The Fall of Constantinople (Greek: Άλωση της Κωνσταντινούπολης, Alōsē tēs Kōnstantinoupolēs; Turkish: İstanbul'un fethi) was the capture of Constantinople, the capital of the Byzantine Empire, which occurred after a siege by the Ottoman Empire, under the command of 21-year-old Ottoman Sultan Mehmed II, against the defending army commanded by Byzantine Emperor Constantine XI Palaiologos. The siege lasted from Friday, 6 April 1453 until Tuesday, 29 May 1453 (according to the Julian calendar), when the city was conquered by the Ottomans.

The capture of Constantinople (and two other Byzantine splinter territories soon thereafter) marked the end of the Roman Empire, an imperial state which had lasted for nearly 1,500 years. It was also a massive blow to Christendom, and the Ottomans thereafter were free to advance into Europe without an adversary to their rear. After the conquest, Mehmed made Constantinople the Ottoman Empire's new capital. Several Greek and non-Greek intellectuals fled the city before and after the siege, migrating particularly to Italy. It is argued that they helped fuel the Renaissance. Some mark the end of the Middle Ages by the fall of the city and empire.

Constantinople (Greek: Κωνσταντινούπολις, Konstantinoúpolis; Latin: Constantinopolis; Ottoman Turkish: قسطنطینیه, Qostantiniyye; and modern Turkish: İstanbul) was the capital of the Eastern Roman Empire, the Latin and the Ottoman Empires. It was founded in AD 330, at ancient Byzantium as the new capital of the Roman Empire by Constantine I, after whom it was named. The city was the largest and wealthiest European city of the Middle Ages, and shared the glories of the Byzantine Empire, which was eventually reduced to the city and its environs.

Although besieged on numerous occasions by various peoples, it was taken only in 1204 by the army of the Fourth Crusade, in 1261 by Michael VIII, and in 1453 by the Ottoman Sultan Mehmed II. An inner wall was erected by Constantine I, and the city was surrounded by a triple wall of fortifications, begun during the 5th century by Theodosius II. The city was built on seven hills as well as on the Bosporus, and thus presented an impregnable fortress enclosing magnificent palaces, domes and towers. The Church of Hagia Sophia, the sacred palace of the emperors, the hippodrome, and the Golden Gate were among the largest of the many churches, public edifices, and monuments lining the arcaded avenues and squares.