Callistus I of Constantinople was a 14th-century Byzantine prelate and monk who served as the Ecumenical Patriarch of Constantinople during two separate terms between 1350 and 1363. A dedicated follower of Gregory Palamas, he spent twenty-eight years living as a monk on Mount Athos. He is remembered for his significant role in establishing the orthodoxy of Hesychasm through a major synod in 1351. As a theologian and writer, he contributed to the spiritual traditions of the Eastern Orthodox Church through his ascetic writings.