Rancho Santa Rosa was a 47,815-acre (193.50 km2) Mexican land grant in present day Riverside County, California given in 1846 by Governor Pio Pico to Juan Moreno. At the time of the US patent, Rancho Santa Rosa was a part of San Diego County. Riverside County was created by the California Legislature in 1893 by taking land from both San Bernardino and San Diego Counties. The site is now registered as a California Historical Landmark.

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.