The Diocese of Moesia was one of the twelve dioceses

Roman diocese

A Roman or civil diocese was one of the administrative divisions of the later Roman Empire, starting with the Tetrarchy. It formed the intermediate level of government, grouping several provinces and being in turn subordinated to a praetorian prefecture....

 in which Diocletian

Diocletian

Diocletian |latinized]] upon his accession to Diocletian . c. 22 December 244  – 3 December 311), was a Roman Emperor from 284 to 305....

 (r. 284–305) divided the Roman Empire

Roman Empire

The Roman Empire was the post-Republican period of the ancient Roman civilization, characterised by an autocratic form of government and large territorial holdings in Europe and around the Mediterranean....

 during his administrative reforms. It encompassed most of the central Balkans

Balkans

The Balkans is a geopolitical and cultural region of southeastern Europe...

 and the Greek peninsula, stretching from the Danube

Danube

The Danube is a river in the Central Europe and the Europe's second longest river after the Volga. It is classified as an international waterway....

 to Crete

Crete

Crete is the largest and most populous of the Greek islands, the fifth largest island in the Mediterranean Sea, and one of the thirteen administrative regions of Greece. It forms a significant part of the economy and cultural heritage of Greece while retaining its own local cultural traits...

. The diocese was split in two, forming the Diocese of Macedonia

Diocese of Macedonia

The Diocese of Macedonia was a diocese of the later Roman Empire, forming part of the praetorian prefecture of Illyricum. Its capital was Thessalonica....

 in the south and the Diocese of Dacia

Diocese of Dacia

The Diocese of Dacia was a diocese of the later Roman Empire, in the area of modern Serbia and western Bulgaria. It was subordinate to the Praetorian prefecture of Illyricum...

 in the north, probably under Constantine I

Constantine I

Constantine the Great , also known as Constantine I or Saint Constantine, was Roman Emperor from 306 to 337. Well known for being the first Roman emperor to convert to Christianity, Constantine and co-Emperor Licinius issued the Edict of Milan in 313, which proclaimed religious tolerance of all...

 (r. 306–337), although it is not attested until ca. 370. The two new diocese were grouped into the new praetorian prefecture of Illyricum

Praetorian prefecture of Illyricum

The praetorian 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 , and, after 379, Thessalonica...

 in the second half of the 4th century, which essentially covered the same area as the Diocese of Moesia.

Administration

At the time of Diocletian, the diocese comprised 11 provinces

Roman province

In Ancient Rome, a province was the basic, and, until the Tetrarchy , largest territorial and administrative unit of the empire's territorial possessions outside of Italy...

:

• Achaea
  Achaea (Roman province)
  Achaea, or Achaia, was a province of the Roman Empire, consisting of the Peloponnese, eastern Central Greece and parts of Thessaly. It bordered on the north by the provinces of Epirus vetus and Macedonia...
  
  
• Crete
• Dacia Aureliana
  Dacia Aureliana
  Dacia Aureliana was a province of the Roman Empire found by Emperor Aurelian, after his retreat from Dacia Traiana in 271. Between 271/275 and 285, it occupied most of what is today Bulgaria. Its capital was in Serdica...
  
  
• Dardania
• Epirus Nova
• Epirus Vetus
• Insulae
• Macedonia
  Macedonia (Roman province)
  The Roman province of Macedonia was officially established in 146 BC, after the Roman general Quintus Caecilius Metellus defeated Andriscus of Macedon, the last Ancient King of Macedon in 148 BC, and after the four client republics established by Rome in the region were dissolved...
  
  
• Moesia Superior
• Praevalitana
  Praevalitana
  Praevalitana was an ancient Roman province. It included parts of present-day Albania, Montenegro and Serbia.-History:...
  
  
• Thessalia