San Diego County is a county located 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 the second-most populous county in California and the fifth-most populous in the United States. Its county seat is San Diego, the eighth-most populous city in the United States. It is the south-westernmost county in the 48 contiguous United States.
San Diego County comprises the San Diego-Carlsbad Metropolitan Statistical Area. The San Diego-Carlsbad, CA Metropolitan Statistical Area as the 17th most populous metropolitan statistical area and the 18th most populous primary statistical area of the United States as of July 1, 2012. San Diego is also part of the San Diego–Tijuana metropolitan area, the largest metropolitan area shared between the United States and Mexico. Greater San Diego ranks as the 39th largest metropolitan area in the Americas.