The Diocese of Illyricum, before 395 known as the Diocese of Pannonia, is a diocese of the Late Roman Empire.

The seat of the vicarius (governor of the diocese) is Sirmium.The Diocese includes the Roman provinces of Pannonia Prima, Pannonia Valeria, Pannonia Savia, Pannonia Secunda, Noricum Mediterraneum, Noricum Ripensis and Dalmatia.Also under its jurisdiction are the Exarch of Sirmium, the Metropolis of Lauriacum, Vindomana, Sirmium, Salona, Salisburgium and the "locus incertus" (the "unknown location").