The Castillo de San Marcos (Spanish for "St. Mark's Castle") is the oldest masonry fort in the continental United States; it is located on the western shore of Matanzas Bay in the city of St. Augustine, Florida.

It was designed by the Spanish engineer Ignacio Daza, with construction beginning in 1672, 107 years after the city's founding by Spanish Admiral and conquistador Pedro Menéndez de Avilés, when Florida was part of the Spanish Empire. The fort's construction was ordered by Governor Francisco de la Guerra y de la Vega after a raid by the English privateer Robert Searles in 1668 that destroyed much of St. Augustine and damaged the existing wooden fort. Work proceeded under the administration of Guerra's successor, Manuel de Cendoya in 1671, and the first coquina stones were laid in 1672. The construction of the core of the current fortress was completed in 1695, though it would undergo many alterations and renovations over the centuries.