Pope Theodore I (Latin: Theodorus I) was Pope from 24 November 642 to his death in 649.
According to the Liber Pontificalis, he was a Greek inhabitant of Jerusalem whose father Theodorus had been a bishop in the city. He was among the many Syrian clergy who fled to Rome following the Muslim conquest of the Levant.
He was made a cardinal deacon (possibly around 640) and a full cardinal by Pope John IV.
His election was supported by the Exarch of Ravenna and he was installed on 24 November 642, succeeding John IV. The main focus of his pontificate was the continued struggle against the heretical Monothelites. He refused to recognize Paul as the Patriarch of Constantinople, because his predecessor, Pyrrhus, had not been correctly replaced. He pressed Emperor Constans II to withdraw the Ecthesis of Heraclius. While his efforts made little impression on Constantinople, it increased the opposition to the heresy in the West; Pyrrhus even briefly recanted his heresy (645), but was excommunicated in 648. Paul was excommunicated in 649. In response, Paul destroyed the Roman altar in the palace of Placidia and exiled or imprisoned the papal nuncios. But he also sought to end the issue with the Emperor by promulgating the Type of Constans, ordering that the Ecthesis be taken down and seeking to end discussion on the doctrine.
Theodore planned the Lateran Council of 649 to condemn the Ecthesis, but died before he could convene it. His successor, Pope Martin I, did so instead. Theodore was buried in St. Peter's Basilica.
His feast day in the Orthodox Church is on 18 May.