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map of the Praetorian prefecture of Illyricum, before 379 AD

The Praetorian prefecture of Illyricum (Latin: Praefectura Praetorio per Illyricum, also termed simply the Prefecture of Illyricum) was one of four large Praetorian prefectures into which the Late Roman Empire was divided. The administrative centre of the prefecture was initially Sirmium, and after 379 Thessalonica. [1] [2] [3] It took its name from the older province of Illyricum, which in turn was named after ancient Illyria.

There is disagreement in various sources about exact date of the establishment of the prefecture. According to some sources, it was organized in 318 (or in 324) by the emperor Constantine I, [4] [5] while according to other sources, it was organized in 356^{[citation needed]} by the emperor Constantius II from the easternmost provinces of the Praetorian prefecture of Italy.^{[citation needed]}

The prefecture originally included two dioceses: the Diocese of Pannonia and the Diocese of Moesia. Later, the Diocese of Moesia was divided into the Diocese of Dacia and the Diocese of Macedonia. The prefecture encompassed Pannonia, Noricum, Crete, and most of the Balkan peninsula except Thrace.

In 361, the emperor Julian abolished the Prefecture and integrated its provinces into the Prefecture of Italy, but it was reestablished under Gratian in 375. During this time, the Prefecture belonged to the Western Empire, but its possession was contested by the Eastern Empire.

In 379, the Prefecture of Illyricum was divided into Western and Eastern Illyricum. Western Illyricum was made a civil diocese under the jurisdiction of a vicar of the Prefecture of Italy residing at Sirmium, while Eastern Illyricum would remain a separate Prefecture under the Eastern Empire. The division was not finalized until the death of Theodosius II in 395.

In 437, however, as part of the dowry of Licinia Eudoxia, Valentinian III ceded the eastern part of the Diocese of Illyricum (ex Diocese of Pannonia) to the Prefecture of Illyricum, thus the seat of the prefecture was again moved to Sirmium. [6] However, the new provinces were soon devastated by the incursions of the Huns under Attila, and the provinces soon reverted to the Western Empire.

• Apraemis, prefect of the Prefecture of Illyricum with residence in Sirmium (before 441).

• The Times History of Europe, Times Books, London, 2001.

• Illyricum (Roman province)
• Diocese of Illyricum
• Illyria
• Roman provinces

• Map - The Roman Empire in AD 337
• Catholic Encyclopedia - Greece
• Catholic Encyclopedia - Thessalonica
• Catholic Encyclopedia - Illyria
• Southern Pannonia during the age of the Great Migrations