Callistus II Xanthopoulos or Xanthopulus (Medieval Greek: Κάλλιστος Ξανθόπουλος; died after 1397) was a Byzantine Hesychast monk and spiritual writer who reigned as Ecumenical Patriarch of Constantinople in 1397. He was Patriarch through the reign of the Byzantine Emperor Manuel II Palaiologos, and through his short Patriarchal reign Constantinople was under siege by the Ottoman Sultan Bayezid I. Within the Orthodox Church, his memory is celebrated on 22 November.^[1][2]

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His surname indicates that he was from the monastery of Xanthopoulos. The majority of Patriarchs in the 14th century were monks in the Hesychast tradition.^[3]