The paper deals with the seismic behavior and retrofitting of a historical masonry building called Treasury Building of Hagia Sophia (Skeuophylakion: a building where eucharistic elements were collected and prepared for services) which has experienced lots of earthquakes since it was constructed in 361 and suffered structural damage since then. A detailed finite element (FE) model of the treasure building was created based on the elaborate technical drawings of the structure. A macro modeling approach was adopted and equivalent material parameters were obtained from in-situ tests and the calibration of the FE model was used as the input of the model. Four different models were created considering both the physical and the geometrical nonlinearities in order to understand the seismic behavior of the existing structure and also to account for several retrofit effects on the seismic behavior of the structure. The treasure building was analyzed under very strong earthquake ground motions it might have been exposed during its life time to understand its response and the cause of the observed manner and paths of the observed cracks. It was found out that the plastic strain distribution obtained from the seismic analyses of the first two models and those of the significant cracks observed in situ on the dome and at the tips of arcs are completely compatible. Hence, it was concluded that these cracks are due to the earthquakes occurred until now. The results of seismic analysis of the third and fourth models show that the use of belts (rings) at the location close to the top elevation of arches of windows for retrofitting the structure improves the performance and seismic behavior of the structure considerably. (C) 2017 Elsevier Ltd. All rights reserved.