<p>Efficient decontamination of radioactive cesium (¹³⁷Cs⁺) from nuclear wastewater is essential for the sustainable and safe use of nuclear power. Here, we report the effective applicability of sulfur-encapsulated mordenite (S-MOR), in which sulfur was introduced into the micropore of MOR via simple vacuum sublimation for the efficient removal of Cs⁺ under high-salinity conditions. Our characterization and Cs⁺ ion-exchange property in distilled water revealed that S-NaA with 10 wt% sulfur (10&#xa0;S-MOR) is the most suitable S-MOR candidate among various S-MORs because S-MOR prepared by vacuum sublimation with more than 15 wt% of sulfur nearly blocks its micropores, resulting in the poor Cs⁺ ion-exchange performance. The optimized 10&#xa0;S-MOR demonstrated superior Cs⁺ selectivity, faster Cs⁺ ion-exchange kinetics, and improved maximum adsorption capacity compared to both MOR and the well-known commercial Cs⁺ adsorbent, Prussian blue (PB), under high-salinity conditions including groundwater and seawater. These enhancements were attributed to the additional Lewis acid-based interaction between sulfur within the micropore of 10&#xa0;S-MOR and Cs⁺, as interpreted by the hard-soft acid-base theory. Due to its excellent kinetics and selectivity for Cs⁺ removal, and simple procedure, our 10&#xa0;S-MOR holds significant promise for deployment in treating water contaminated with radioactive cesium.</p>