Abstract: The interaction at the geosynthetic–reinforced soil interface is a key factor in determining the stability of reinforced soil structures, with soil particle morphology significantly influencing this interaction. To study the influence of gravel morphology on the shear characteristics of the gravel–geogrid interface, gravel samples with different particle size ranges (4–6, 8–10, and 12–14 mm) and roundness values (0.35, 0.55, and 0.75) were prepared using a high-speed centrifugal mill. Monotonic direct shear tests were performed using a large indoor direct shear apparatus to investigate the effect of particle morphology on the shear characteristics of the interface. Discrete element software was used to create a computational model for these tests, with geogrid mesoscopic parameters calibrated through pull-out experiments. The model’s contact parameters were validated by comparing the shear stress and volumetric behavior predictions with experimental results. The evolution of anisotropy in particle arrangement at the geosynthetic–reinforced soil interface during monotonic shearing was analyzed as well as the micromechanisms behind the macroscopic shear characteristics of the gravel–geogrid interface. The study revealed that the reinforced soil interface exhibited typical shear softening behavior during monotonic shearing, where shear stress increased initially, then decreased, and eventually reached a stabilized residual phase as shear displacement progressed. Increasing the roundness of the backfill led to declines in both peak and residual shear stress. For a median particle size of 9 mm, an increase in roundness from 0.35077 to 0.75068 caused a reduction in peak shear stress by 15.26%, 15.46%, and 15.79% under σ₀ = 30, 60, and 90 kPa, respectively, and a decrease in residual shear stress by 23.05%, 29.10%, and 20.94%, respectively. For interfaces with constant roundness, larger particle sizes resulted in increased internal friction angles and apparent cohesion. When roundness was 0.35, increasing the median particle size from 5 to 13 mm led to rises of 10.77% and 144.19% in peak friction angle and peak apparent cohesion, respectively, and increases of 6.88% and 180.00% in residual friction angle and residual apparent cohesion, respectively. Volumetric analysis showed pronounced contraction and dilation behavior for lower-roundness interfaces compared to higher-roundness ones. Similarly, under equivalent vertical loads, larger particle sizes exhibited more significant contraction and dilation responses than smaller sizes. Microscopically, under a constant vertical load, the shear contact force exhibited a “peanut-shaped” distribution, while the tangential contact force exhibited a “petal-shaped” distribution. With increasing shear displacement, the main axes of particle contact normals, normal contact forces, and tangential contact forces shifted. Larger particles with lower roundness exhibited greater deflection angles in these axes, indicating stronger microscopic anisotropy, which corresponded to the dilation observed in macroscopic tests.