Old San Juan (Viejo San Juan) is the historic core of San Juan. Although this eight by ten block district is part of San Juan, it is quite geographically and culturally distinct from the rest of the city. It occupies the western half of the islet of San Juan, which it shares with Puerta de Tierra. As this is a tourist destination, English is relatively common, but not universally spoken.

Founded in 1509, San Juan became a walled city protected by multiple forts. It guarded an important entrance into the Spanish Main, and withstood multiple attacks by the British and Dutch (some partially successful). It was considered the Gibraltar of the West Indies. Due to its military significance, the government kept the growing population within the city walls until 1897, when a few bastions were demolished. The forts received some naval bombardment from US naval ships during the Spanish American War (1898). Much of the district's architecturally historic fabric is intact, including the impressive fortifications.

Many tourists are caught unaware of the sun exposure received from simply walking around the sites of this tropical city. Sunblock is available at many stores in town. Wear good walking shoes to deal with the hills and cobblestone streets. Around the perimeter of the district the trade winds make it surprisingly comfortable, but along the interior streets it get much hotter, with closely spaced, multi-story buildings cutting off any breeze. Brief rain showers are quite common, so watch your step, as the cobblestones can get quite slippery.

Old San Juan is a common stop for cruise ships, yet it is definitely not a beach resort. It is a real neighborhood, where people work and live. Men typically wear collared shirts and long pants, and businessmen wear suits. Women tend to wear skirts or dresses and often high heeled shoes. Although visitors sometimes dress more casually; a collared shirt, shorts with pockets and belt, and shoes are minimally appropriate for adults at most attractions.