Thin film composite membranes can selectively separate mono- and divalent ions from water via solution-diffusion of each species through a dense but ultrathin, highly cross-linked polymer "skin" layer; water is transported across the membrane faster than associated salts. Changing the selectivity of the "skin" layer typically requires adjusting the monomer chemistries that make up the polymer "skin" layer, but doing so also impacts a host of other membrane properties. Here, we employ electrostatic layer-by-layer deposition of inorganic nanoparticles to enhance the permselectivity of an existing commercial nanofiltration membrane. We chose this approach because it is simple and robust and does not require any change to the underlying chemistry of the thin film composite (TFC) membrane. We found that a single layer of nanoparticles was sufficient to increase the permselectivity of the membrane by nearly 50%, compared to the virgin TFC membrane. In order to understand the mechanism for permselectivity enhancement, we developed a modified solution-diffusion model to account for the additional hydraulic resistance of the nanoparticle layer, which can faithfully capture the effect of nanoparticle layer thickness on the observed water and salt flux of the modified TFC membrane.