information:classification, structural:expansion
San Juan (/ ˌ s æ n ˈ hw ɑː n /, Spanish:; Spanish for "Saint John ") is the capital city and most populous municipality in the Commonwealth of Puerto Rico, an unincorporated territory of the United States. As of the 2020 census, it is the 57th-largest city under the jurisdiction of the United States, with a population of 342,259. San Juan was founded by Spanish colonists in 1521, who called it Ciudad de Puerto Rico ("City of Puerto Rico", Spanish for rich port city).

Puerto Rico's capital is the second oldest European-established capital city in the Americas, after Santo Domingo, in the Dominican Republic, founded in 1496, and is the oldest European-established city under United States sovereignty. Several historical buildings are located in the historic district of Old San Juan; among the most notable are the city's former defensive walls, Fort San Felipe del Morro and Fort San Cristóbal, and La Fortaleza, the oldest executive mansion in continuous use in the Americas. These historic sites were declared a UNESCO World Heritage Site in 1983.

Today, San Juan is Puerto Rico's most important seaport and is the island's financial, cultural, and tourism center. The population of the metropolitan statistical area, including San Juan and the municipalities of Bayamón, Guaynabo, Cataño, Canóvanas, Caguas, Toa Alta, Toa Baja, Carolina and Trujillo Alto, is about 2.443 million inhabitants; thus, about 76% of the population of Puerto Rico now lives and works in this area. San Juan is also a principal city of the San Juan-Caguas-Fajardo Combined Statistical Area. The city has been the host of events within the sports community, including the 1979 Pan American Games; 1966 Central American and Caribbean Games; events of the 2006, 2009 and 2013 World Baseball Classics; the Caribbean Series and the Special Olympics and MLB San Juan Series in 2010.