Abstract <p>Membrane fouling is a major challenge in practical membrane applications, which cannot be completely avoided but can be minimized. A promising approach to membrane modification is the introduction of hydrophilic polymers or polyelectrolytes into the coagulation bath (CB) during membrane fabrication via the phase inversion method. This study systematically investigates the influence of polyacrylic acid (PAA) molecular weight (100 000, 250 000, and 450 000 g mol^–1) on the structure, surface physicochemical properties, transport characteristics, and fouling resistance of polysulfone (PSf) membranes modified with the amphiphilic block copolymer polyethylene glycol-polypropylene glycol (Synperonic F108). Aqueous PAA solutions (0.4–2.0 wt %) were used as the CB. The results demonstrate that ultrafiltration PSf/Synperonic F108/PAA membranes exhibit significantly enhanced surface hydrophilicity (water contact angle decreases from 53° to 10°–32°), reduced surface roughness, and a more negatively charged surface compared to unmodified membranes. The use of PAA solutions in the CB leads to a denser membrane structure, characterized by a decrease in the average pore size and number of pores in the selective layer. As a result, the pure water permeability declines from 195 to 21–66 L m^–2 h^–1, while selectivity improves substantially. The rejection coefficients increase from 56 to 76–94% for polyvinylpyrrolidone (PVP K30, <i>M</i>_<i>n</i> = 40 000 g mol^–1) and from 55 to 92–94% for lysozyme. The modified membrane structure and permeability also significantly enhance fouling resistance during filtration of a humic acid model solution: the flux recovery ratio (FRR) improves from 74 to 92–100%, and the total flux decline (DT) decreases from 30 to 0–5%, while maintaining high efficiency in iron and color removal.</p>