A novel electrolyte-responsive membrane, RC-g-PSBMA, was successfully prepared from regenerated cellulose (RC) membrane through surface-initiated atom transfer radical polymerization (ATRP) of a zwitterionic monomer, sulfobetaine methacrylate (SBMA). Different degrees of polymerization for the grafted SBMA polymers (i.e., PSBMA) on the RC membrane were easily obtained by adjusting the ATRP reaction conditions. The electrolyte-responsive behavior of RC-g-PSBMA was first evaluated through the permeation experiments with sodium chloride (NaCl) solutions of different concentrations. It was found that the permeability of RC-g-PSBMA showed a clear dependence on NaCl concentration in the solutions. To further examine the potential of RC-g-PSBMA for protein purification, bovine serum album (BSA) was chosen as a model protein and polystyrene nanoparticles (NPs) of different sizes were used as representative impurities in the solutions. The rejection rates of BSA and NPs by RC-g-PSBMA were examined with the solutions containing BSA and NPs at different NaCl concentrations. The results showed that the rejection rates of BSA were at a very low level regardless of the concentration of NaCl in the solutions, indicating that the membrane allowed BSA to permeate. However, the rejection rates of NPs of different sizes by RC-g-PSBMA changed remarkably with the concentration of NaCl in the solutions. The study has demonstrated the possibility to separate BSA from NPs of different sizes by using the same membrane but simply altering the concentration of NaCl in the solutions. Membranes with such properties will have a great potential for protein purification as well as for many other separation applications.