Old San Juan is a historic district located at the "northwest triangle" of the islet of San Juan in San Juan. Its area roughly correlates to the Ballajá, Catedral, Marina, Mercado, San Cristóbal, and San Francisco sub-barrios (sub-districts) of barrio San Juan Antiguo in the municipality of San Juan, Puerto Rico. Old San Juan is the oldest settlement within Puerto Rico and the historic colonial district of the city of San Juan. This historic district is a National Historic Landmark District named Zona Histórica de San Juan and is listed on the United States National Register of Historic Places as the Old San Juan Historic District. Several historical buildings and structures, particularly La Fortaleza, the city walls, and El Morro and San Cristóbal castles, have been inscribed in the UNESCO World Heritage Site list since 1983. Historically the mixed-use commercial and residential real estate in the main streets of Calle Cristo and Calle Fortaleza from Calle Tanca to the Governor’s Mansion is the most valuable in the area and it has kept its value and increased steadily through several years despite the past economic turmoils.

San Juan Islet is a 3-square-mile (7.8 km2) islet or small island on San Juan Bay in the Atlantic coast of northern Puerto Rico. Home to Old San Juan, it is the site of the oldest permanent European settlement in Puerto Rico (1521), and the second oldest European settlement in the West Indies after Santo Domingo (1496). Due to its strategic location in the Caribbean during the Spanish colonization of the Americas, it is home to a city wall and a number of militaristic buildings such as El Morro Castle. Today, it is also home to many of Puerto Rico's government buildings such as the territory's capitol building.