The scattering characteristics of random rough surfaces illuminated with a 3D converging–diverging Gaussian beam are investigated by applying the conventional Harvey–Shack theory in conjunction with 2D plane-wave decomposition. The Gaussian beam is assumed to have an arbitrary angle of incidence and to be linearly s-polarized. Using data obtained from laser BRDF measurements on isotropic random rough surfaces with low surface roughness, we demonstrate that the Gaussian beam Harvey–Shack theory is in better accordance with the experimental data than the conventional Harvey–Shack theory. The two models become identical for a large beam waist radii but are significantly different for smaller ones.