The fall of Constantinople, also known as the conquest of Constantinople, was the capture of the capital of the Byzantine Empire by the Ottoman Empire. The city was captured on 29 May 1453 as part of the culmination of a 53-day siege which had begun on 6 April.

The attacking Ottoman Army, which significantly outnumbered Constantinople's defenders, was commanded by the 21-year-old Sultan Mehmed II (later nicknamed "the Conqueror"), while the Byzantine army was led by Emperor Constantine XI Palaiologos. After conquering the city, Mehmed II made Constantinople the new Ottoman capital, replacing Adrianople.

The conquest of Constantinople and the fall of the Byzantine Empire was a watershed of the Late Middle Ages, marking the effective end of the last remains of the Roman Empire, a state which began in roughly 27 BC and had lasted nearly 1500 years. Among many modern historians, the fall of Constantinople is considered the end of the medieval period. The city's fall also stood as a turning point in military history. Since ancient times, cities and castles had depended upon ramparts and walls to repel invaders. The Walls of Constantinople, especially the Theodosian Walls, were some of the most advanced defensive systems in the world at the time. These fortifications were overcome with the use of gunpowder, specifically in the form of large cannons and bombards, heralding a change in siege warfare.