Abstract: Natural mordenite exhibits poor adsorption performance for radionuclides in aqueous environments due to its high Si/Al ratio and low specific surface area. To improve its adsorption capacity, natural mordenite was hydrothermally modified at temperatures ranging from 80 to 140 °C, resulting in the formation of zeolite phases with lower Si/Al ratios and higher specific surface areas. In this study, natural mordenite was used as a precursor and subjected to hydrothermal treatment. The composition, morphology, and structural characteristics of the pristine and modified samples were investigated by X−ray diffraction (XRD), Fourier−transform infrared spectroscopy (FTIR), X−ray fluorescence (XRF), scanning electron microscopy coupled with energy−dispersive X−ray spectroscopy (SEM−EDS), N₂ adsorption−desorption analysis, and laser particle size analysis. The adsorption performance of the modified materials was evaluated using Sr²⁺, Co²⁺, and Mn²⁺ as simulated radionuclides. The results show that hydrothermal treatment at 100 °C and 140 °C converts natural mordenite into zeolite P and analcime, respectively. The Si/Al ratio decreases from 4.47 for natural mordenite to 2.69 and 2.50 after hydrothermal treatment at 100 °C and 140 °C, respectively. Meanwhile, the specific surface area increases from 17 m²/g to 23 m²/g and 29 m²/g, respectively. Natural mordenite exhibits low adsorption capacities of 15, 6, and 5 mg/g for Sr²⁺, Co²⁺, and Mn²⁺, respectively. Compared with natural mordenite, the adsorption capacities of the samples obtained at 100 °C increase by 4.0, 4.2, and 5.3 times for Sr²⁺, Co²⁺, and Mn²⁺, respectively, while those of the samples obtained at 140 °C increase by 3.4, 5.2, and 2.8 times, respectively. In complex systems containing K⁺, Na⁺, Ca²⁺, and Mg²⁺, the hydrothermally modified samples exhibit significantly improved selective adsorption performance compared with natural mordenite. The selectivity coefficients of Sr²⁺, Co²⁺, and Mn²⁺ over K⁺ and Mg²⁺ are all greater than 1, ranging from 1.05 to 24.22. The sample obtained at 140 °C exhibits higher selective adsorption ability for the three target ions than the sample obtained at 100 °C, whereas the latter shows higher adsorption capacity. The adsorption performance of hydrothermally modified mordenite is closely related to its Si/Al ratio, specific surface area, and framework structure. Overall, the sample obtained by hydrothermal treatment at 100 °C exhibits superior adsorption capacity, indicating its potential for the efficient utilization of natural mordenite in radionuclide removal applications.