This thesis' subject is the seismic behavior of an existing masonry residential building in Petrinja, constructed in the early 1970s, prior to the adoption of current building regulations. The thesis begins with an overview of the history of masonry structures, starting from the earliest stone constructions, through the introduction of brick as a building material, to modern construction methods and the use of advanced materials. The following section of the thesis describes the 2022 Petrinja earthquake, along with an analysis of its impact on local infrastructure, with a particular emphasis on damage to masonry residential buildings. It is highlighted that the majority of residential units in Croatia are masonry buildings, with many constructed before the implementation of Eurocode 6, the current standard for the design of masonry structures. In accordance with these regulations, the thesis includes a calculation of the vertical and horizontal load bearing capacity of the specified confined masonry building, along with an additional analysis of its seismic resistance according to Eurocode 8. Following the analytical calculation, two numerical models were developed using the 3Muri software, a specialized tool for the calculation and analysis of masonry structures with a focus on seismic resistance. The first model represents unreinforced masonry, while the second model represents confined masonry. The 3Muri software enabled a detailed simulation of the structure's behavior using the pushover analysis. In the last section of the thesis, the results of the analytical calculation are compared with the results of the numerical models developed in 3Muri. The conclusions of the thesis emphasize the importance of proper design and reinforcement of masonry buildings to enhance their seismic resistance and ensure the safety of residents. The thesis also highlights the usefulness of the 3Muri software in evaluating and improving the seismic performance of existing buildings.