San Diego ( SAN dee-AY-goh, Spanish: [san ˈdjeɣo]) is a city on the Pacific Ocean coast in Southern California located immediately adjacent to the Mexico–United States border. With a population of over 1.3 million residents, the city is the eighth-most populous in the United States and the second-most populous in the state of California after Los Angeles. The city is the seat of San Diego County, which has a population of nearly 3.3 million people as of 2021. San Diego is known for its mild year-round Mediterranean climate, extensive beaches and parks, its long association with the United States Navy, and its recent emergence as a healthcare and biotechnology development center.

Historically home to the Kumeyaay Native Americans, San Diego has been referred to as the Birthplace of California, since it was the first site visited and settled by Europeans on what is now the West Coast of the United States. Upon landing in San Diego Bay in 1542, Juan Rodríguez Cabrillo claimed the area for Spain, forming the basis for the settlement of Alta California 200 years later. The Presidio and Mission San Diego de Alcalá, founded in 1769, formed the first European settlement in what is now California. In 1821, San Diego became part of the newly declared Mexican Empire, which reformed as the First Mexican Republic two years later. California was conquered by the U.S. in 1848 following the Mexican–American War and was admitted to the union as a state in 1850.

San Diego's main economic engines are military and defense-related activities, tourism, international trade, research, and manufacturing. The city is the economic center of the San Diego–Tijuana conurbation, the second-most populous transborder metropolitan area in the Western Hemisphere (after Detroit–Windsor), home to an estimated 4.9 million people as of 2012. The primary border crossing between San Diego and Tijuana, the San Ysidro Port of Entry, is the busiest international land border crossing in the world outside of Asia (fourth-busiest overall). The city's airport, San Diego International Airport, is the busiest single-runway airport in the world.