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.