Callistus II of Constantinople was a Byzantine monk and spiritual writer who served as the Ecumenical Patriarch in 1397. Born around 1301 in Constantinople, he was a practitioner of the Hesychast tradition, which was common among many high-ranking clergy during the 14th century. His brief tenure as patriarch occurred during the reign of Emperor Manuel II Palaiologos, a period when the city faced a siege by the Ottoman Sultan Bayezid I.