San Diego County, officially the County of San Diego (Spanish: 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. 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. 4K Resolution.