John V Palaiologos or Palaeologus (Greek: Ίωάννης Ε' Παλαιολόγος) was a Byzantine emperor, who succeeded his father in 1341 at age eight.
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.
Matthew Kantakouzenos was crowned co-emperor on 10 December 1354. From his Thracian domain, centred on Gratzianous, he led several wars against the Serbs. An attack, which he prepared in 1350, was frustrated by the defection of his Turkish auxiliaries. However, with five thousand Turks he tried to re-establish his former appanage along the Serbian-Byzantine border by attacking this region but failed to take Serres and soon was defeated in battle in late 1356 or early 1357 by a Serb army under Vojvoda Vojihna, the holder of Drama, a major fortress in the vicinity. The Serbs captured Matthew with the intention of releasing him when he had raised the large ransom they demanded. However John V, who had rapidly moved in to occupy Matthew's lands, offered Vojihna an even larger sum to turn Matthew over to him.
After imprisoning Matthew first on Tenedos, then on Lesbos under the watchful eye of Francesco I Gattilusio, John forced him to renounce the imperial title, John then released him to go to the Morea, where he joined his brother Manuel, who was ruling there (1361).