John V Palaiologos or Palaeologus (Greek: Ίωάννης Ε' Παλαιολόγος) 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.
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. 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.
The Ottomans, who had been allied with the Kantakouzenoi, continued to press John. 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. John V appealed to the West for help, proposing to end the schism between the Byzantine and Latin churches by submitting the patriarchate to the supremacy of Rome.
In 1390, his grandson John VII briefly usurped the throne, but he was quickly overthrown. John V was finally succeeded to the imperial throne by his son Manuel. His younger son Theodore had already acceded to the Despotate of Morea in 1383.