Metrolink (reporting mark SCAX) is a commuter rail system in Southern California, serving Los Angeles, Orange, Riverside, San Bernardino, and Ventura counties, as well as to Oceanside in San Diego County. The system consists of eight lines and 69 stations operating on 545.6 miles (878.1 km) of track. This includes Arrow, which Metrolink operates under a contract with the San Bernardino County Transportation Authority (SBCTA).

In 2024, the system had a ridership of 5,647,700, or about 21,300 per weekday as of the second quarter of 2025.

Metrolink connects with Los Angeles County's Metro Rail and Metro Busway systems, North County Transit District's Coaster commuter rail and Sprinter hybrid rail services, and with Amtrak's Pacific Surfliner, Coast Starlight, Southwest Chief, Sunset Limited, and Texas Eagle inter-city rail services. Metrolink owns several hundred miles of track; however, it also shares track with freight railroads.

The system, founded in 1991 as the Southern California Regional Rail Authority (SCRRA) and adopting "Metrolink" as its moniker, started operation in 1992. Average weekday ridership was 42,928 as of 2017. The SCRRA contracts with Alstom both to operate the trains and to maintain the rolling stock (locomotives and passenger cars).

In addition to suburban communities and cities, Metrolink also serves several points of interest such as downtown Los Angeles, downtown San Bernardino, Burbank, Hollywood Burbank Airport, Cal State LA, Angel Stadium, and the San Clemente Pier. Special service has also been extended to the Pomona Fairplex, the Ventura County Fairgrounds, and Auto Club Speedway for certain events.

Weekend service is offered on all routes except the Riverside Line.

The system currently consists of eight lines:

Metrolink's fare structure is based on a flat fee for boarding the train and an additional distance cost with fares calculated in 25-cent increments of driving distance between stations.