Display omitted •A novel NF membrane building block PEC NPs armed with sulfated groups was fabricated.•Hydrophilicity and water permeability of SPECMs were enhanced.•SPECMs possessed good salt/dye separation and antifouling properties in a long-term process. Polysaccharide nanofiltration (NF) membrane with traditional modification normally suffers from poor water permeability as a result of its tight packing of polymeric chains. In this work, a novel membrane building block, polyelectrolyte complex (PEC) nanoparticles (NPs) armed with adjustable content of sulfated groups was developed using chitosan and dextran sulfate sodium. The sulfated polyelectrolyte complex membranes (SPECMs) were first prepared by solution-casting and glutaraldehyde crosslinking process, and their structural characteristics and surface properties were systematically investigated. Intrinsic aggregation structure combined with numerous sulfate groups attenuates packing density of polymeric chains and promotes hydrophilicity, endowing SPECMs with high flux and perm-selectivity. SPECMs achieved a water permeability of 6.71Lm−2h−1bar−1, which was ∼2.3 times higher than pristine sulfated chitosan membrane. The selectivity for NaCl/Na2SO4 separation and NaCl/methyl blue dye separation were as high as 13.1 and 850.0, respectively. Moreover, SPECMs featured efficient dye antifouling property with low flux decline ratio (6.8%) and high recovery ratio (96.8%), which was primarily attributed to its hydrophilicity and smooth surface. The prominent perm-selectivity associated with antifouling property suitably position SPECMs for practical small organic molecule/inorganic salt mixture separation in a long-term process.