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 Marinids 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. 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.

The Moroccan citadel of Ceuta, on the southern side of the Strait of Gibraltar, had been seized in 1415 in a surprise attack by the Kingdom of Portugal. (See Conquest of Ceuta). The Marinids had tried to recover it in 1418–1419, but failed. The assassination of the Marinid sultan in 1420 sent Morocco reeling into political chaos and internal disorder for the next few years, giving the Portuguese time to entrench themselves in Ceuta.

Whatever its original objectives, the capture of Ceuta had profited the Portuguese little. The Marinids had cut off all of Ceuta's trade and supplies from the landward side. Ceuta became little more than a large, empty, windswept fortress-city, with an expensive Portuguese garrison that had to be continually re-supplied from across the sea. There had been no follow-up Portuguese campaigns in North Africa, with the result that the Ceuta garrison had little to do, beyond waiting and eating through the king's treasury. There were growing calls in the Portuguese court to simply withdraw the troops and abandon Ceuta.

In 1416, King John I of Portugal placed his son, the Portuguese Prince Henry the Navigator, Duke of Viseu, in charge of supplying and provisioning Ceuta. As a result, Henry was disinclined to abandoning the city, and instead urged an expansion of Portuguese holdings in Morocco.

In 1432, Henry the Navigator proposed to his father King John I of Portugal an ambitious project to allow him to lead a war of conquest of Marinid territories, or at least carve out a wider regional enclave in the north. The king called on the royal council, including the rest of his sons – the Ínclita Geração – for consultation. Henry's brothers, the princes Edward of Portugal, Peter of Coimbra, John of Reguengos, their half-brother Afonso of Barcelos and Afonso's grown sons, Ferdinand of Arraiolos and Afonso of Ourém, almost unanimously pronounced themselves against the project. They cited the lack of Portuguese manpower and the huge expense of conquering and holding such a large area, and questioned the purpose and legal basis of the conquest. Moreover, they subtly expressed doubts about Henry's ability to lead such an expedition, and suggested that if Henry was intent on military glory or crusade, then he perhaps ought to enter the service of the Crown of Castile and campaign on the Granadan frontier instead. (Indeed, such a proposal was submitted to Castile a month later (July 1432) by a Portuguese emissary, but was rejected out of hand by the Castilian strongman Álvaro de Luna.)