beach (a county park), motorists get their first magnificent full view of the Pacific Ocean north of the Golden Gate. North of Trinidad, the highway narrows to one lane in each direction after crossing Big Lagoon to pass inland of Stone Lagoon and follow the coastal bar between Freshwater Lagoon and the Pacific Ocean south of Orick, California. After a 14-mile (23 km) freeway through Redwood National and State Parks running inland east of the Prairie Creek Redwoods State Park boundary, the highway again narrows to one lane in each direction at the Klamath River. The original placement of the highway near giant Coast Redwoods led to increased awareness of the destruction of the redwoods after decades of extensive logging, which ultimately led to the establishment of Redwood National Park in 1968. The original highway segment through Prairie Creek Redwoods State Park is now a scenic alternate similar to Avenue of the Giants, named Newton B. Drury Scenic Parkway in honor of the fourth director of the National Park Service and executive director of the Save-the-Redwoods League

North of the town of Klamath just inside Del Norte County, the highway closely follows the Pacific coast again. In Crescent City, US 101 once again separates into a one-way couplet (L and M Streets) for nine blocks. As it leaves Crescent City, US 101 becomes a divided freeway for the last time in California (built slightly to the west of the original two-lane alignment, now called Parkway Drive). As the 3-mile divided freeway portion ends, US 101 intersects the southern terminus of U.S. Route 199, which heads northeast as the Redwood Highway, terminating in Grants Pass, Oregon. Highway 101 (no longer called the "Redwood Highway" at this point) is reduced to two lanes and continues north along the California coast until it reaches the Oregon border.

Instead of terminating in Los Angeles, Highway 101 used to travel all the way south through San Diego to the United States–Mexico border in San Ysidro. However, this part was deleted on July 1, 1964 in favor of Interstate 5. The only remnant of the old route is a mileage sign at the Santa Barbara/Ventura County line, which lists the distance to San Diego, even though Highway 101 ends in Los Angeles.

The old alignment in San Diego County from Oceanside to Del Mar is now known as San Diego County Route S21. It is signed unofficially in many places as "Historic Route 101". Before I-5's implementation in the 1960s, long sections of 101 in this area (through USMC's Camp Pendleton and south to the northern reaches of San Diego) were three lanes, with the center lane being a 'passing lane' for both northbound and southbound drivers. Horrific and fatal head on collisions were not uncommon. In newspaper accounts, it was often referred to as the "suicide lane".

Significant portions of 101 were originally known as the Royal Road or El Camino Real. The name, El Camino Real, continues in widespread use from South San Francisco to San Diego for surface routes, most of which are close to, and parallel to 101.

The primary control city that is listed on freeway signs along northbound 101 through the Central Coast region is San Francisco. Although San Jose surpassed San Francisco population decades after the highway was built, there has been no push to change all the signs.

Under the California Streets and Highways Code § 401, the Golden Gate Bridge is legally not part of Highway 101. The portion of Route 101 starting from Los Angeles ends at "the approach to the Golden Gate Bridge" and then resumes at "a point in Marin County opposite San Francisco" to the Oregon state line. The bridge itself is maintained by the Golden Gate Bridge, Highway and Transportation District instead of Caltrans.

A segment of the Redwood Highway, US 101, located in the Redwood National and State Parks near Klamath was added to the National Register of Historic Places in 1979.

In 2003, the segment of US 101 between Morgan Hill and San Jose, also known as the Sig Sanchez Freeway, expanded to eight lanes between Cochrane Road and SR 85 exits. Originally, the ten-mile segment was only two lanes in each direction. The improved segment is to alleviate the consistent congestion that has expanded as far south as Masten Avenue coming from Gilroy, and as far north as Bernal Road coming from San Jose.

The US 101 Highway is part of the auto tour route of the Juan Bautista de Anza National Historic Trail, a National Park Service unit in the United States National Historic Trail and National Millennium Trail programs.

In 2005, Caltrans began posting signs on roads that overlap with the historic 1776 Juan Bautista de Anza trail route, so that California drivers can now follow the trail.

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.