This contribution assesses the effect of particle surface roughness on the shear wave velocity (VS) and the small-strain stiffness (G0) of soils using both laboratory shear plate dynamic tests and discrete element method (DEM) analyses. Roughness is both controlled and quantified to develop a more comprehensive understanding than was achieved in prior contributions that involved binary comparisons of rough and smooth particles. Glass beads were tested to isolate surface roughness effects from other shape effects. VS and G0 were accurately determined using a new design configuration of piezo-ceramic shear plates. Both the experimental and the DEM results show that increasing surface roughness reduces G0 particularly at low stress levels; however, the effect is less marked at high pressures. For the roughest particles, the Hertzian theory does not describe the contact behaviour even at high pressures; this contributes to the fact that the exponent in the G0 – mean effective stress relationship exceeds 0.33 for sand particles. Particle-scale analyses show that the pressure-dependency of the surface roughness effects on G0 can be interpreted using roughness index α which enables the extent of the reduction in G0 due to surface roughness to be estimated.