Three-dimensional (3D)-printed membranes via stereolithography (SLA) are promising in oil-water separation, which is the key in the purification of industrial oily wastewater. To achieve gravity-driven oil-water separation, the membrane material needs to be simultaneously hydrophilic/oleophobic. However, most of the state-of-the-art materials for SLA do not meet the requirement. While water-adsorbing hydrogel is simultaneously hydrophilic/oleophobic and there have been reports on 3D printing of hydrogels in biomedical applications, the hydrogel is too soft for membrane application. Here, we report a simple approach to tackle the issue: a hydrogel coating on SLA-based plastic membranes. The coating is fabricated, using [2-(methacryloyloxy) ethyl] dimethyl-(3-sulfopropyl) ammonium hydroxide as the zwitterionic monomer and acrylamide as the comonomer, via in situ polymerization on SLA-based plastic membranes. The contact angle tests and Fourier transform infrared spectrum show that such a membrane readily adsorbs water and becomes simultaneously hydrophilic/oleophobic. The oil-water separation tests indicate that the water-adsorbed membrane is highly efficient in gravity-driven oil-water separation in 31 repeating cycles. Our results indicate the great potential of 3D-printed membranes in oil-water separation.