¹³⁷Cs and ⁹⁰Sr, both byproducts of the uranium and plutonium fission processes, make up the majority of high-level waste from nuclear power plants. ⁶³Ni is a byproduct of the erosion-corrosion process of the reactor components in nuclear energy plants. The concentrations of these ions in solution determine the Waste Class (A, B, or C); thus, their selective removal in the presence of large excesses of nonradioactive ions is necessary to reduce waste volume and cut disposal costs. We report the new material K_2xMg_xSn_3–xS₆ (x = 0.5–1, KMS-2) and its application for the ion exchange of Cs⁺, Sr²⁺, and Ni²⁺ in varying conditions. This compound crystallizes in the hexagonal space group P6₃/mmc with cell parameters a = b = 3.6749(8) Å and c = 16.827(4) Å. The difference in crystal structure between KMS-2, the previously reported Mn analog K_2xMn_xSn_3–xS₆ (KMS-1), and their parent SnS₂ is also described. Distribution coefficients for KMS-2 are high for Cs⁺ (7.1 × 10³ mL/g) and Sr²⁺ (2.1 × 10⁴ mL/g) at neutral pH (∼ 6 ppm, V/m ∼1000 mL/g). We also report on the comparative study of Ni²⁺ ion exchange with both KMS-1 and KMS-2. Additional competitive reactions using Cs⁺, Sr²⁺, and Ni²⁺ in high concentrations of salt solution and at different pH values are reported.