The Diocese of the East is 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 is 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 is at Antioch, and its governor has 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 is established after the reforms of Diocletian (r. 284–305), and is subordinate to the praetorian prefecture of the East.
The diocese includes originally all Middle Eastern provinces of the Empire: 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 are grouped into the separate Diocese of Egypt under Valens (r. 364–378).
During the course of the 4th century, the several provinces are 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 dates in the reign of Justinian I (r. 527–565), when Theodorias, the region around Laodicea, is split off from Syria I.In 535, as part of his administrative reforms, Justinian I abolishes the diocese, and the comes Orientis became the provincial governor of Syria I, while retaining his previous rank of vir spectabilis and his salary.
The entire area of the former diocese comes under Sassanid Persian occupation in the 610s and 620s, during the Byzantine–Sassanid War of 602–628.
Shortly after the Byzantine victory in the war and the recovery of the region, it is again lost, this time permanently, to the Muslim conquests.
