San Diego is a city in the U.S. state of California on the coast of the Pacific Ocean and immediately adjacent to the Mexican border. With a 2020 population of 1,386,932, San Diego is the eighth most populous city in the United States and second most populous in California. The city is the county seat of San Diego County, the fifth most populous county in the United States, with 3,338,330 estimated residents as of 2019.