Arpos was an ancient city in the region that is now European Turkey. Its exact location is not known, but it was near Rodosto (the modern city of Tekir Dagh, Turkey), west of Istanbul.

In the fourth century, Arpos was the principle city of the region south of Heraclea.

Arpos was a metropolitan see of the Roman Catholic Church from the time of Emperor Manuel I Comnenus (ruled 1143 - 1180) called Theodosiopolis. Either Theodosius I or Theodosius II had attempted to rename Arpos Theodosiopolis, but only the Catholic Church seems to have adopted the name for any length of time.

After the capture of Constantinople during the Fourth Crusade the city of Arpos was alternately held by Tsar Kaloyan of Bulgaria and the Crusaders. The Crusaders referred to Arpos as Naples or Napoli. The Crusaders made Theodosius Branas (who Geoffroi de Villehardouin refers to as Vemas) the Lord of Arpos.

In 1206, Tsar Kaloyan destroyed Arpos, but it was later rebuilt and continued to be ruled by Branas.