St Theodore (Russian: Святой Феодор начертанный) (died 820) was a monk of the Eastern Orthodox Church who was known for his strong opposition to the Byzantine Iconoclasm.
Theodore was born in Constantinople to a family with strong connections to the Eastern Orthodox Church.
He clashed with successive emperors (Constantine VI, Nicephorus I, Leo V the Armenian, and Michael II the Stammerer) on the issue of iconoclasm (then for the second time in ascendancy in the Empire but strongly opposed by St Theodore).
For this he suffered imprisonment and exile: he is the author of many works in which he defends Orthodoxy.
He died in exile in Bithynia in the Akrita Monastery which he had founded.