Hadrianopolis is an important Southern Paphlagonian ancient city within the borders of Eskipazar district of Karabük. As a result of the excavations that have been going on since 2003, the ruins of seven buildings, date to the Early Byzantine period were discovered in the city. The one of them called as The Cross-in-Square Planned Building in Hadrianopolis Citadel, is also the subject of this article. Its location, architectural characteristics and findings indicate that it was an administrative or military building. Its construction has been completed in two stages, the first construction was in the Roman period, probably around the 2nd-3rd century AD. and it was destroyed at the beginning of the Early Byzantine period between the 4th and 5th centuries and rebuilt with the cross-in-square plan. The cross-in-square plan was common in Byzantine architecture mostly in religious buildings such as churches, martyrium and mausoleums. The cross-in square planned building adressed in this article had an administrative or military function. Besides the rarity of its type of plan in the Early Byzantine period, its military or administrative function makes it more particular as a cross-in-squared planned building of this era. In this study, the history, architectural characteristics and findings of the building were explained in detail, and suggestions about its function were presented and the importance of the building in Byzantine architecture is tried to be emphasized.