The Diocese of the East (Latin: Dioecesis Orientis, Greek: Ἐῴα Διοίκησις) was a diocese of the later Roman Empire, incorporating the provinces of the western Middle East, between the Mediterranean Sea and Mesopotamia. During late Antiquity, it was one of the major commercial, agricultural, religious and intellectual areas of the Empire, and its strategic location facing the Sassanid Empire and the unruly desert tribes gave it exceptional military importance.
Its capital was at Antioch, and its governor had the special title of comes Orientis ("Count of the East", of the rank vir spectabilis and later vir gloriosus) instead of the ordinary "vicarius". The diocese was established after the reforms of Diocletian, and was subordinate to the Praetorian prefecture of the East. It presumably lasted until its territories were finally overrun by the Muslim conquest in the 630s. After this, the armies (by this point known in Greek as Themata) of the East retreated into central Anatolia, where they would form the basis of what would eventually evolve into the Theme of Anatolikon.
The diocese included originally all Middle Eastern provinces: Isauria, Cilicia, Cyprus (until 536), Euphratensis, Mesopotamia, Osroene, Syria, Phoenice, Palaestina Prima, Palaestina Secunda, Arabia, and the Egyptian provinces Aegyptus, Augustamnica, Thebais, Libya Superior and Libya Inferior, which were grouped into the separate Diocese of Egypt under Valens. During the course of the 4th century, the several provinces were split, resulting in the new provinces of Cilicia I and Cilicia II, Syria I and Syria Salutaris, Phoenice Libanensis (east of the Mt. Lebanon), Palaestina I, Palaestina II and Palaestina Salutaris (or Palaestina III). The last creation of a new province dated in the reign of Justinian I, when Theodorias, the region around Laodicea, was split off from Syria I.
List of Comites Orientis