The Resisto 5.9 Tube is an advanced, steel-based dry retrofit system developed by Progetto Sisma S.r.l., located in Italy’s Motor Valley. Designed as an anti-seismic “coat,” the system is compatible with both masonry and reinforced concrete (RC) frame structures, enhancing their seismic resilience. The primary load-bearing components of the Resisto 5.9 Tube consist of galvanized steel profiles arranged as vertical and horizontal members, supported by diagonal braces and custom connection plates. In collaboration with the Department of Structures for Engineering and Architecture at the University of Naples Federico II, an extensive experimental program was conducted to assess the Resisto 5.9 Tube’s performance on RC frames with masonry infill walls. The re-search included full-scale, in-plane tests on two-dimensional RC frames to evaluate their seismic response before and after applying the retrofit. Initial tests focused on the in-plane behavior of infilled RC frames, while subsequent tests, after the application of the seismic retrofit, examined both in-plane and out-of-plane seismic responses. This provided a comprehensive assessment of the retrofit’s effectiveness. To complement the experimental results, numerical simulations were developed to create a calibrated model for further analysis of the system’s behavior under seismic loads. This integrated experimental and numerical approach validates the Resisto 5.9 Tube as a practical and adaptable solution for seismic retrofitting of RC structures with masonry infills, demonstrating significant improvements in their in-plane seismic performance.