Byzantine monk, theologian, and chronicler, coauthor of a report on the situation of the Eastern Church during the turbulent reign of Photius (858–867 and 878–886), the controversial patriarch of Constantinople. This theological chronicle, or “Letter of Appeal,” constituted one of the most important documents on the resulting break (the Photian schism) between the Western and Greek churches.

An opponent of Photius, Theognostos supported the reigning patriarch, Ignatius, who was deposed by the emperor Michael III in 858 and was replaced by Photius as patriarch. After helping Ignatius to compose the “Letter of Appeal” as a theological apology for his position, Theognostos delivered it to Pope Nicholas I at Rome c. 861 and solicited his aid in returning Ignatius to office. When Nicholas condemned Photius in 863, a bitter dispute ensued, causing a gulf between Byzantium and Rome. Theognostos returned to Constantinople in 868 with a papal message for the new emperor, Basil I, who, seeking a reconciliation with Rome, in turn deposed Photius. Patriarch Ignatius then made Theognostos superior of the Pege monastery and administrator of Hagia Sophia.

The “Letter of Appeal,” Theognostos' anthology on the lives of the saints, and an address on the Virgin Mary have been published in the series Patrologia Graeca, J.-P. Migne (ed.), vol. 105 (1866).