Efficient Cs+-Sr2+ separation, highly desirable for radionuclide recovery in medical and industrial applications, was achieved by the ion exchange technique over a novel microporous silver selenidostannate, [NH3CH3](0.5)[NH2(CH3)(2)](0.25)Ag1.25SnSe3 (AgSnSe-1). This material was synthesized in deep eutectic solvent (DES), where the alkylammonium cations play significant structure-directing roles in the construction of micropores that allow for selective ion exchange toward Cs+ against Sr2+. The much greater K-d(Cs )(1.06 X 10(4) mL g(-1)) over K-d(Sr) (87.7 mL g(-1)) contributes to an outstanding separation factor SFcs/sr of similar to 121.4 that is top-ranked among inorganic materials. An ion exchange column filled with AgSnSe-1 exhibits a remarkable separation effect for 10 000 bed volumes of continuous flow, with removal rates of similar to 99.9% and similar to 0 +/- 5.5% for Cs+ and Sr2+, respectively. AgSnSe-1 exhibits excellent beta and gamma radiation resistances and a chemical stability over a broad pH range of 1-12. The Se leaching level below the safe guideline value for drinking water highlights the environmental-friendly nature of AgSnSe-1. The high Cs+ exchange performance is almost unaffected by Na+, Mg2+, and Ca2+ cations. The Cs+-laden product AgSnSe-1Cs can be facilely eluted for recycling us; highlighting the great potential of open framework metal selenides in nuclear waste treatment and renewable energy utilization.