A three-dimensional electrostatic patchy charge model is developed to compute surface diffusion coefficients as a function of background ionic strength for sodium counter-ions adsorbed at the inner Helmholtz plane (IHP) of a charged montmorillonite aqueous interface. The model preserves the discrete nature of the montmorillonite lattice charge by arranging discrete patches of different charge densities at the surface. The salinity dependence of the surface diffusion coefficients arises from an electrical energy barrier to translation of counter-ions along the IHP which is due to the oscillating electrostatic potential profile at the surface. This energy barrier is computed numerically using a three-dimensional finite-element program.We discuss the assumptions implicit in the patchy charge model, and we point out the advantages of performing the three-dimensional calculation over other widely used approaches to modeling heterogeneously charged surfaces. The influence of the critical model parameters on the behavior of the surface diffusion coefficients with changing electrolyte strength is explored. The predicted dependence of the surface diffusion coefficients on ionic strength from the patchy charge calculation yields excellent agreement with experimental data on the conductivity of water-saturated shaly sands.