Purification of water through oil–water separation is essential for preserving the ecosystem and protecting human health. Although a conventional polypropylene depth filter can effectively purify water, modifying the wettability of a filter for oil–water separation is difficult owing to its low reactivity. In this study, we developed a superhydrophilic polypropylene filter with a hydrogel layer that could enable effective oil–water separation by using plasma treatment and dip coating, which enabled an even distribution of the coating solution across the filter. The fabricated filter was superhydrophilic with a water contact angle of 0o. It showed a high repulsive force with oil in water with an underwater oil contact angle of 142.9o. When such filter was applied to an oil–water separation device, it effectively purified water with low oil content (< 15 ppm) at a flow rate of 300 mL/min. These results demonstrate potential applications of such filters in areas such as wastewater treatment and oil spill cleanup.