John VI Kantakouzenos or Cantacuzene was a Byzantine Greek nobleman, 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. Deposed by his former ward, he was forced to retire to a monastery under the name Joasaph Christodoulos and spent the remainder of his life as a monk and historian. At age 90 or 91 at his death, he was the longest-lived of the Roman emperors.

The title of grand domestic was given in the 11th–15th centuries to the commander-in-chief of the Byzantine army, directly below the Byzantine Emperor. It evolved from the earlier office of the domestic of the Schools, and came to rank as one of the senior dignities in the Byzantine state during the last centuries of its existence. From Byzantium, it was also adopted by the breakaway Empire of Trebizond, as well as by the 14th-century Serbian Empire.