The Sea Walls of Constantinople consisted of a single line of fortifications along the shores of the Propontis (Marmara) Sea and the Golden Horn. They are, for the most part, poorly preserved today, due in part to the construction of a railroad and coastal road in the 19th and 20th century. The Palace of Boukoleon and the so-called Monastery of Christ Philanthropos, which were built along the walls of the Marmara, are partially preserved today. In addition, several inscriptions can be found, particularly along the Marmara walls.

The ancient city Byzantion had sea walls, due to its strategic location that linked the Balkans with the eastern provinces by land, and the Aegean and the Black Sea by sea. It is likely that the sea walls later built more or less follow the same line of the wall along the Marmara. On several occasions, it was attacked, most notably by Septimius Severus who reportedly destroyed its walls and then later had them rebuilt at the end of the second century. The city that Constantine made his new capital was based on what had been developed under Septimius Severus, though Constantine expanded the city to the west when he built a new city wall. It is unclear, though, whether he expanded the sea walls to meet his new land walls. The city was again expanded during the reign of Theodosius II around 412, when a new land wall was built beyond the Constantinian wall. The city walls were built in part because of the Gothic threat, which had made it increasingly clear that the empire was less secure. Following the completion of the land wall, the prefect Cyrus Panopolites reportedly built walls along the Marmara Sea and the Golden Horn, though they are only clearly recorded in the 8th century. A large harbor, known as the Harbor of Theodosius, was built at the mouth of the Lykos River, possibly during the reign of Theodosius I. Eventually the sea walls had three harbors on the shore of the Marmara Sea. There were also two major harbors – the Neorion and the Prosphorion – on the Golden Horn, which later could no longer be used as the Golden Horn silted up. These harbors were included in the concessions of the Italian maritime republics (Venice, Genoa, Amalfi, and Pisa) before the Fourth Crusade. The Kynegon Harbor, which was in front of the Gate of the Kynegos, was probably used by emperors and dignitaries going to the palace or churches at Blachernai. This was probably part of an elaborate triple gate that included the Kynegion (Balat) Gate  (the location of a marble relief of Nike).

The sea walls frequently needed to be repaired due to earthquakes and storms, which were often commemorated by inscriptions. There was an extensive series of inscriptions, dating from the fifth to the fifteenth centuries, on the walls of Constantinople. While a large number of them have since disappeared, many inscriptions can still be found along the walls, particularly on the Marmara walls. Three inscriptions, once located along the Marmara, can now be found in the Istanbul Archaeological Museums.

The Ottoman periodically restored the walls as well. For example, during the reign of Murad IV (1623-1640), the walls were extensively repaired due to damage caused by earthquakes in 1509, 1574, and 1595. French general Sebastiani restored the walls when the city was threatened by the British navy in 1807. The walls of the Golden Horn were repaired several times during the Ottoman era, but they largely disappeared over the course of the 19th century. Significant sections of the walls along the Marmara and around Topkapı Palace began to be demolished first when the railway was built in 1871, then again in 1959, as part of the construction of a new coastal road (later called Kennedy Street). During the occupation of Istanbul after World War I, the French army camping near Topkapı Palace excavated the region in 1922-1923, bringing to light the ruins of several structures in the Mangana region.

Today the wall along the Marmara is in better condition in certain sections, while most sections of the Golden Horn wall no longer exist. The location of the wall along the Golden Horn is also harder to determine, in part because there were two ports - Prosphorion and Neorion - that gradually filled up over the course of the Medieval Era. There are traces of older walls dating to the Roman era or even earlier along the Marmara near Topkapı Palace. The walls are generally further from the sea than they once were, because the shoreline of Istanbul has expanded over time. The construction of the coastal highway on the Marmara, for example, expanded the shoreline by around 60 meters. One noteworthy example of this can be seen by the Palace of Boukoleon, which originally was the location of a small harbor.

(See also: Palace of Boukoleon, Golden Horn Chain, Christ Philanthropos, Harbor of Theodosius, Marble Tower)