San Diego County, technically San Diego County, is a county in the U.S. state of California's southwestern corner. The population is over 3,095,000, making it the second-most populous county in California and the fifth-largest in the United States. San Diego is the county seat. There are more than 70 miles of coastline in San Diego County. The US also has 16 naval and military bases. Navy, United States Marine Corps, and the United States. The San Diego County Coast Guard. San Diego County stretches from north to south from the southern frontiers of the Orange and Riverside Counties to Mexico-U.S. The Tijuana and Tecate border with Baja California. San Diego County stretches from the Pacific Ocean to the Imperial County, from west to east.