Gregory III of Constantinople was an Eastern Orthodox priest who served as the Ecumenical Patriarch from 1445 to 1450. Born in Crete around 1390, he became a significant figure in the theological efforts to reunite the Eastern and Western churches. He was a close advisor to Emperor John VIII Palaiologos and played a central role in the negotiations at the Council of Florence. Due to intense internal opposition regarding these unionist policies, he eventually left his position and moved to Rome.