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.

An islet is a very small, often unnamed island. Most definitions are not precise, but some suggest that an islet has little or no vegetation and cannot support human habitation. It may be made of rock, sand and/or hard coral; may be permanent or tidal ; and may exist in the sea, lakes, rivers or any other sizeable bodies of water.