San Diego County is a county located near the Pacific Ocean in the far southwest of the U.S. state of California. The state of California estimates San Diego County's population as of 2008 to be 3,146,274 people, making it the second most populous county in California, ahead of its northwestern neighbor Orange County. The county seat is the city of San Diego. San Diego County was one of twenty-seven original counties formed when California became a state on 18 February 1850.
San Diego County lies just north of the Mexican border—sharing a border with Tijuana—and lies south of Orange County. It is home to miles of beaches, a mild Mediterranean climate and 16 military facilities hosting the United States Navy, the United States Coast Guard and the United States Marine Corps.
European settlement in what is now San Diego County began with the founding of the Mission San Diego de Alcala by the Spanish, in 1769.
San Diego County became part of the United States as a result of the Treaty of Guadalupe Hidalgo in 1848, ending the U.S.-Mexican War. This treaty designated the new border as terminating at a point on the Pacific Ocean coast which would result in the border passing one Spanish league south of the southernmost portion of San Diego Bay, thus ensuring that the United States received the natural harbor.
San Diego County was one of the original counties of California, and was created at the time of statehood in 1850. It was named after San Diego Bay, which had been rechristened in 1602 by Sebastián Vizcaíno in honor of the Franciscan St. Didacus of Alcalá, known in Spanish as San Diego de Alcalá de Henares, and whose name was borne by Vizcaíno's flagship.
As originally created in 1850 San Diego County was quite large and included most of southeastern California south and east of Los Angeles County. As such it included major parts of what are now Inyo, San Bernardino, Riverside, and Imperial counties.
The later part of the 19th century witnessed numerous realignments of county boundaries. The most recent changes were the creation of Riverside County, in 1893, and Imperial County, in 1907.