<p>This study investigates the seismic performance of reinforced concrete (RC) frames infilled with deformable masonry panels that include horizontal sliding joints. These innovative infill systems are designed to mitigate the seismic deficiencies commonly observed in conventional RC buildings with rigid infills, where the strong interaction between the frame and the infill often causes brittle and undesirable failures. The study analyses the response of three RC structural frames with three, five, and seven storeys, by varying the infill properties and the design ductility classes, to evaluate how these parameters influence the overall seismic response. A further case study is introduced to reflect the common practice of including an opening, such as a door or window, in the masonry infill, leading to a reduced infill stiffness. The structural performance is assessed through linear, non-linear static, and non-linear dynamic time-history analyses, considering different seismic performance limit states. Particular attention is given to ductility verification, collapse mechanisms, and the evaluation of the behaviour factor capacity. Comparative results demonstrate that deformable infills incorporating sliding joints can effectively enhance structural performance and energy dissipation under seismic loading. The findings support their possible integration into performance-based seismic design methodologies and confirm that simplified analyses, such as response spectrum and pushover analyses, can provide reliable estimates of seismic demand when such ductile infill contributions are properly accounted for.</p>