The Battle of Tangier, sometimes referred to as the Siege of Tangiers and, by the Portuguese, as the Disaster of Tangier (Portuguese: Desastre de Tânger), refers to the attempt by a Portuguese expeditionary force to seize the Moroccan citadel of Tangier and its defeat by the armies of the Marinid Sultanate, in 1437.

The Portuguese expeditionary force, led by Prince Henry the Navigator, Duke of Viseu, set out from Portugal in August 1437, intending to seize a series of Moroccan coastal citadels. The Portuguese laid siege to Tangier in mid-September. After a few failed assaults on the city, the Portuguese force was attacked and defeated by a large Moroccan relief army led by vizier Abu Zakariya Yahya al-Wattasi of Fez. The Moroccans encircled the Portuguese siege camp and starved it to submission. To preserve his army from destruction, Henry negotiated a treaty promising to return the citadel of Ceuta (captured earlier in 1415) to Morocco in return for being allowed to withdraw his troops. As it turns out, the terms of the treaty never were fulfilled; the Portuguese decided to hold on to Ceuta and allowed the Portuguese hostage, the king's own brother Ferdinand the Holy Prince, to remain in Moroccan captivity, where he perished in 1443.

The Tangier fiasco was a tremendous setback for the prestige and reputation of Henry the Navigator, who personally conceived, promoted and led the expedition. Simultaneously, it was an enormous boon to the political fortunes of the vizier Abu Zakariya Yahya al-Wattasi, who was transformed overnight from an unpopular regent to a national hero, allowing him to consolidate his power over Morocco.

This was the first of four attempts by the Portuguese to seize the city of Tangier in the 15th century.