Praetorian Prefecture of Illyricum

The praetorian prefecture of Illyricum (Latin: praefectura praetorio per Illyricum, Greek: ἐπαρχότης/ὑπαρχία τοῦ Ἰλλυρικοῦ, also termed simply the Prefecture of Illyricum) was one of four praetorian prefectures into which the Late Roman Empire was divided.

The administrative centre of the prefecture was Sirmium (318-379), and, after 379, Thessalonica. It took its name from the older province of Illyricum, which in turn was named after ancient Illyria, and in its greatest expanse encompassed Pannonia, Noricum, Crete, and most of the Balkan peninsula except for Thrace.