Byzantine monk and patriarch of Constantinople, who directed the opposition to the reunion of Greek and Latin churches decreed by the Second Council of Lyon in 1274. His efforts in reforming the Greek Orthodox Church encountered opposition from clergy and hierarchy.

A monk who emigrated to a monastery in the hallowed region of Mt. Athos, Greece, Athanasius journeyed to the Holy Land and lived as a solitary on Mt. Galesios, Palestine, where he was ordained priest. Later he returned to Mt. Athos and founded a monastery. Because of his anti-unionist activities after the reunification decree of the Council of Lyon, he was compelled by the patriarch John XI Beccus (1275–82) to seek Palestinian refuge. With the accession of the anti-unionist emperor Andronicus II (Andronicus II Palaeologus) in 1289, however, Athanasius was chosen patriarch of Constantinople and initiated a sweeping ecclesiastical reform. He imposed strict discipline on the clergy, charged his bishops to live in their own dioceses, and restricted the wanderings of monks. Much of the source material on his reform effort, as well as on Byzantine social and economic conditions of the times, is recorded in a collection of 126 letters.

Athanasius' severe measures evoked opposition from the clergy, and Emperor Andronicus permitted his resignation. Popular support restored him to his patriarchal office; however, after his expulsion of the Latin Church's Franciscan monks from Constantinople in 1307, the unionist faction finally succeeded in forcing his retirement early in 1310 to the monastery of Xerolophus in Constantinople.