San Diego County is a county located near the Pacific Ocean in the far southwest of the U.S. state of California. It is the southwesternmost county in the contiguous 48 states. The population in 2000 was 2,813,835. A July 2008 estimate placed the population at 3,001,072 people, making it the third most-populous county in California, just behind its northern neighbor Orange County. The population in 2009 was estimated at 3,208,466, making it the fifth most-populous county in the United States. The county seat and largest city is San Diego.

The county contains the American metropolitan statistical area San Diego-Carlsbad-San Marcos. In addition, San Diego County is part of the San Diego – Tijuana metropolitan area, an area with over 5 million people and the largest bi-national metropolitan area shared between the United States and Mexico. San Diego County is also part of the Southern California Border Region, also referred to as San Diego-Imperial, the smallest but most economically diverse region in the state.

San Diego County lies just north of the Mexican border, sharing a border with Tijuana. It lies south of Orange County and Riverside County and west of Imperial County. It is home to miles of beaches, a mild Mediterranean to semi-arid climate, and 16 military facilities hosting the United States Navy, the United States Coast Guard and the United States Marine Corps.