San Diego (pronounced /ˌsændiˈeɪgoʊ/), named after Saint Didacus (Didacus of Alcalá) in Spanish, is the second-largest city in California and the ninth largest city in the United States, located along the Pacific Ocean on the west coast of the United States. The California Department of Finance estimates the city's population at 1,353,993 as of January 1, 2009. This coastal city is also the county seat of San Diego County as well as the economic center of the San Diego–Carlsbad–San Marcos metropolitan area. As of 2008, this metropolitan area is the 17th-largest in the United States with a population of 3,001,072 and the 38th-largest metropolitan area in the Americas when including Tijuana, Mexico. According to Forbes the city of San Diego ranks as the fifth wealthiest in the United States. Tourism remains San Diego's third-biggest industry, behind manufacturing and the military.

San Diego's economy is largely composed of agriculture, biotechnology/biosciences, computer sciences, electronics manufacturing, defense-related manufacturing, financial and business services, ship-repair and construction, software development, telecommunications, and tourism. The presence of the University of California, San Diego (UCSD) with the affiliated UCSD Medical Center promotes research in biotechnology.

The area of San Diego has long been inhabited by the Kumeyaay Indians. The first European to visit the region was Portuguese-born explorer Juan Rodríguez Cabrillo sailing under the Spanish Flag, who sailed his flagship San Salvador from Navidad, New Spain. In 1542, Cabrillo claimed the bay for the Spanish Empire and named the site San Miguel. In November of 1602, Sebastián Vizcaíno was sent to map the California coast. Arriving on his flagship San Diego, Vizcaíno surveyed the harbor and what are now Mission Bay and Point Loma and named the area for the Catholic Saint Didacus, a Spaniard more commonly known as San Diego. On November 12, 1602, the first Christian religious service of record in Alta California was conducted by Fray Antonio de la Ascensión, a member of Vizcaíno's expedition, to celebrate the feast day of San Diego.