San Diego County, officially the County of San Diego, is a county in the southwestern corner of the state of California, in the United States. As of the 2010 census, the population was 3,095,313, making it California's second-most populous county and the fifth-most populous in the United States. Its county seat is San Diego,[8] 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.