The Walls of Constantinople are a series of defensive stone walls that have surrounded and protected the city of Constantinople (today Istanbul in Turkey) since its founding as the capital of the Eastern Roman Empire by Constantine the Great. With numerous additions and modifications during their history, they were the last great fortification system of antiquity, and one of the most complex and elaborate systems ever built.

Initially built by Constantine the Great, the walls surrounded the new city on all sides, protecting it against attack from both sea and land. As the city grew, the famous double line of the Theodosian Walls was built in the 5th century. Although the other sections of the walls were less elaborate, when well manned, they were almost impregnable for any medieval besieger, saving the city, and the Byzantine Empire with it, during sieges from the Avars, Arabs, Rus', and Bulgars, among others (see Sieges of Constantinople). The advent of gunpowder siege cannons rendered the fortifications vulnerable, leading to the fall of Constantinople to the Ottomans on 29 May 1453 after a prolonged siege.