John V (John Palaeologus) (pāl´ēŏl´əgəs), 1332–91, Byzantine emperor (1341–91), son and successor of Andronicus III. Forced to fight John VI (John Cantacuzene), who usurped the throne during his minority, he came into power in 1354. In his reign the Ottoman Turks took Adrianople and Philippolis, conquered Serbia, and exacted tribute from the emperor. John vainly tried to heal the schism between East and West in order to secure Western aid against the Turks. He professed (1369) the Roman Catholic faith at Rome; while returning to Constantinople he was briefly imprisoned for debt in Venice. In 1371 he recognized the suzerainty of the Ottoman sultan Murad I. Deposed (1376) by his son Andronicus IV, he was restored in 1379. In 1390 his grandson, John VII, briefly usurped the throne. John V was succeeded by his son Manuel II.

John V (John the Magnanimous), 1689–1750, king of Portugal (1706–50), son and successor of Peter II. Before his accession the Methuen Treaty (1703) with England had brought Portugal into the War of the Spanish Succession, but after a major defeat at Almansa (1707), the Portuguese played little part in the fighting. After the war, John sought to maintain Portugal's alliance with England and to keep peace, except in giving assistance (1716–17) to the Venetians against the Ottomans. Enriched by gold from Brazil, John was a patron of arts and letters, had a sumptuous court, and erected beautiful buildings in Lisbon. However, his wealth also made him independent of the Cortes, so he ruled with increasing absolutism. He has been criticized for subservience to the church, from which he drew most of his ministers, especially in later years. John was succeeded by his son Joseph.