The Diocese of Egypt is a diocese of the later Roman Empire, incorporating the provinces of Egypt and Cyrenaica.

Its capital was at Alexandria, and its governor has the unique title of praefectus augustalis (Augustal Prefect, of the rank vir spectabilis) instead of the ordinary vicarius.

The diocese is initially part of the Diocese of the East, but in the year 370, it becomes a separate entity, which lasts until its territories are finally overrun by the Muslim conquest of Egypt in the 640s.The diocese is included in the Praetorian prefecture of the East and includes originally five provinces: Aegyptus Iovia, later renamed Aegyptus, comprised the western Nile Delta, and had Alexandria as its capital; Aegyptus Herculia, later renamed Augustamnica, comprised the eastern Delta, with Pelusium as capital; Thebais, which was bounded to the south by the First Cataract of the Nile, with Ptolemais Hermiou as capital; Libya Inferior (or Interior), corresponding to Marmarica, with Paraetonium as capital; Libya Superior (or Exterior), corresponding to Cyrenaica, with Ptolemais as capital.The attributes Iovia and Herculia were related to the tetrarchs Diocletian and Maximian respectively, and are later changed to remove the pagan connotations.By the early 6th century, the provinces have increased with the creation of: Aegyptus I Aegyptus II Augustamnica I, with Pelusium as capital; Augustamnica II Thebais Superior Thebais Inferior Arcadia, with capital OxyrhyncusDuring the reforms of Justinian I in the late 530s, the administrative structure changes again.

The post of Augustal Prefect (vicar of the diocese) is abolished, and five independent governors (duces), who combine military and civilian authority, are appointed instead.

Two of them, the dux Alexandriae and the dux Thebaidos, also hold the title augustalis (dux et augustalis).