This paper reports on an experimental study of six full-scale partially grouted masonry walls (PG-MWs), and evaluated the accuracy of international design codes based on the test results. The main test parameters were the horizontal spacing between vertical grout cells and the horizontal reinforcement ratio. All test walls had identical thickness, height, and length, and were constructed using concrete masonry units (CMUs). The walls were subjected to a constant axial load and incrementally increasing in-plane lateral cyclic displacement applied to the top of each wall. The horizontal spacing between the vertical grouted cells ranged from 610 mm (24 in.) to 1,219 mm (48 in.), and the horizontal reinforcement ratio ranged from 0.127% to 0.254%. Results showed that the strength of the PG-MWs was very sensitive to the spacing between vertical grouted cells, and that the contribution of shear reinforcement to the shear strength was lower than what is commonly adopted in design codes, especially for the walls with higher ratios of horizontal reinforcement. The accuracy of current design codes in predicting the strength of the tested walls was investigated, and possible modifications were proposed. Disregarding the effect of grout spacing resulted in an inaccurate estimation of the strength in the current design codes. Consequently a simple grout spacing modification factor was introduced which could significantly improve the strength prediction in PG-MWs. Using a test database of PG-MWs collected from the literature, it was shown that the proposed method could effectively improve the shear strength prediction and reduce the scatter of the predicted strength.