The Duchy of Athens (Greek: Δουκᾶτον Ἀθηνῶν, Doukaton Athinon; Catalan: Ducat d'Atenes) was one of the Crusader states set up in Greece after the conquest of the Byzantine Empire during the Fourth Crusade, encompassing the regions of Attica and Boeotia, and surviving until its conquest by the Ottoman Empire in the 15th century.
The first duke of Athens (as well as of Thebes, at first) was Otto de la Roche, a minor Burgundian knight of the Fourth Crusade. Although he was known as the "Duke of Athens" from the foundation of the duchy in 1205, the title did not become official until 1260. Instead, Otto proclaimed himself "Lord of Athens" (in Latin Dominus Athenarum, in French Sire d'Athenes). The local Greeks called the dukes "Megas Kyris" (Greek: Μέγας Κύρης, "Great Lord"), from which the shortened form "Megaskyr", often used even by the Franks to refer to the Duke of Athens, is derived.
Athens was originally a vassal state of the Kingdom of Thessalonica, but after Thessalonica was captured in 1224 by Theodore, the Despot of Epirus, the Principality of Achaea claimed suzerainty over Athens, a claim disputed by the de la Roche in the War of the Euboeote Succession. Like the rest of Latin Greece, however, the Duchy recognized the suzerainty of Charles I of Sicily after the Treaties of Viterbo in 1267.
From 1395 to 1402 the Venetians briefly controlled the Duchy. In 1444 Athens became a tributary of Constantine Palaeologus, the despot of Morea and heir to the Byzantine throne. In 1456, after the Fall of Constantinople (1453) to the Ottoman Empire, Turahanoğlu Ömer Bey conquered the remnants of the Duchy. Despite the Ottoman conquest, the title of "Duke of Athens and Neopatras" continued in use by the kings of Aragon, and through them by the Kings of Spain, up to the present day.