Abu al-Hajjaj Yusuf ibn Ismail (29 June 1318 – 19 October 1354), known by the regnal name al-Muayyad billah (lit. 'He who is aided by God'), was the seventh Nasrid ruler of the Emirate of Granada on the Iberian Peninsula. The third son of Ismail I (r. 1314–1322), he was Sultan between 1333 and 1354, after his brother Muhammad IV (r. 1325–1333) was assassinated.
Coming to the throne at age fifteen, he was initially treated as a minor and given only limited power by his ministers and his grandmother Fatima. In February 1334, his representatives secured a four-year peace treaty with Granada's neighbours Castile and the Marinid Sultanate. Aragon joined in the treaty in May. After gaining more control of the government, in 1338 or 1340 he expelled the Banu Abi al-Ula family, who had masterminded the murder of his brother and had been the leaders of the Volunteers of the Faith—North African soldiers who fought for Granada. After the treaty expired, he allied himself with Abu al-Hasan Ali (r. 1331–1348) of the Marinids against Alfonso XI of Castile (r. 1312–1350). After winning a major naval victory in April 1340, the Marinid–Granadan alliance was decisively defeated on 30 October in the disastrous Battle of Río Salado. In its aftermath, Yusuf was unable to prevent Castile from taking several Granadan castles and towns, including Alcalá de Benzaide, Locubín, Priego and Benamejí. In 1342–1344, Alfonso XI besieged the strategic port of Algeciras. Yusuf led his troops in diversionary raids into Castilian territory, and later engaged the besieging army, but the city fell in March 1344. A ten-year peace treaty with Castile followed.