The Château d'If is a fortress (later a prison) located on the island of If, the smallest island in the Frioul archipelago situated in the Mediterranean Sea about 1.5 kilometres (7⁄8 mile) offshore in the Bay of Marseille in southeastern France. It is famous for being one of the settings of Alexandre Dumas' adventure novel The Count of Monte Cristo. "If" is the French word for the Yew tree.
The "château" is a square, three-story building 28 m (92 ft) long on each side, flanked by three towers with large gun embrasures. It was built in 1524-31 on the orders of King Francis I, who, during a visit in 1516, saw the island as a strategically important location for defending the coastline from sea-based attacks.