ABSTRACT: The shear behaviour of masonry walls subjected to in-plane lateral forces is strongly dependent on the quality of the mortar used in the joints and on the strength of the blocks. In heritage buildings, where masonry was fabricated by using weak materials, collapse due to sliding forces acting along the loaded diagonals of the walls, following the joints directions, are frequent during earthquakes. This typical vulnerability of Unreinforced Masonry (URM) walls in historical buildings can be mitigated by using new strengthening techniques that result simply and quick to be applied, if compared with traditional techniques or with epoxy-bonded FRP sheets (Fiber Reinforced Polymers). Different innovative strengthening techniques based on the use of Fiber Reinforced Mortars (FRM) were studied and compared in the paper. Different types of fibrous grids were grouted to the wall surfaces by using cementitious or non-cementitious mortars. Totally thirty unstrengthened and FRM-strengthened half scale masonry walls were tested until failure by using a shear-diagonal test set-up. Results of the diagonal shear tests revealed the effectiveness of the strengthening systems, that contributed to increase the ultimate load and at the same have helped to establish a dissipative behaviour. This pseudo-ductility was due to diffuse cracking of the mortar that developed after the peak load. High load levels were maintained during the progress of the cracks that extended well beyond the initial compressed strut. Conclusions will illustrate how the mechanical properties of the URM walls subjected to diagonal shear can be upgraded through the use of the tested strengthening systems.