San Diego is a city on the Pacific coast of Southern California, adjacent to the Mexico–United States border. With a population of over 1.4 millio…San Diego is a city on the Pacific coast of Southern California, adjacent to the Mexico–United States border. With a population of over 1.4 million, it is the eighth-most populous city in the United States. San Diego is the seat of San Diego County. It is known for its mild Mediterranean climate, extensive beaches and parks, long association with the United States Navy, and recent emergence as a healthcare and biotechnology development center.