The praetorian prefecture of Illyricum (Latin: praefectura praetorio per Illyricum; Greek: ὑπαρχία τῶν πραιτωρίων τοῦ Ἰλλυρικοῦ, also termed the prefecture of Illyricum) was one of four praetorian prefectures into which the Late Roman Empire was divided.
The administrative center of the prefecture was Sirmium from 375 to 379, then Thessalonica. The prefecture took its name from the older province of Illyricum, which in turn was named after ancient Illyria. In its greatest expanse it encompassed Pannonia, Noricum, Crete, and most of the Balkan peninsula except for Thrace.
