A fortification on the island of If in the Frioul archipelago in the Mediterranean Sea. Initially, the structure was built as a fort to defend Marseille from attacks from the sea. Construction took place in 1524–1531 by order of King Francis I. From the end of the 16th century, the castle began to be used to isolate and guard especially dangerous criminals. The dungeons contained Huguenots, politicians, leaders of the Paris Commune, and persons who posed a danger to France.

In the 1830s, the Château d’If officially ceased to be a prison, but in 1871, the leaders of the Paris Commune were held here, and its leader Gaston Crémieux was shot on the island of If. In the 19th century, the castle was turned into a museum, and in 1926 it was given the status of an architectural monument.

The castle gained wide fame thanks to the French writer Alexandre Dumas. In his novel The Count of Monte Cristo, written in 1844–45, the protagonist, Edmond Dantes, was imprisoned in the Chateau d’If.