Biofouling settlement on ship hulls causes negative impact on ship operations as it increased fuel consumption and maintenance requirement, ultimately resulting in significant economic losses. To mitigate this problem, antifouling paint is commonly applied to ship hulls. Commercial antifouling paints commonly contain SeaNine 211, a co-biocide that is recognized for its harmful effect on the marine environment. Therefore, it is necessary to reduce SeaNine 211 and incorporate a non-toxic co-biocide to maintain or even enhance the effectiveness of antifouling paint. In this study, cerium oxide (CeO2) is used as an alternative to SeaNine 211, offering biofilm-inhibition properties, result from their antibacterial activity that contribute to antifouling paint effectiveness. Cerium oxide was added into the antifouling paint at concentration of 0.1%, 0.3%, and 0.5% w/w, while the Seanine 211 concentration was reduced from the commercial standard 2% to 1% and 0% w/w. Mild steel coupons were used as a substrate, multipurpose epoxy anticorrosion and cerium oxide-modified antifouling paint as primer and top coat, respectively, were applied on mild steel coupons. The coated coupons were exposed to Indonesian tropical sea at Tanjung Pasir Bay, Tangerang for 1 month (January – February 2025) where marine biofouling typically occurs more aggressively than in subtropical regions. Biofouling resistance after exposure was evaluated using fouling rating scale based on NSTM Chap. 081 combined with ASTM D6990–20. The physical performance of antifouling coating system was evaluated using two approaches: electrochemical impedance spectroscopy (EIS) and coating physical property assessments, which included physical deterioration rating (PDR), softness rating, gloss test, and adhesion test. The results of this study indicate that cerium oxide is a suitable co-biocide to reduce the required SeaNine 211 concentration in antifouling paints. © The Author(s), under exclusive licence to Springer Nature Switzerland AG 2026.