A novel water purification membrane has been prepared by layer-by-layer (LBL) assembly of tailored, core–shell star block copolymers. The star block copolymer consists of a hydrophobic polystyrene core (PS) and a plurality of charged hydrophilic arms (poly(N,N-dimethylaminoethyl methacrylate): PDMAEMA or polymethacrylic acid: PMAA). The high density of functional groups per unit volume of the PS-PDMAEMA and PS-PMAA star polymers enabled the formation of thin films with densely packed spherical morphologies, even with a low number of LBL cycles. The hydrophilic arms at the membrane surface helped mitigate membrane fouling, whereas the hydrophobic PS core suppressed undesirable swelling of the LBL-films in water, retaining good film durability. The star polymer films composed of 3.5 alternating layers of PS-PDMAEMA and PS-PMAA demonstrated an efficient nanofiltration capability of >98% rejection toward the small carcinogenic textile dye, Congo Red, with high water flux (ca. 40 LMH at 50 psi) at very low operation pressure.