Nanoporous poly(styrene)-b-poly(isoprene)-b-poly(styrene) (PS-PI-PS) was prepared from PS-PI-PS-PLA (PLA = poly(d,l-lactide)) tetrablock terpolymer precursors. Hydroxy end-functionalized PS-PI-PS was synthesized by sequential anionic polymerizations and used to initiate the ring-opening polymerization of d,l-lactide. A combination of small-angle X-ray scattering, transmission electron microscopy and scanning electron microscopy data supports a core-shell cylinder morphology with PLA as the core component. The inherently robust mechanical properties associated with PS-PI-PS block copolymers provide the materials with enhanced flexibility and toughness compared to analogous nanoporous polystyrene samples prepared by etching of PS-PLA diblocks. Composite membranes containing nanoporous PS-PI-PS on top of microporous poly(ether sulfone) (PES) were prepared by direct spin-coating and also by a salt-plate coating/film-transfer strategy. Nanopores (dpore ≈ 15 nm) were created by a combination of reactive ion etching (RIE) and basic PLA hydrolysis. Resulting composite PS-PI-PS/PES membranes exhibited high selectivity and permeability comparable to that of commercial ultrafiltration membranes.