San Diego is a city located on the Pacific coast of Southern California, situated adjacent to the Mexico–United States border. It serves as the county seat of San Diego County and stands as the eighth-most populous city in the United States. The urban area forms part of the San Diego–Tijuana region, which is the second-most populous transborder metropolitan area in the Western Hemisphere. Its geographic position places it directly across from Tijuana, Mexico, making it a significant hub for international interaction and trade.

Historically home to the Kumeyaay people, the area was first visited by Europeans in 1542 when Juan Rodríguez Cabrillo claimed the bay for Spain. Permanent European colonization began in 1769 with the establishment of the Presidio of San Diego and Mission San Diego de Alcalá, marking the first European settlement in what is now California. The city later became part of the Mexican Empire in 1821 before being ceded to the United States in 1848 following the Mexican–American War. Both the Presidio and the Mission are designated as National Historic Landmarks.

The city is known for its mild Mediterranean climate, extensive beaches, and parks, alongside a long association with the United States Navy, which maintains its Pacific Fleet there. Notable sites include the San Diego Zoo, Safari Park, and Scripps Institution of Oceanography, reflecting its role in wildlife conservation. The urban landscape features rocky tidepools, seaside cliffs, and desert hills inland. The city lacks clearly defined districts but is characterized by many neighborhoods and a strong maritime heritage.

San Diego attracts visitors with its ideal climate, miles of beaches, and laid-back attitude, while also offering a rich cultural scene. The city is a center for wireless, electronics, healthcare, and biotechnology development, alongside its military and defense sectors. Tourism is a major economic driver, supported by the San Ysidro Port of Entry, the busiest international land border crossing in the world outside of Asia. The area also hosts several universities, including UC San Diego and San Diego State University.

As of 2020, the city had a population of 1,386,932, while the metropolitan area exceeded 3.3 million residents. The economy is driven by military activities, tourism, international trade, research, and manufacturing. San Diego International Airport is the busiest single-runway airport in the United States. The region experiences a mild Mediterranean climate, which has long attracted travelers seeking to escape the hustle of nearby Los Angeles, located approximately 100 miles to the northwest.