Byzantine emperor who spent his reign appealing to the West for help against the final assaults by the Ottoman Turks (Ottoman Empire) on the Byzantine Empire.

Son of Manuel II Palaeologus, John was crowned coemperor with his father in 1408 and took effective rule in 1421. He was sole emperor after his father's death in 1425. He ruled the area immediately surrounding Constantinople, while his brothers governed remnants of the fragmented empire in the Greek Peloponnese and in the districts on the Black Sea.

When the Turks took Thessalonica (modern Thessaloníki, Greece) in March 1430, John turned to the West for help. In 1437 he went to Italy, where he brought about a union between the Byzantine and Latin churches at the Council of Ferrara-Florence (Ferrara-Florence, Council of) (1439). Western efforts against the Turks failed, and the union stirred dissension among the Byzantines, who refused to submit their church to the papacy. John's spirit was broken, and intrigues over the succession, coupled with news of the Turkish victory over the Hungarians in the Second Battle of Kosovo (Kosovo, Battle of) in October 1448, hastened his death. Although his efforts at unification failed, John's trip to Italy was not without some benefit and was an important event in the history of the Italian Renaissance. Italian artists, including Benozzo Gozzoli (Gozzoli, Benozzo), captured the splendour of John and his entourage. Of greater significance was the exchange of ideas between members of the entourage, which included the great scholar George Gemistus Plethon (Gemistus Plethon, George), and Italian humanists. Italian scholars also benefited from access to Greek manuscripts, which were freely bought and sold at the council's meeting places.