John V Palaiologos or Palaeologus (Greek: Ἰωάννης Παλαιολόγος, Iōánnēs Palaiológos; 18 June 1332 – 16 February 1391) was Byzantine emperor from 1341 to 1391, with interruptions.

His long reign was marked by constant civil war, the spread of the Black Death and several military defeats to the Ottoman Turks, who rose as the dominant power of the region.

His long reign was marked by the gradual dissolution of imperial power amid numerous civil wars and the continuing ascendancy of the Ottoman Turks.

The Hungarian monarch then offered him help on the condition that John join the Catholic Church, or at least achieve recognition by the Patriarch of the Pope's supremacy.

Suleyman Paşa, the son of the Ottoman sultan, led their forces in Europe and was able to take Adrianople and Philippopolis and to exact tribute from the emperor.

In October 1369, John, having travelled through Naples to Rome, formally converted to Catholicism in Saint Peter's Basilica and recognized the Pope as supreme head of the Church.

The same year, John V ordered the strengthening of the Golden Gate in Constantinople, utilizing marble from the decayed churches in and around the city.

Upon completion of this construction, Bayezid I demanded that John raze these new works, threatening war and the blinding of his son Manuel, whom he held in captivity.