Riverside County is a county located in the southeastern part of the U.S. state of California, stretching from Orange County to the Colorado River, which is the border with Arizona. This county is part of the Riverside-San Bernardino Area, in a region of Southern California known as the Inland Empire.

The population of Riverside County was 1,545,387 in 2000, and the 2006 population has been estimated at 2,026,803 . The county seat is the city of Riverside.

Geographically, the county is desert. Most of Joshua Tree National Park is located in the county. Riverside County lies inland of Los Angeles, and south of San Bernardino. Large numbers of Los Angeles workers have moved to the county in recent years to take advantage of relatively affordable housing. Alongside neighboring San Bernardino County, it is one of the fastest growing parts of the Inland Empire. This spawned a wave of toll road construction in the area in the 1990s, starting with the addition of toll commuter lanes to the California State Route 91 freeway, the main traffic artery to the western metropolitan area. In addition, smaller, but significant, numbers of people have been moving into southern Riverside County from the San Diego metropolitan area. The towns of Temecula and Murrieta account for 20% of increase in population of Riverside County between 2000 and 2007.

Such famous golf resorts as Indian Wells, La Quinta, Rancho Mirage, Palm Springs and Palm Desert are located in Riverside County. Indio is the center of an important date growing region.