An experimental and numerical research, aimed at evaluating the in-plane seismic performance of an innovative external steel modular system for the strengthening of load-bearing masonry walls, is currently underway at the EUCENTRE Foundation in Pavia. This paper presents, in particular, the results of the experimental part of this research campaign. Solid clay brick and hollow clay "doppio UNI" masonry typologies, representing common solutions in Italian existing buildings, were considered. After the mechanical characterization of the masonry and of the components of the reinforcement system, cyclic in-plane pseudo-static tests on full-scale specimens were performed in order to investigate the influence of the proposed system on the in-plane response of the walls, compared to the unreinforced conditions. The main seismic parameters of the masonry walls (i.e. elastic stiffness, lateral resistance, displacement capacity and energy dissipation) were assessed depending on the achieved damage mechanism. The proposed reinforcement system provided very promising results, improving the in-plane seismic performance of masonry walls at serviceability and especially at ultimate conditions, with a significant increase of deformation capacity above all in the case of shear mechanisms.