Emerging technologies for treating oily wastewater employ ultrafiltration membranes that incorporate hydrophilic nanoparticles for enhanced effectiveness and efficiency. In this study, cellulose acetate (CA) mixed-matrix membranes with zwitterionic nanoparticles—polydopamine-sulfobetaine methacrylate or P(DA-SBMA)—were fabricated through wet-phase inversion. The ratio of SBMA to DA monomers was varied to alter the size of P(DA-SBMA) nanoparticles. Adding P(DA-SBMA) to a CA solution led to thermodynamic instability, which resulted in turn in instantaneous demixing during the phase separation in water. The effect was favorable on the membrane porosity and mean pore radius. Moreover, the membrane hydrophilicity was improved because of the abundance of hydrophilic functional groups present in P(DA-SBMA) nanoparticles. A suitable size and content of P(DA-SBMA) nanoparticles embedded in the CA matrix produced a modified CA membrane that delivered optimal water flux of 583.64 ± 25.12 L m−2 h−1. Flux recovery and reversible fouling were enhanced by 8.85% and 11.10%, respectively, whereas irreversible fouling decreased by 8.85%. Different oil-in-water emulsions containing diesel oil, dodecane, food-grade oil, toluene, and hexane were subjected to tests, and high separation efficiencies (95–99%) were achieved. Overall, the fabricated membranes were effective in treating different types of oily wastewater.