Determination of interface shear strength between soil and geosynthetic reinforcement is crucial in the design of many geosynthetic-reinforced soil systems. To study the mechanical interaction between a subgrade soil (glacial till) and an aggregate base (No. 53 aggregate) layer with and without a geogrid at the interface, a series of direct interface shear tests were performed using a large-scale direct shear apparatus. The soil and aggregate layers were prepared in the large direct shear box at their optimum water contents (wc_opt = 16.4% and 8.2% for the soil and aggregate, respectively) and compacted to relative compaction values of 93–98%. Seven biaxial geogrids with varying aperture areas and junction strengths were used in the direct interface shear tests; each test was performed for three different normal stresses of 50, 100 and 200 kPa. For a given geogrid aperture area, the average peak and end-of-test interface shear strength coefficients, defined as the ratio of the interface shear strength of the soil–aggregate system with a geogrid at the interface to that without a geogrid at the interface corresponding to peak and end-of-test states, respectively, increase with increasing junction strength of the geogrid. Based on the results obtained from this study, the geogrid aperture area, the normalized geogrid aperture area, the normalized geogrid aperture length, and the geogrid junction strength needed to optimize the peak interface shear strength of soil–aggregate–geogrid systems are 825 mm², 4.7, 5.4, and 11.5 kN/m, respectively. These values are restricted to the materials and test conditions used in this study.