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Reverse osmosis membrane elements suffer oxidative degradation during operation thereby resulting in increased salt passage. The present article demonstrates the novel approach to bind a chitosan–tannic acid layer above the degraded reverse osmosis membrane to make a supramolecular assembly over loosened polyamide layer to increase the selectivity performance. Chitosan being naturally occurring linear polysaccharide, its biodegradability, nontoxicity, antimicrobial property make it a promising candidate for membrane surface modification. Tannic Acid functions as cross‐linking agent, contains many polyphenolic compound, and was found suitable to bind with chitosan and form a supramolecular layer over polyamide. The present article showed the increase in salt rejection upto 33.17% of oxidative degraded reverse osmosis membrane by chitosan–tannic acid treatment. The selectivity enhancement was significant in case of membrane modified with chitosan–tannic acid supramolecular assembly. The modified membranes exhibited lower leaching as compared to only tannic acid treated membrane. They were characterized for morphology, surface features and chemical structure by scanning electron micrographs, atomic force micrographs, Fourier transform infrared spectroscopy. Moreover, the differential scanning calorimetry analysis was done to evaluate the glass transition temperature of modified membrane. Stress–strain behavior of modified membranes were analyzed. This article proposes as novel approach to enhance the selectivity of degraded membrane.