John VIII Palaiologos or Palaeologus (Greek: Ἰωάννης Παλαιολόγος, romanized: Iōánnēs Palaiológos; 18 December 1392 – 31 October 1448) was the penultimate Byzantine emperor.

Ruling from 1425 to 1448, he attempted to bring about the reunification of the Orthodox and Catholic churches and prioritized the protection of Constantinople against the Ottoman Empire.

[4][5][6] In June 1422, John VIII Palaiologos supervised the defense of Constantinople during a siege by Murad II,[7] but had to accept the loss of Thessalonica, which his brother Andronikos had given to Venice in 1423.

The union was ratified at the Council of Florence in 1439, which John attended with 700 followers including Patriarch Joseph II of Constantinople and George Gemistos Plethon, a Neoplatonist philosopher influential among the academics of Italy.

The union failed due to opposition in Constantinople, but through his prudent conduct towards the Ottoman Empire he succeeded in holding possession of the city.