Membrane surface modification with polydopamine (PDA) coatings can reduce fouling in oily water filtration due, at least in part, to enhanced surface hydrophilicity. In this study, polysulfone (PSf) UF membranes were coated with PDA. PDA coating conditions (solution concentration and deposition time) were varied, and the effect of coating conditions on membrane molecular weight cutoff (MWCO) and hydraulic permeability was measured. Membrane MWCO decreased and PDA film thickness increased as initial dopamine coating solution concentration or deposition time increased. The MWCO decrease confirmed that PDA restricted the membrane pores. While the PDA coating thickness on membrane surfaces grew progressively with increasing initial dopamine concentration or coating time, coating inside the membrane pores was limited by the finite membrane pore size. A tradeoff between selectivity and hydraulic permeability of unmodified and PDA-modified membranes was noted. This tradeoff is reminiscent of that observed in other separation membranes. Zydney’s hindered solute transport model of flow through porous membranes was used to estimate changes in membrane mean pore size and pore size distribution. Based on the modelling results, membrane mean pore radius increased at low initial dopamine concentrations or short deposition times and decreased at high initial dopamine concentrations or long deposition times with increasing initial dopamine concentration or increasing PDA coating time. The pore size distribution narrowed as the membranes were modified with PDA. The porosity to thickness ratio of PDA-modified membranes remained unchanged or was only slightly higher than that of unmodified membranes.