The Diocese of Moesia is one of the twelve dioceses in which Diocletian (r. 284–305) divides the Roman Empire during his administrative reforms.

It encompasses most of the central Balkans and the Greek peninsula, stretching from the Danube to Crete.

The diocese ias split in two, forming the Diocese of Macedonia in the south and the Diocese of Dacia in the north, probably under Constantine I (r. 306–337), although it is not attested until ca.

370.

The two new diocese are grouped into the new praetorian prefecture of Illyricum in the second half of the fourth century, which essentially covers the same area as the Diocese of Moesia.