Claim: San Diego city is in Southern California.

San Diego County 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 , 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 is 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 38th largest metropolitan area in the Americas .   San Diego County has 70 mi of coastline . Most of the county has a mild Mediterranean climate to semiarid climate , though there are mountains that receive frost and snow in the wintertime .   There are also 16 naval and military installations of the U.S. Navy , U.S. Marine Corps , and the U.S. Coast Guard in San Diego County . These include the Naval Base San Diego , Marine Corps Base Camp Pendleton , Marine Corps Air Station Miramar , and Naval Air Station North Island .   From north to south , San Diego County extends from the southern borders of Orange County and Riverside County to the Mexico -- United States border and Baja California . From west to east , San Diego County stretches from the Pacific Ocean to its boundary with Imperial County .