San Bernardino County, officially the County of San Bernardino, is a county located in the southern portion of the U.S. state of California. As of the 2012 U.S. Census, the population was 2,081,313, up from 1,709,434 in the 2000 census. San Bernardino County is part of the Inland Empire area of Southern California, which also includes Riverside County. At just over 20,000 square miles (52,000 km2), San Bernardino County is slightly larger than the states of New Jersey, Connecticut, Delaware and Rhode Island combined. It is the largest county in the United States (Alaska has boroughs). It is the only county in California bordered by both Nevada and Arizona, and is one of only two counties in California bordering more than one U.S. state (the other being Modoc County, bordering Nevada and Oregon in the northeast corner of the state).

The Mojave National Preserve covers some of the eastern desert, especially between Interstate 15 andInterstate 40. The desert portion also includes the cities of Needles next to the Colorado River and Barstow at the junction in Interstate 15 and Interstate 40. Trona is at the northwestern part of the county west of Death Valley. This national park, mostly within Inyo County, also has a small portion of land within the San Bernardino County. The largest metropolitan area in the Mojave Desert part of the county is Victor Valley, with the incorporated localities of Adelanto, Apple Valley, Hesperia, and Victorville. Further south, a portion of Joshua Tree National Park overlaps the county near Twentynine Palms. Additional places near and west of Twentynine palms include Yucca Valley, Joshua Tree, and Morongo Valley.

The mountains are home to the San Bernardino National Forest, and include the communities of Crestline, Lake Arrowhead, Running Springs, Big Bear City, Forest Falls, and Big Bear Lake.

The San Bernardino Valley is at the eastern end of the San Gabriel Valley. The San Bernardino Valley includes the cities of Ontario, Chino, Chino Hills,Upland, Fontana, Rialto, Colton, Grand Terrace, Rancho Cucamonga, San Bernardino, Loma Linda, Highland, Redlands, and Yucaipa.