Marcopolis is a Catholic titular see

Titular see

A titular see in various churches is an episcopal see of a former diocese that no longer functions, sometimes called a "dead diocese". The ordinary or hierarch of such a see may be styled a "titular bishop", "titular metropolitan", or "titular archbishop"....

. The original diocese was in Asia Minor

Asia Minor

Asia Minor is a geographical location at the westernmost protrusion of Asia, also called Anatolia, and corresponds to the western two thirds of the Asian part of Turkey...

, a suffragan of Edessa. The native name of this city is not known, but it owes its Greek name to the Emperor Marcus Aurelius.

Marcopolis is described at the beginning of the seventh century by the geographer George of Cyprus. The site of this city has not been found.

Bishops

In the Notitiæ episcopatuum of Antioch

Antioch

Antioch on the Orontes was an ancient city on the eastern side of the Orontes River. It is near the modern city of Antakya, Turkey.Founded near the end of the 4th century BC by Seleucus I Nicator, one of Alexander the Great's generals, Antioch eventually rivaled Alexandria as the chief city of the...

 (sixth century) it is alluded to as a see of Osrhoene. Two of its early bishops are known:

• Cyrus, who attended the First Council of Ephesus in 431;
• Caioumas, present at the Council of Chalcedon
  Council of Chalcedon
  The Council of Chalcedon was a church council held from 8 October to 1 November, 451 AD, at Chalcedon , on the Asian side of the Bosporus. The council marked a significant turning point in the Christological debates that led to the separation of the church of the Eastern Roman Empire in the 5th...
  
   in 451.

Eubel mentions four other titulars between 1340 and 1400, and a fifth from 1441 to 1453.