born Dec. 17/18, 1392
died Oct. 31, 1448, Constantinople
Byzantine emperor (142148).
The son of Manuel II Palaeologus , he was crowned coemperor with his father in 1408 and took effective control of the empire in 1421. He became sole emperor after his father's death in 1425. Of the diminished and fragmented empire, he ruled only Constantinople and the surrounding area. The city was besieged by the Ottoman Turks (1422), and, when Thessaloníki fell to Turkish forces (1430), John appealed to the West for help. He united the Byzantine and Latin churches (1439), but joint efforts against the Turks failed, and the Byzantines refused to submit to the pope. John died amid intrigues over succession.