John V Palaiologos or Palaeologus (Greek: Ίωάννης Ε' Παλαιολόγος, Iōannēs V Palaiologos; 18 June 1332 – 16 February 1391) was a Byzantine emperor, who succeeded his father in 1341 at age eight.

John V was the son of Emperor Andronikos III and his wife Anna, the daughter of Count Amadeus V of Savoy by his second wife Maria of Brabant. His long reign was marked by the gradual dissolution of imperial power amid numerous civil wars and the continuing ascendancy of the Ottoman Turks.

John V came to the throne at age eight. His reign began with an immediate civil war between his designated regent, his father's friend John Kantakouzenos, and a self-proclaimed council of regency composed of his mother Anna, the patriarch John XIV Kalekas, and the megas doux Alexios Apokaukos. During this civil war in 1343 Anna pawned the Byzantine crown jewels for 30,000 Venetian ducats. From 1346 to 1349, the Black Plague devastated Constantinople.

Victorious in 1347, John Kantakouzenos ruled as co-emperor until his son Matthew was attacked by John V in 1352, leading to a second civil war. John V asked the ruler of Serbia, Stefan Dušan for help, and Dušan obliged by sending 4,000 Serbian horsemen to his aid. Matthew Kantakouzenos asked his father for help, and 10,000 Ottoman Turks showed up at Demotika (Didymoteicho) in October 1352 and engaged the forces of John V's Serbian allies in an open field battle that resulted in the destruction of the allies and a victory for the more numerous Turks in the service of the Byzantines. The Ottoman Empire thus acquired its first European territory, at Çimpe and Gallipoli. Able to retake Constantinople in 1354, John V removed and tonsured John VI; by 1357, he had deposed Matthew as well, who had been captured by the Serbs and was ransomed to John V.

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