Riverside County is a county located in the southern portion of the U.S. state of California. As of the 2020 census, the population was 2,418,185, making it the fourth-most populous county in California and the 10th-most populous in the United States. The name was derived from the city of Riverside, which is the county seat.

Riverside county was a major focal point of the Civil Rights Movements in the US, especially the African-American sections of Riverside and heavily Mexican-American communities of the Coachella Valley visited by Cesar Chavez of the farm labor union struggle.

Riverside County is organized as a General Law County under the provision of the California Government Code. The county has five supervisorial districts, and one supervisor is elected from each district every four years.

In 1999, the County Board of Supervisors approved a multimillion-dollar planning effort to create the Riverside County Integrated Plan (RCIP) which was to encompass a completely new General Plan, regional transportation plan (CETAP) and Habitat Conservation Plan. The resultant General Plan adopted in 2003 was considered groundbreaking for its multidisciplinary approach to land use and conservation planning.