Polyvinylidene fluoride (PVDF) membranes are commonly used in ultrafiltration because of their good chemical resistance and mechanical strength. However, the hydrophobic nature of PVDF often leads to fouling, which gradually lowers membrane performance during operation. In this study, PVDF membranes were modified using tannic acid (TA) and Fe³⁺ to improve antifouling behaviour and membrane stability. Commercial tannic acid and gambir-derived tannins were used as modifiers, followed by Fe³⁺ treatment to strengthen the surface-modification layer via metal–phenolic coordination. The results showed that tannic acid incorporation increased membrane hydrophilicity and permeability. However, during repeated filtration cycles, a gradual decline in flux was still observed, indicating that improved hydrophilicity alone could not fully maintain stable membrane performance. Membranes treated with Fe³⁺ exhibited slightly lower initial flux values, but their filtration performance remained more stable, with flux retention above 85% after five cycles. In addition, all modified membranes maintained humic acid rejection values above 90%, indicating that membrane selectivity remained stable. The improved flux retention observed in Fe³⁺-modified membranes indicates that interfacial stabilization plays a critical role in sustaining long-term filtration performance. The combination of tannic acid and Fe³⁺ offers a simple way to balance permeability and membrane stability in PVDF membranes. Furthermore, Gambir-derived tannins performed comparably to commercial tannic acid, indicating that they could serve as a more sustainable alternative for membrane modification.