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 new capital of the 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, they were, when well-manned, almost impregnable for any medieval besieger. They saved the city, and the Byzantine Empire with it, during sieges by the Avar-Sassanian coalition, Arabs, Rus', and Bulgars, among others. The advent of gunpowder siege cannons rendered the fortifications vulnerable, but cannon technology was not sufficiently advanced to capture the city on its own, and the walls could be repaired between reloading. Ultimately, the city fell from the sheer weight of numbers of the Ottoman forces on 29 May 1453 after a two-month seige.

The walls were largely maintained intact during most of the Ottoman period until sections began to be dismantled in the 19th century, as the city outgrew its medieval boundaries. Despite lack of maintenance, many parts of the walls survived and are still standing today. A large-scale restoration program has been underway since the 1980s.