Bithynia was a region of northwest Asia Minor, opposite Constantinople. Bithynia became a separate province in the early 4th century. Besides its Capital, Nicomedia, Bithynia contained a few important cities, including Nicaea, Chalcedon and Prousa and rich agricultural land. Although its cities were eclipsed by the growth of Constantinople, Bithynia prospered from its location on the trade and military routes between Constantinople and Anatolia. The suburban coastal region east of Constantinople flourished particularly as the seat of many rich villas. Bithynia became part of the theme of Opsikion in the 7th century, then was divided between that theme and the Optimatoi. The civil province of Bithynia continued to exist into the 8th century, when Slav captives were settled there.
Frequent later references are to the geographical region. Texts of the 13th century mention a district called Mesothynia, which apparently denotes the peninsula of Nicomedia. Bithynia preserves the remains of numerous fortifications but is especially noted for its churches (including churches at Sige, Medikion, Pelekete and Nicaea). It was also a monastic center that grew in importance in the Iconoclastic period at Mt. Auxentios (modern Uludağ). Ecclesiastically, Bithynia was divided into three provinces after 451: Nicomedia, Nicaea, and Chalcedon.