The siege of Constantinople in 1203 was a Crusader siege of the capital of the Byzantine Empire, in support of the deposed emperor Isaac II Angelos and his son Alexios IV Angelos. It marked the main outcome of the Fourth Crusade.

To take the city by force, the Crusaders first needed to cross the Bosphorus. About 200 ships, horse transports and galleys would undertake to deliver the crusading army across the narrow strait, where Alexios III had lined up the Byzantine army in battle formation along the shore, north of the suburb of Galata. The Crusaders' knights charged straight out of the horse transports, and the Byzantine army fled south. The Crusaders followed south, and attacked the Tower of Galata, which held one end of the chain that blocked access to the Golden Horn. The Tower of Galata held a garrison of mercenary troops of English, Danish, and Italian origin. As the crusaders laid siege to the Tower, the defenders routinely attempted to sally out with some limited success, but often suffered bloody losses. On one occasion the defenders sallied out but were unable to retreat back to the safety of the tower in time, the Crusader forces viciously counterattacked, with most of the defenders being cut down or drowning in the Bosporus in their attempts to escape. The Golden Horn now lay open to the Crusaders, and the Venetian fleet entered.