El Rio is a small, rural unincorporated town in Ventura County, California, United States, on the northeast side of the 101 Freeway and Oxnard, and south of the Santa Clara River. The town was founded in 1875 and named New Jerusalem by the owner of general store who became the first postmaster in 1882. In 1885, a school and Santa Clara Catholic Church were built. Located near where the coastal railroad line crosses the Santa Clara River, the town's name was changed to El Rio around 1900. The former wagon river crossing route became the Ventura Freeway which separates the community from the commercial developments that grew up on the southwest side of the freeway. That portion of the historic community and sections closer to the river have been annexed to the city of Oxnard. Vineyard Avenue is the central business district for the largely residential community.

Ventura County is a county located in the southern part of the U.S. state of California. As of the 2020 census, the population was 843,843. The largest city is Oxnard, and the county seat is the city of Ventura.