Oil–water separation is a worldwide challenge because of increasingly oily wastewater, as well as frequent oil spill accidents. We report in this work the fabrication of a zwitterionic polyelectrolyte brush (poly(3-(N-2-methacryloxyethyl-N,N-dimethyl) ammonatopropanesultone), abbreviated as PMAPS)-grafted poly(vinylidene fluoride) (PMAPS-g-PVDF) membrane via a surface-initiated atom transfer radical polymerization (SI-ATRP) technique for separating oil from water. By tuning polymerization time, a superhydrophilic and underwater superoleophobic PMAPS-g-PVDF membrane was successfully prepared. The membrane can be applied for thoroughly separating dispersed oil from water with ultrahigh separation efficiency >99.999% in terms of rejection coefficient. The oil contents after one-time separation of a selection of oil–water mixtures are all less than 10 ppm and some of them are even lower than 2 ppm. These values are much lower than the standard minimum requirements of the international regulations for wastewater discharging. The oil-adhesion force between an oil droplet and the membrane surface was measured to be less than 1 μN. Due to the ultralow oil adhesion, the membrane exhibits an excellent antifouling property to oil and is easily recyclable.