An experimental investigation about of the behavior of two confined clay brick masonry walls subjected to in-plane lateral load is presented. The specimens were built with identical characteristic, except that one of them was reinforced with diagonal strips of glass fiber reinforced polymer (GFRP). The objective was to study the effects of GFRP reinforcement on the behavior of the system. Additionally, to valid the experimental results, diagonal compression tests on masonry assemblages non-strengthened and strengthened with GFRP were carried out. The diagonal tension strength is one the mechanical properties with the greatest impact on the behavior of masonry walls. In some countries around the world experimental studies have been carried out on a wide variety of techniques for the reinforcement and rehabilitation of masonry walls, in order to increase their resistance to diagonal tension and thereby improve their behavior against in-plane forces. It is widely recognized that the use of composite materials as alternative reinforcement increases the lateral resistance and deformation capacity of the masonry walls. However, there is little information in the existing literature that allows to establish technical specifications for the explicit calculation of the contribution of this type of reinforcement to the lateral resistance of the system. Both specimens were tested in cantilever under reversible cyclic lateral load. To simulate gravity load, constant vertical load was applied on the top of the walls. The results indicate that GFRP reinforcement lightly increase the lateral resistance and deformation capacity of the masonry wall.