(Ἰωάννης ὁ Καντακουζήνος), emperor of Constantinople (A. D. 1342-1355), often called Joannes VI. His full name was Joannes Angelus Comnenus Palaeologus Cantacuzenus. He was the eldest son of Joannes Cantacuzenus, the chief of a great Greek family, and Theodora Palaeologina, and was born early in the beginning of the 14th century. [See the genealogical table of the Cantacuzeni,Vol. I. p. 595.] His history is intimately connected with that of his ward and rival Joannes VI. Palaeologus. John Cantacuzenus, the subject of this article, early distinguished himself in the service of his relative, the emperor Andronicus Palaeologus the elder, who appointed him prefect of the sacred bed-chamber. United, by friendship and harmony of sentiments, to the emperor's grandson, Andronicus the younger, he took the part of the latter in his rebellion against his grandfather; and it was to his valour, wisdom, and exertions, that the younger Andronicus owed his final success and the undisputed crown of Constantinople. In reward for his services, he was appointed magnus domesticus. Aetolia and Lesbos, both in the hands of usurpers, were re-united by him to the empire; and his influence was so great, that he, rather than Andronicus, was the real sovereign of the Greeks. His administration was wise : he enforced the laws with firmness, but also with forbearance; and at a time when every public functionary was a robber of the people, he alone escaped the charge of peculation and fiscal oppression. The emperor bestowed upon him unbounded confidence, and was so fondly attached to him, that he proposed to share the throne with him. This Cantacuzenus refused, from motives both of modesty and prudence. Andronicus, on his deathbed (A. D. 1341), appointed him guardian of his infant son, John, in whose name he was to govern the empire.

No sooner had Cantacuzenus begun to exercise his eminent functions, than he was checked by two ambitious intriguers, the admiral Apocauchus and the patriarch of Constantinople, John of Apri, who aspired to the regency, and for that purpose persuaded the widow of the late emperor, Anna, princess of Savoy, to claim the guardianship of her son, although it was lawfully vested in Cantacuzenus. The conspirators found many adherents ; and from a system of calumny and petty annoyance, proceeded to bold attacks. During a temporary absence from the capital, Cantacuzenus was suddenly charged with high treason; and his enemies being his judges also, he was found guilty, sentenced to death, and deprived of his estates and emoluments. Under such circumstances he had no alternative but rebellion or death : yet he hesitated till his friends showed him that even by submission and imploring the clemency of his adversaries, he could not save his life. Accordingly Cantacuzenus took up arms, not against the infant emperor, but against his powerful councillors, and assumed the title of emperor. On the 21st of March, 1342, he was crowned with great solemnity, together with his wife, Irene, at Adrianople, by Lazarus, patriarch of Jerusalem. His adherents not being numerous, he sought assistance at the court of Stephen Duscham, kral or king of Servia; and having reason to suspect the faith of this prince, he reluctantly concluded an alliance with Umur Bey, the Turkish prince of Aidin (Lydia, Maeonia and Caria). During the transactions which led to this alliance Cantacuzenus was at the Servian court, and his wife was at Didymoticum. Umur Bey sailed over to Greece with a fleet of 380 vessels, and an army of 28,000 men; and after having left a strong garrison at Didymoticun, marched upon Servia. An early and very severe winter compelled him to return to Asia without having had an interview with Cantacuzenus; but the two princes met in the following year, 1343, at Clopa, near Thessalonica, and in their operations against Apocauchus and his party, Greece and Thrace were dreadfully ravaged. Bribed by Apocauchus, Umur Bey ceased assisting Cantacuzenus, who, however, found a more powerful ally in the person of Urkhan, sultan of the Turks Osmanlis, to whom he gave his daughter in marriage. During five years Greece was desolated by a civil war. In 1346, however, Cantacuzenus became the more powerful; and having made a sort of reconciliation with the dowager empress, Anna, he advanced upon Constantinople, after re-enforcing his army by a body of Latin mercenaries. In January, 1347, he took the capital with scarcely any resistance, the gates having been opened by Facciolati, an Italian captain, who was the secret adherent of Cantacuzenus; and Apocauchus was slain in the tumult. Being now sole master, Cantacuzenus consented to acknowledge John Palaeologus as co-emperor, on condition that until the majority of the young prince, who was then fifteen years, and would be of age at twenty-five, according to the Greek law, he should be the sole ruler ; and as a guarantee for the future harmony between the two princes, he married his daughter Helena to his youthful colleague. In the same year Cantacuzenus was crowned a second time in the capital, by Isidorus, patriarch of Constantinople.

The reign of John Cantacuzenus was not blessed with peace. In the year of his accession, the plague made great havoc among the inhabitants of the capital and other towns. The Genoese of Pera, who enjoyed great privileges, despised the imperial authority, took up arms, and laid them down only after having obtained still greater privileges; and during the same time Duscham, the kral of Servia, made an inroad into Thrace, but was fortunately compelled, by severe defeats, to sue for peace. The emperor's relations with the Turks were amicable for several years. In his history (4.16) Cantacuzenus alludes to a project formed by Merjan, an eunuch in the service of sultan Urkhan, to poison his young colleague; but it would seem as if the story had been invented by himself, for the purpose of frightening young Palaeologus, and thus bringing him under a still closer watch. His friendship with Urkhan was, however, not very sincere, for he sent ambassadors to pope Clement VI. promising to bring the Greek church under the papal authority if the holy father would preach a crusade against the Turks; but Clement declined the proposition, knowing that the Greeks and Latins would agree upon religion only so long as the crusaders did upon a common plan of attack, and an equal mode of division in case of success. Meanwhile, dissensions arose between Cantacuzenus and Palaeologus, who grew tired of his inactivity, and listened to the advice of the former party of Apocauchus, although he was kindly treated and allowed full domestic freedom by his father-in-law, which, it would seem, was quite enough for so young a man. Suspecting some treachery, Cantacuzenus sent him to reside at Thessalonica, and employed Anne of Savoy, though in vain, as mediator between her son and him : the young prince emancipated himself from the surveillance of the officers charged with guiding and watching him, and in 1353 raised the standard of rebellion. Defeated in a pitched battle by the united forces of Cantacuzenus and Urkhan, Palaeologus took refuge with the Latins in Tenedos; and in order to exclude him for ever from the throne, the emperor proclaimed his son, Matthaeus, coemperor, and his future successor. However well calculated this step might have been had the emperor enjoyed universal popularity, it proved disastrous under contrary circumstances, as the Greeks felt much more sympathy with the house of the Palaeologi than with the Cantacuzeni, and the emperor soon learned that the people's attachment to a distinguished person is often much less strong than their love of a distinguished family. Numerous bands organised themselves for the support of the son of their late emperor, but the forces upon which the latter could rely with more security were the mercenary band and the ships of Gasteluzzi or Gatteluzzi, a noble Genoese who promised to help him to the crown on condition of obtaining the hand of his sister and the grant of some lands. The descendants of Gasteluzzi became sovereign princes, and were conspicuous in the latter part of Byzantine history. Palaeologus and Gasteluzzi made sail for Constantinople; and pleading distress and want of provisions as pretext for their admission within the Golden Horn, the chain across the entrance of the port was lowered by the watch of the harbour, who were either bribed by Palaeologus, or were not aware that the ships had hostile intentions. The inhabitants of Constantinople now took up arms against Cantacuzenus, who, although he asserts the contrary, was apparently forsaken by most of his adherents, abdicated (January, 1355), and four days after his abdication renounced the world, and assumed the monastic habit.