Interstate 405 (I-405, locally referred to as the 405) is a major north–south auxiliary Interstate Highway in Southern California. The entire route is known as the northern segment of the San Diego Freeway. I-405 is a bypass auxiliary route of I-5, running along the southern and western parts of the Greater Los Angeles urban area from Irvine in the south to Sylmar in the north.

I-405, heavily traveled by both commuters and freight haulers along its entire length, is the busiest and most congested freeway in the United States. The freeway's annual average daily traffic between exits 21 and 22 in Seal Beach reached 374,000 in 2008, making it the highest count in the nation. It has played a crucial role in the development of dozens of cities and suburbs along its route through Los Angeles and Orange counties. It also serves Los Angeles International Airport, Long Beach Airport, and Orange County's John Wayne Airport.

The entirety of I-405 is defined in section 615 of the California Streets and Highways Code as Route 405, and that the highway is from "Route 5 near El Toro to Route 5 near San Fernando". This definition corresponds with the Federal Highway Administration (FHWA)'s route logs of I-405.

I-405 is part of the California Freeway and Expressway System and is part of the National Highway System, a network of highways that are considered essential to the country's economy, defense, and mobility by the Federal Highway Administration. The entire freeway is known as the San Diego Freeway, and parts of it are less commonly known as the Sepulveda Freeway (after Sepulveda Boulevard). In 2024, the California State Legislature passed Assembly Bill 2698, adding subdivision (b) to section 615 of the California Streets and Highways Code that designates the segment from Bolsa Chica Road to Bolsa Avenue near the Little Saigon district in Orange County as the Little Saigon Freeway, in recognition of it being the oldest and largest Little Saigon enclave in the country.

I-405 begins at the El Toro Y interchange in southeastern Irvine in Orange County, splitting from its parent I-5 and inheriting that route's San Diego Freeway title; I-5 continues north as the Santa Ana Freeway. The freeway passes immediately south of the Irvine Spectrum Center mall before intersecting with State Route 133 (SR 133). It then continues through Irvine, passing north of the University of California, Irvine, and then along the northern boundary of John Wayne Airport. After passing the airport, the freeway enters Costa Mesa and has an interchange with SR 55. It passes South Coast Plaza before a partial interchange with SR 73, which serves as a partially-tolled bypass of I-405 between Costa Mesa and Laguna Niguel.

The freeway then travels through Fountain Valley and along the edges of Westminster and Huntington Beach before entering Seal Beach, where it begins to run concurrently with SR 22. It continues along the northern edge of Seal Beach, passing between Naval Weapons Station Seal Beach and Joint Forces Training Base - Los Alamitos, before SR 22 splits from I-405 and continues west while the freeway turns north. I-405 then intersects the southern end of I-605 before crossing the San Gabriel River and entering Los Angeles County.

I-405 enters Los Angeles County in the city of Long Beach. It passes to the north of California State University, Long Beach, and then along the south of Long Beach Airport. The freeway then intersects with I-710 before entering Carson (and crossing through a small sliver of the city of Los Angeles before reentering Carson). It passes near California State University, Dominguez Hills, and Dignity Health Sports Park, home of Major League Soccer club LA Galaxy. A weigh station for both directions is located in Carson between the Avalon Boulevard and Main Street exits.

The freeway then intersects with I-110 as it briefly reenters the city of Los Angeles by passing through the Harbor Gateway, a strip of land connecting San Pedro to the rest of the city. I-405 then continues to roughly parallel the contour of the coastline as it passes through the South Bay communities of Torrance, Lawndale, Redondo Beach, Hawthorne, and El Segundo. The freeway then encounters I-105 on the southeastern corner of Los Angeles International Airport. It passes to the east of the airport, serving it with exits at the Imperial Highway and Century Boulevard.