Mission Viejo () is a commuter city located within Orange County, California, United States in the Saddleback Valley. Mission Viejo is considered one of the largest master-planned communities ever built under a single project in the United States and is rivaled only by Highlands Ranch, Colorado in its size. Its population as of 2019 was estimated at 94,381.
Mission Viejo is suburban in nature and culture. The city consists of residential property, although there are a number of offices and businesses within the limits of the city. The city is known for its tree-lined neighborhoods, receiving recognition from the National Arbor Day Foundation. The city's name is a reference to Rancho Mission Viejo, a large Spanish land grant from which the community was founded.
Mission Viejo was purchased by John Forster, also known as "Don Juan," an Englishman by birth who became a Mexican citizen. During the Mexican–American War, Forster provided fresh horses to United States military forces which were used on the march of San Diego to invade Los Angeles.
Mission Viejo was a hilly region primarily used as cattle and sheep grazing land, since it was of little use to farmers. This city was one of the last regions of Orange County to be urbanized due to its geologic complexity.