John VI Kantakouzenos, Cantacuzenus, or Cantacuzene (Greek: Ἰωάννης ΣΤʹ Καντακουζηνός, Iōannēs ST′ Kantakouzēnos; Latin: Johannes Cantacuzenus; c. 1292 – 15 June 1383) was a Greek , statesman, and general. He served as Grand Domestic under Andronikos III Palaiologos and regent for John V Palaiologos before reigning as Byzantine emperor in his own right from 1347 to 1354. Usurped by his former ward, he retired to a monastery under the name Joasaph Christodoulos and spent the remainder of his life as a monk and historian.

Born in Constantinople, John Kantakouzenos was the son of Michael Kantakouzenos, governor of the Morea; Donald Nicol speculates that he may have been born after his father's death and raised as an only child. Through his mother Theodora Palaiologina Angelina, he was related to the then-reigning house of Palaiologos. He was also related to the imperial dynasty through his wife Eirene Asanina, a second cousin of Emperor Andronikos III Palaiologos. Kantakouzenos became a close friend to Andronikos III and was one of his principal supporters in Andronikos's struggle against his grandfather, Andronikos II Palaiologos. On the accession of Andronikos III in 1328, he was entrusted with the supreme administration of affairs and served as Grand Domestic throughout his reign. He was named regent to Andronikos's successor, the 9-year-old John V, upon the emperor's death in June 1341.

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