The Diocese of Pannonia (Latin: Dioecesis Pannoniarum, lit. "Diocese of the Pannonias"), from 395 known as the Diocese of Illyricum, was a diocese of the Late Roman Empire. The seat of the vicarius (governor of the diocese) was Sirmium.
Disputed by the two halves of the Empire in the following years, the Diocese of Pannonia was one of the two dioceses in the eastern quarters of the Tetrarchy not belonging to the cultural Greek half of the empire (the other was Dacia), and so it was transferred back to the Western Empire at the death of Theodosius I in 395 and was joined to the Prefecture of Italy as the Diocese of Illyricum.
In the 9th century, Diocese of Pannonia was also a name of the ecclesiastical territory of the Christian church whose archbishop was Saint Methodius.[1]
As found in the Notitia Dignitatum. Provincial administration reformed and dioceses established by Diocletian, c. 293. Permanent praetorian prefectures established after the death of Constantine I. Empire permanently partitioned after 395. Exarchates of Ravenna and Africa established after 584. After massive territorial losses in the 7th century, the remaining provinces were superseded by the theme system in c. 640–660, although in Asia Minor and parts of Greece they survived under the themes until the early 9th century.