John VIII (1390 - Constantinople October 31 1448), surnamed Palaeologus, Byzantine emperor, the oldest son of Manuel II, became sole emperor in 1425. He was married twice, first to Anna daughter of Grand Duke Basil I of Moscow in 1414, then to Sophia of Montferrat in 1421, when he was made co-emperor with his father Manuel. Neither marriage produced any children.

To secure protection against the Ottoman Turks he visited the pope and consented to the union of the Greek and Roman churches, which was ratified at Florence in 1439. He was accompanied by George Gemistos Plethon, a Neoplatonist philosopher who very influential among the academics of Italy and influenced the western European Renaissance, which was just beginning. The proposed church union failed, but by his prudent conduct towards the Ottoman Empire he succeeded in holding possession of Constantinople, and in 1432 withstood a siege by Sultan Murad II.

He nominated his brother Constantine XI as his successor. Despite the machinations of their younger brothers Demetrius and Thomas, their mother Helena Dragas was able to secure Constantine's succession.