San Diego (Wikipedia) San Diego (Spanish for ‘Saint Didacus’; /ˌsæn diˈeɪɡoʊ/ SAN dee-AY-goh, Spanish: [san ˈdjeɣo]) is a city on the Pacific Ocean coast of Southern California located immediately adjacent to the Mexico–United States border. With a 2020 population of 1,386,932, it is the eighth-most populous city in the United States, the second-most populous city in California and the seat of San Diego County, the fifth-most populous county in the United States, with 3,286,069 estimated residents as of 2021. The city is known for its mild year-round Mediterranean climate, natural deep-water harbor, extensive beaches and parks, long association with the United States Navy, and recent emergence as a healthcare and biotechnology development center. San Diego is the second-largest city in California after Los Angeles.