patriarch of Alexandria (535–566), theologian, and leader of the Monophysites (monophysite) in Egypt and Syria, who were reputed for their asceticism and also for their mystical prayer.

Through the support of the Byzantine empress Theodora, Theodosius was elected patriarch as the candidate of the moderate wing of the Monophysites, a group of Christians who believed that Christ had one nature rather than both divine and human natures. With his colleague Severus, the learned patriarch of Antioch, Theodosius favoured mysticism and a belief in a spiritualized Christ. He opposed the teachings of Eutyches (a monk who argued that the humanity of the incarnate Christ was unlike that of other men) and the extreme Monophysites and their candidate for the patriarchate. Lacking popular support, however, Theodosius fled Alexandria and took refuge in a neighbouring monastery until late May 535, when imperial troops drove the extremist Monophysite party from Alexandria. Although judged to be the legitimate patriarch, he was repudiated by a wide segment of Alexandria's population who sympathized with the more uncompromising Monophysites.

The Byzantine emperor Justinian I summoned Theodosius to Constantinople in December 536 in an attempt to win him over to the orthodox position on Christology as expressed in 451 by the Council of Chalcedon. Although Theodosius' moderate Monophysitism could not be reconciled with the conciliar decree, he was not forcefully constrained to abjure his views. He was, nevertheless, effectively prevented from administering his patriarchate by being detained for the remainder of his life under imperial surveillance at Constantinople together with like-minded Monophysite clergy. During his detention, Theodosius continued to play a leading role in the Monophysite church. After the death of Severus of Antioch, its principal spokesman, Theodosius emerged as its head throughout the Byzantine Empire. While avoiding outright confrontation with the orthodox emperor, he still succeeded in aiding and influencing independent churches in Antioch, Syria (the Jacobite church), and in Egypt (the Coptic church) and advancing their missionary activity. At the accession of Emperor Justin II in 565, Theodosius was given permission to return to Alexandria, but he died before departure and was buried at Constantinople with patriarchal honours. In his Byzantine exile Theodosius wrote treatises against the heresies of the tritheists, believers in three gods, and the Agnoetae, believers in Christ's fallible knowledge. His extant works include Coptic sermons and expositions of moderate Monophysite doctrine addressed to leading Byzantine figures. These writings are contained in the series Patrologia Graeca, J.-P. Migne (ed.), vol. 86 (1866).