In this study, the Early Byzantine Building of “Church B” from Hadrianoupolis, the ancient city in the town of Eskipazar (Karabuk province) is described with the purpose of addressing the historical process of the church with new data and evaluate its architectural and ornamental characteristics. Church B, has been the subject of some scientific puplications to present day. However, it hasn’t been adequately appreciated in terms of Byzantine art and architecture. Within the scope of this study, the plan design, architectural sculpture, building materials, wall masonsary, liturgical furniture and floor mosaic of the church will be evaluated, and its position and importance in Byzantine art and architecture will be discussed. The Church B was built in the city centre of Paphlagonian Hadrianoupolis, in the area called Dut dibi.  The church has been referred to as Hadrianoupolis Church B in puplications until present day, in this study, it was designated Hadrianoupolis The Four River Church due to the mosaic panel containing four sacred rivers. The church was built in a frequently used three-nave basilica plan with a semicircle shaped apsis, protruding to the east, and two rectangular narthexes in its west. To the west of the apsis is a rectangular bema that is bordered by a plane templon on three sidesway. The excavations revealed that the naos, bema and apse floors of the church were adorned with mosaics. In addition to the findings from excavations, it was concluded that church was built in the V.-VI. centuries based on its architectural and ornamental characteristics.