The increasing demand for water treatment in arid and semiarid areas has led to the exploration of the potential integration of polymeric nanofiltration membranes for selective separation of monovalent and multivalent ions from surface waters with high salinity. This study investigates the modification of semiaromatic-based polyamide nanofiltration membrane (Fortilife-XNTM) using polyelectrolyte multilayers through the layer-by-layer (LbL) technique to enhance ion selectivity and cation rejection. Poly(diallyl dimethylammonium chloride) (PDADMAC) and poly(sodium-4-styrene sulfonate) (PSS) were used to coat the membranes, with varying bilayer numbers to assess their impact on membrane performance. Membranes were characterized and tested with synthetic solutions mimicking Llobregat river water (Spain) to determine membrane performance in terms of water flux and rejections. Besides, data was fitted to the Solution-Electro-Diffusion-Film model to determine membrane permeances to species. Results showed that the membrane coated with 5.5 bilayers exhibited the highest selectivity for monovalent and divalent cations, with significant improvements in ion rejection. Specifically, Mg(II) rejection increased to 93 %, and selectivity for K/Mg and Na/Mg rose by 42 % and 60 %, respectively. Furthermore, the selectivity for K/Ca and Na/Ca improved by 92 % and 140 %, with Ca(II) rejection reaching 91 %. The modified membranes exhibited water permeability comparable to the unmodified membrane, with the (PDADMAC/PSS)4.5 membrane showing the highest permeability, owing to its optimal balance of roughness (77 nm) and contact angle (20 degrees). These findings highlight the effectiveness of polyelectrolyte multilayers in enhancing the selectivity capabilities of nanofiltration membranes for salinity reduction in surface water treatment for drinking water production as substitution of reverse osmosis.