Aliso Viejo (Spanish for "Old Sycamore") is a city in the San Joaquin Hills of southern Orange County, California. It had a population of 52,176 as of the 2020 census, up from 47,823 as of the 2010 census. It became Orange County's 34th city on July 1, 2001, the only city in Orange County to be incorporated since 2000. It is bordered by the cities of Laguna Beach on the west and southwest, Laguna Hills on the east, Laguna Niguel on the southeast, and Laguna Woods on the north. It is similarly named to another nearby city, Mission Viejo.