Which Byzantine emperor was on the throne during the Great Schism of 1054?

The Byzantine emperor on the throne during the Great Schism of 1054 was Constantine IX Monomachos. He ruled from 1042 until his death in January 1055, placing him at the center of the political crisis that divided the Catholic and Eastern Orthodox churches.

The immediate dispute involved Pope Leo IX’s representatives and Michael I Cerularius, the patriarch of Constantinople. Constantine initially welcomed the papal legates because he hoped cooperation with Rome might help secure Byzantine interests in southern Italy. Their quarrel with the patriarch escalated, however, and in July 1054 the legates placed a papal bull of excommunication on the altar of Hagia Sophia. Cerularius responded by condemning the legates.

The breach was not an instant, complete separation of every Christian community, and later historians often treat 1054 as a symbolic milestone in a longer process. Constantine X Doukas, Isaac I Komnenos, and Michael VI came to power afterward.