This paper studies the effect of using glass fiber-reinforced polymer (GFRP) flexural reinforcement on the shear strength of reinforced masonry (RM) beams. For this purpose, two sets of shear-critical RM beams were tested, one set being two courses high, the other being five courses high. Each set consisted of two RM beams, one reinforced with GFRP and one reinforced with high-strength steel. The reinforcement ratio was kept constant for all beams. The test results showed that the use of GFRP reinforcement resulted in considerably reduced shear strength compared to beams reinforced with steel. Furthermore, the failure shear stresses of the beams decreased significantly due to increases in depth indicating that size effect is real in reinforced masonry. The shear behavior of the RM beams was seen to be fundamentally similar to that of reinforced concrete beams. As shear design expressions in most of the masonry design codes, such as TMS 402-2016 code, are intended to be used for designing masonry beams reinforced with conventional steel, the test results showed that these expressions can yield unsafe shear predictions for GFRP-reinforced or high-strength steel-reinforced masonry beams. The general method of the Canadian Standards Association (CSA) S304-2014 code, on the other hand, was able to capture safely the variations in the shear strength of GFRP-reinforced masonry. The general method is based on the modified compression field theory and can account for all influential factors affecting the shear strength of reinforced masonry beams. The results of this paper highlight the necessity of revising masonry design codes to address the size effect and the effect of reinforcement stiffness.