<p>This study explores the reinforcement potential of modified nanosilica (mNS), derived from rice husk ash, in a 50/50 blend of chlorinated ethylene propylene diene monomer (Cl-EPDM) and chlorinated acrylonitrile butadiene rubber (Cl-NBR). The nanosilica was surface-modified using a 2:1 ratio of bis[3-(triethoxysilyl)propyl]tetrasulfide (TESPT) to polyoxyethylene(20)sorbitan monolaurate (TWEEN-20). Rubber nanocomposites were prepared via conventional two-roll mill mixing, and the effects of varying mNS content were assessed on processing behaviour, mechanical performance, swelling, oil resistance, morphology, compression set, and dynamic properties. Results showed that incorporation of mNS improved tensile strength, stress at 100% elongation, tear strength, hardness, abrasion resistance, and resistance to swelling and oil uptake, while reducing elongation at break, rebound resilience, and heat build-up. Notably, optimal mechanical enhancement was observed at 6 phr mNS loading. Swelling studies conducted at different temperatures and with various solvents revealed a decline in mole percent uptake with increasing mNS content. Overall, the use of TESPT/TWEEN-20-modified nanosilica significantly enhanced the multifunctional performance of Cl-EPDM/Cl-NBR composites.</p>