Claim: Riverside county is in Southern California.

Southern California , often abbreviated as SoCal , is a geographic and cultural region that generally comprises California 's 10 southernmost counties . The region is traditionally described as eight counties , based on demographics and economic ties : Imperial , Los Angeles , Orange , Riverside , San Bernardino , San Diego , Santa Barbara , and Ventura . The more extensive 10-county definition , which includes Kern and San Luis Obispo counties , is also used and is based on historical political divisions .   The 8-county and 10-county definitions are not used for the greater Southern California Megaregion , one of the 11 megaregions of the United States . The megaregion is more expansive , extending east into Las Vegas , Nevada and south across the Mexican border into Tijuana .   Southern California includes the heavily built-up urban area which stretches along the Pacific coast from Ventura through the Greater Los Angeles Area and the Inland Empire , and down to Greater San Diego . Southern California 's population encompasses seven metropolitan areas : the Los Angeles metropolitan area -LRB- Los Angeles and Orange counties -RRB- , the Inland Empire , -LRB- Riverside and San Bernardino counties -RRB- , the San Diego metropolitan area , the Oxnard -- Thousand Oaks -- Ventura metropolitan area , the Santa Barbara metropolitan area , the San Luis Obispo metropolitan area , and the El Centro area . The Los Angeles area has over 12 million inhabitants , while the Riverside-San Bernardino area has over 4 million inhabitants and the San Diego area has over 3 million inhabitants . For Community Supported Agriculture -LRB- CSA -RRB- metropolitan purposes , the five counties of Los Angeles , Orange , Riverside , San Bernardino , and Ventura , are all combined to make up the Greater Los Angeles Area with over 17.5 million people . With over 22 million people , Southern California contains roughly 60 percent of California 's population .   The Colorado Desert and the Colorado River are located on Southern California 's eastern border with Arizona , and the Mojave Desert is located north on California 's Nevada border . Southern California 's southern border is part of the Mexico -- United States border .