The Los Angeles River (also known as the L.A. River) is a river that starts in the San Fernando Valley, in the Simi Hills and Santa Susana Mountains, and flows through Los Angeles County, California, from Canoga Park in the western end of the San Fernando Valley, nearly 48 miles (77 km) southeast to its mouth in Long Beach. Several tributaries join the once free-flowing and frequently flooding river, forming alluvial flood plains along its banks. It now flows through a concrete channel on a fixed course.

Environmental groups and park advocates support the removal of concrete and the restoration of natural vegetation and wildlife. There are also plans for a series of parks along the river's city frontage in Los Angeles. The Los Angeles River also flows through several Los Angeles County communities and has been featured in many Hollywood films.

Before the opening of the Los Angeles Aqueduct, the river was the primary source of fresh water for the city. Although the Los Angeles region still gets some of its water from the river and other local sources, most comes from several aqueducts serving the area. The river suffers pollution from agricultural and urban runoff.

Los Angeles (ⁱ/lɒs ˈændʒələs/ loss-AN-jə-ləs; Spanish: [los ˈaŋxeles], which is written Los Ángeles; British pronounciation /lɒs ˈændʒəliːz/ loss-AN-jə-leez) with a population at the 2010 United States Census of 3,792,621, is the most populous city in the state of California, and the second most populous in the United States, after New York City. It has an area of 468.67 square miles (1,213.8 km²), and is located in Southern California. Often known by its initials L.A., the city is the focal point of the larger Los Angeles-Long Beach-Santa Ana metropolitan statistical area and Greater Los Angeles Area region, which contain 12,828,837 and nearly 18 million people respectively as of 2010, making it one of the most populous metropolitan areas in the world and the second largest in the United States. Los Angeles is also the seat of Los Angeles County, the most populated and one of the most ethnically diverse counties in the United States, while the entire Los Angeles area itself has been recognized as the most diverse of the nation's largest cities. The city's inhabitants are referred to as "Angelenos".