Los Angeles County, sometimes abbreviated as LA County, is the most populous county in the United States, with 9,757,129 residents estimated in 2024. Its population is greater than that of 40 individual U.S. states. Comprising 88 incorporated cities and 101 unincorporated areas within a total area of 4,083 square miles (10,570 km²), it accommodates more than a quarter of Californians and is one of the most ethnically diverse U.S. counties. The county's seat, Los Angeles, is the second most populous city in the United States, with 3,878,704 residents estimated in 2024. The county is globally known as the home of the U.S. motion picture industry since its inception in the early 20th century.

Los Angeles County is one of the original counties of California, created at the time of statehood in 1850. The county originally included parts of what are now Kern, San Bernardino, Riverside, Inyo, Tulare, Ventura, and Orange counties. In 1851 and 1852, Los Angeles County stretched from the coast to the state line of Nevada. As the population increased, sections were split off to organize San Bernardino County in 1853, Kern County in 1866, and Orange County in 1889.

Before the 1870s, Los Angeles County was divided into townships (many of which were amalgamations of one or more old ranchos):

As shown by the map below, Los Angeles County is bordered on the north by Kern County, on the east by San Bernardino County, on the southeast by Orange County, on the south by the Pacific Ocean, and on the west by Ventura County.

According to the U.S. Census Bureau, the county has an area of 4,751 square miles (12,310 km²), of which 4,058 square miles (10,510 km²) (85%) is land and 693 square miles (1,790 km²) (15%) is water. Los Angeles County borders 70 miles (110 km) of coast on the Pacific Ocean and encompasses mountain ranges, valleys, forests, islands, lakes, rivers, and desert. The Los Angeles River, Rio Hondo, Ballona Creek, the San Gabriel River and the Santa Clara River flow in Los Angeles County, while the primary mountain ranges are the Santa Monica Mountains and the San Gabriel Mountains. The western extent of the Mojave Desert begins in the Antelope Valley, in the northeastern part of the county.

Most of the population of Los Angeles County resides in the south and southwest, with major population centers in the Los Angeles Basin, San Fernando Valley, and San Gabriel Valley. Other population centers are found in the Santa Clarita Valley, Pomona Valley, Crescenta Valley and Antelope Valley.

The county is divided west-to-east by the San Gabriel Mountains, which are part of the Transverse Ranges of southern California, and are contained mostly within the Angeles National Forest. Most of the county's highest peaks are in the San Gabriel Mountains, including Mount San Antonio 10,068 feet (3,069 m) at the Los Angeles–San Bernardino county lines, Mount Baden-Powell 9,399 feet (2,865 m), Mount Burnham 8,997 feet (2,742 m) and Mount Wilson 5,710 feet (1,740 m). Several lower mountains are in the northern, western, and southwestern parts of the county, including the San Emigdio Mountains, the southernmost part of Tehachapi Mountains and the Sierra Pelona Mountains.

Los Angeles County includes San Clemente Island and Santa Catalina Island, which are part of the Channel Islands archipelago off the Pacific Coast.