The removal of trace Se(VI) from water is a great challenge because its adsorption or ion-exchange is significantly inhibited by other coexisting anions at much greater levels. To address this problem, the millimeter-sized nanocomposite nZVI@D201 was fabricated by in situ preparation of nanoscale zerovalent iron (nZVI) confined in the pore channels of a polymeric anion exchanger (D201). Preferable removal of trace Se(VI) in the presence of sulfate by nZVI@D201 over D201, nZVI and their mixture was attributed to the significant roles of pore confinement effect and the Donnan membrane effect from the polymeric host. Moreover, the removal of trace Se(VI) by nZVI@D201 was insignificantly affected by pH (3-10), dissolved oxygen, coexisting anions, and humic acid at their environmental levels. The XPS spectrum revealed that the Se immobilized in nZVI@D201 was mainly Se(IV) (84.9%), indicating the synergistic removal mechanism involving ion-exchange, adsorption, and reduction. Through the periodic/complete regeneration, nZVI@D201 could be sustainably utilized for Se(VI) removal. In addition, column experiments showed that nZVI@D201 exhibited great potential for practical trace Se(VI) removal in fixed-bed systems.