San Juan National Historic Site in the Old San Juan section of San Juan, Puerto Rico, is a National Park Service-managed historic site which preserves and interprets the Spanish colonial-era fortification system of the city of San Juan, and features structures such as the San Felipe del Morro and San Cristóbal fortresses. This fortification system is the oldest European construction under United States jurisdiction and one of the oldest in the New World. This national historic site, together with La Fortaleza, have been designated a UNESCO World Heritage Site since 1983.

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.