Aqueous phase separation (APS) is a sustainable method for fabricating polymeric membranes without using toxic organic solvents. In this study, membranes were prepared using poly(sodium 4-styrene sulfonate) (PSS) and polyethyleneimine (PEI) via a pH-shift induced APS. The membranes were tested for organic fouling using bovine serum albumin (BSA) as a model foulant in a dead-end filtration setup. The pure water flux, flux recovery ratio (FRR), reversible fouling ratio (RFR), irreversible fouling ratio (IFR), and total fouling ratio (TFR) were evaluated over three filtration cycles. The initial water flux was 255 Lm⁻²h⁻¹ at 1 bar. After exposure to BSA, the flux significantly decreased to approximately 19% of its original value after three cycles. The FRRs remained low (0.19–0.31), while the IFRs were high (0.69–0.81), indicating dominant irreversible fouling. UV–vis analysis showed unstable rejection behavior, including negative rejection values in later cycles, likely due to insufficient cleaning and concentration polarization. The results show that although the PSS–PEI APS membranes exhibited high initial flux, their fouling resistance toward BSA was limited under the tested conditions.