Adding a reinforced stucco layer to the masonry infill walls is a preferred method for strengthening RC frame system structures with an easy-to-apply method that does not require a long time, is economical, and does not require detailed and extensive workmanship. However, no research has been discovered as a result of the extensive literature review that investigates the effects of masonry-infilled RC frames strengthened with a reinforced stucco layer on the seismic performance of openings that must be due to architectural requirements such as doors, windows, installations, and similar ventilation systems. As a result, an experimental study was planned to investigate the effects of the dimensions and location of the opening in the masonry infill walls on the performance of the strengthening method with the reinforced stucco layer. The applied strengthening method increased the ultimate load capacity, initial stiffness, and energy dissipation capacity values of reinforced concrete frames with masonry infill walls by 83%, 226%, and 62%, respectively, but resulted in a 38% decrease in displacement-ductility ratios. The study found that the openings in the masonry infill walls harm the performance of the strengthening technique by adding a rebar-reinforced stucco layer and decreasing the success level. When the opening size increased, and the opening was located at the corner of the masonry wall, the performance of the applied strengthening technique was negatively affected and decreased. Furthermore, nonlinear numerical analyses of the experiments conducted as part of the study were performed using ABAQUS finite element software. The numerical analysis results were compared to the experimental results. It has been determined whether numerical analysis models are compatible with experimental results.