The Diocese of Dacia (Latin: Dioecesis Daciae) was a diocese of the later Roman Empire, in the area of modern Serbia, Montenegro and western Bulgaria. It was subordinate to the praetorian prefecture of Illyricum. Its capital was at Serdica (modern Sofia).
Emperor Aurelian (270-275), confronted with the secession of Gallia and Hispania from the empire since 260, with the advance of the Sassanids in Asia, and the devastations that the Carpians and the Goths had done into Moesia and Illyria, abandoned the province of Dacia created by Trajan and withdrew the troops altogether, fixing the Roman frontier at the Danube. A new Dacia Aureliana was organised south of the Danube our of central Moesia, with its capital at Serdica.
The abandonment of Dacia Traiana by the Romans is mentioned by Eutropius in his Breviarium historiae Romanae, book IX :
|“||The province of Dacia, which Trajan had formed beyond the Danube, he gave up, despairing, after all Illyricum and Moesia had been depopulated, of being able to retain it. The Roman citizens, removed from the town and lands of Dacia, he settled in the interior of Moesia, calling that Dacia which now divides the two Moesiae, and which is on the right hand of the Danube as it runs to the sea, whereas Dacia was previously on the left.||”|
Later, during the administrative reforms of Diocletian and Constantine the Great, the Diocese of Moesia was created, encompassing most of the central Balkans and the Greek peninsula. After a few years, however, the diocese was split in two, forming the Diocese of Macedonia and the Diocese of Dacia, encompassing the provinces of Dacia Mediterranea (the southern, interior portion of Dacia Aureliana), Dacia Ripensis (the northern, Danubian portion of Dacia Aureliana), and Moesia Superior, Dardania and Praevalitana.
The diocese was transferred to the Western Empire in 384 by Theodosius I, probably in partial compensation to the empress Justina for his recognition of the usurpation of Magnus Maximus in Britannia, Gaul and Hispania. However, upon his death in 395, it reverted to the Eastern Empire, forming, together with the Diocese of Macedonia to the south, the praetorian prefecture of Illyricum.