Map - San Diego County, California (San Diego County)

San Diego County, officially the County of San Diego (Condado de San Diego), is a county in the southwestern corner of the U.S. state of California. As of the 2020 census, the population was 3,298,634, making it California's second-most populous county 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 city in the United States. It is the southwesternmost county in the 48 contiguous United States, and is a border county. It is also home to 18 Native American tribal reservations, the most of any county in the United States.

San Diego County comprises the San Diego-Chula Vista-Carlsbad, CA Metropolitan Statistical Area, which is the 17th most populous metropolitan statistical area and the 18th most populous primary statistical area of the United States as of July 1, 2012. San Diego County is also part of the San Diego–Tijuana transborder metropolitan area, the largest metropolitan area shared between the United States and Mexico.

San Diego County has more than 70. mi 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.

There are 16 military installations, of the U.S. Navy, U.S. Marine Corps, and the U.S. Coast Guard in San Diego County. These include Naval Base San Diego, Marine Corps Base Camp Pendleton, Marine Corps Air Station Miramar, Naval Air Station North Island, and Coast Guard Air Station San Diego.

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 Baja California municipalities of Tijuana and Tecate. From west to east, San Diego County stretches from the Pacific Ocean to its boundary with Imperial County, which separated from it in 1907. Since 2010, statewide droughts in California have further strained San Diego County's water security.

The area which is now San Diego County has been inhabited for more than 12,000 years by Kumeyaay (also called Diegueno and Ipai/Tipai), Payómkawichum (Luiseño), Kuupangaxwichem (Cupeño), ʔívil̃uqaletem (Cahuilla), and the Acjachemen (Juaneño) Indians and their local predecessors.

In 1542, the explorer Juan Rodríguez Cabrillo, who may have been born in Portugal but sailed under the flag of Castile, claimed San Diego Bay for the Spanish Empire, and he named the site San Miguel. In November 1602, Sebastián Vizcaíno surveyed the harbor and what are now Mission Bay and Point Loma and named the area for Saint Didacus, a Spaniard more commonly known as San Diego. European settlement in what is now San Diego County began with the founding of the San Diego Presidio and Mission San Diego de Alcalá by Spanish soldiers and clerics in 1769. This county was part of Alta California under the Viceroyalty of New Spain until the Mexican declaration of independence. From 1821 through 1848 this area was part of Mexico.

San Diego County became part of the United States as a result of the Treaty of Guadalupe Hidalgo in 1848, ending the Mexican–American War. This treaty designated the new border as terminating at a point on the Pacific Ocean coast which would result in the border passing one Spanish league south of the southernmost portion of San Diego Bay, thus ensuring that the United States received all of this natural harbor.

San Diego County was one of the original counties of California, created at the time of California statehood in 1850.

At the time of its establishment in 1850, San Diego County was relatively large, and included all of southernmost California south and east of Los Angeles County. It included areas of what are now Inyo and San Bernardino Counties, as well as all of what are now Riverside and Imperial Counties.