San Juan (, Spanish: [sa xwan]; "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 2010 census, it is the 46th-largest city under the jurisdiction of the United States, with a population of 395,326. 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 third oldest European-established capital city in the Americas, after Santo Domingo, in the Dominican Republic, founded in 1496, and Panama City, in Panama, founded in 1521, and is the oldest European-established city under United States sovereignty. Several historical buildings are located in San Juan; among the most notable are the city's former defensive forts, Fort San Felipe del Morro and Fort San Cristbal, and La Fortaleza, the oldest executive mansion in continuous use in the Americas.

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 Bayamn, Guaynabo, Catao, Canvanas, 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.

The damage caused in 2017 by Hurricane Maria was extensive. Significant progress had been made in the capital by April 2019, and particularly by October 2019. This was significant for tourism, which had rebounded by October of that year and was close to the pre-Maria era.

The Dupont Plaza Hotel arson was a fire that occurred at the Hotel Dupont Plaza (now San Juan Marriott Resort & Stellaris Casino) in San Juan, Puerto Rico, on New Year's Eve, December 31, 1986.

The fire was set by three disgruntled employees of the hotel who were involved in a labor dispute with the owners. It claimed between 96 and 98 lives and caused 140 injuries. It is the most catastrophic hotel fire in Puerto Rican history and the second deadliest hotel fire in the history of the United States.