Interstate 105 (I-105, locally referred to as the 105) is an east–west auxiliary Interstate Highway in the Greater Los Angeles urban area of Southern California. It runs from State Route 1 (SR 1) near El Segundo and Los Angeles International Airport (LAX) to Studebaker Road in the City of Norwalk. It is commonly known as the Century Freeway after Century Boulevard which it parallels, and also officially known as the Glenn Anderson Freeway after the late congressman Glenn M. Anderson who advocated for its construction.

The entirety of I-105 is defined in section 405 of the California Streets and Highways Code as Route 105, and that the highway is from "Pershing Drive near El Segundo to Route 605".

Route 105 was never fully constructed as it is defined. Currently, the western terminus of I-105 is at Sepulveda Boulevard (SR 1) and Imperial Highway on the southern edge of Los Angeles International Airport (LAX), near the El Segundo–Los Angeles line. Motorists can still continue west via Imperial Highway over conventional roadway to Pershing Drive. Caltrans actually set Route 105's mile 0 along Imperial Highway about 0.5 miles (0.80 km) west of SR 1, but that segment is neither signed as part of I-105 or included in the Federal Highway Administration (FHWA)'s Interstate Highway route logs. Caltrans does not have any intention to adopt Imperial Highway further west to Pershing Drive as part of the state highway system or construct an extension of I-105 over it. A portion of undeveloped land south of this section of Imperial Highway exists to facilitate a potential extension of the freeway.

East of Sepulveda Boulevard, I-105 intersects with I-405 (San Diego Freeway) at the Lennox–Hawthorne line, then continues east near the northern city limits of Hawthorne and the southern city limits of Inglewood. Upon reaching the Harbor Gateway neighborhood of Los Angeles, I-105 intersects with I-110 at the five-level Judge Harry Pregerson Interchange. I-105 continues east into Willowbrook before entering the city of Lynwood, where it has a four-level interchange with I-710 and crosses the Los Angeles River. I-105 then runs through Paramount before entering Downey. In Downey, I-105 intersects with Lakewood Boulevard (SR 19). I-105 then enters Norwalk, where it meets with I-605. Although Route 105's aforementioned state legal definition sets its eastern terminus at I-605, the HOV lanes extend approximately 0.3 miles (0.48 km) further east to an at-grade intersection with Studebaker Road; this segment is also included in both state and federal records.

The freeway never intersects its parent route, I-5. I-105 complies with numbering conventions by intersecting other auxiliary routes of I-5 that do intersect I-5, namely I-405 and I-605.

Much of the length of the Century Freeway runs parallel to Imperial Highway. It also runs parallel to (and 1 mi (1.6 km) south of) Century Boulevard, from which its original name is derived. Century Boulevard, in turn, is named for its position equivalent to 100th Street in the Los Angeles grid.

The Los Angeles Metro Rail C Line runs in the median of nearly the entire length of I-105. The C Line's eastern terminus is at Norwalk, at the interchange between I-105 and I-605. 2 mi (3.2 km) from the western end of the freeway, the C Line separates onto its own right-of-way at Aviation Boulevard before turning north towards the LAX area, where a future people mover connecting Metro Rail to airport terminals is scheduled to open in 2026.

I-105 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.