The fifth siege of Gibraltar was a second attempt by King Alfonso XI of Castile to retake the fortified town of Gibraltar. It had been held by the Moors since 1333. The siege followed years of intermittent conflict between the Christian kingdoms of Spain and the Moorish Emirate of Granada, which was supported by the Marinid sultanate of Morocco. A series of Moorish defeats and reverses had left Gibraltar as a Moorish-held enclave within Castilian territory. Its geographical isolation was compensated for by the strength of its fortifications, which had been greatly improved since 1333.

Alfonso brought an army of around 20,000 men to dig in to the north of Gibraltar for a lengthy siege. In the New Year of 1350, the;Black Death;– which had been raging through western Europe for the previous two years – appeared in the camp. The outbreak caused panic as increasing numbers of Castilian troops began dying from the plague. The generals, nobles and ladies of the royal household begged Alfonso to call off the siege, but Alfonso refused to abandon the siege and fell victim to the plague on 27 March 1350, becoming the only monarch to die of the disease. His death meant the immediate end of the siege. The Moors recognised that they had had a narrow escape.