San Diego County, officially the County of San Diego, is located in the southwest corner of the U.S. state of California, north to its border with Mexico. As of the 2020 census, the population was 3,298,634; it is the second-most populous county in California and the fifth-most populous in the United States. Its county seat is San Diego, the second-most populous city in California and the eighth-most populous in the United States. It is the southwestern most county in the 48 contiguous United States, and is a border county. It is home to 18 Indian reservations, the most of any county in the United States. There are 16 military installations of the U.S. Navy, Marine Corps, and Coast Guard in the county.
San Diego County comprises the San Diego–Chula Vista–Carlsbad, CA Metropolitan Statistical Area, which is the 18th most populous metropolitan statistical area and the 18th most populous primary statistical area in the United States. San Diego County is also part of the San Diego–Tijuana region, the largest metropolitan area shared between the United States and Mexico. From north to south, San Diego County extends from the southern borders of Orange and Riverside counties to the Mexico–U.S. border and the municipalities of Tijuana and Tecate in Baja California. From west to east, San Diego County stretches from the Pacific Ocean to its boundary with Imperial County, which was separated from it in 1907.
San Diego County has more than 70 miles (113 km) of coastline. This forms the most densely populated region of the county, which has a mild Mediterranean to semiarid climate and extensive chaparral vegetation, similar to the rest of the western portion of Southern California. Precipitation and temperature extremes increase to the east, with mountains that receive frost and snow in the winter. These lushly forested mountains receive more rainfall than the average in Southern California, while the desert region of the county lies in a rain shadow to the east, which extends into the Desert Southwest region of North America.