The aim of this study is to present a methodology on the determination of the seismic performance of existing reinforced concrete (RC) structures subjected to earthquake effects. This paper focuses on the assessment of existing RC structures with a nonlinear analysis (pushover analysis) method. The research is presented with the case study of an existing RC structure. The investigations on this RC structure is done with the field surveys, lab studies and numerical analyses. The first stage of the study includes the assessment of the compliance between the architectural and structural design projects of the related structure. Necessary checks have been made whether the projects of the structure are consistent with its existing state. Secondly, on-site mechanical tests were conducted on the structural members, and laboratory tests were performed on the samples collected from the structure. In the last stage, the seismic performance of the structure was determined based on the three-dimensional finite element model prepared for the existing state of the structure. The analysis results are discussed considering the target performance level parameter. The results have shown the importance of designating the appropriate target performance level in the evaluation of the seismic performance of RC structures.