137 Cs + and 90 Sr 2+ are highly soluble, highly environmentally mobile, and strongly radiotoxic. Efficiently sequestering 137 Cs + and 90 Sr 2+ ions from aqueous solutions remains a serious challenge, especially from acidic environments because of the effects of protonation and poor stabilities of materials. Here, efficient removal of Cs + and Sr 2+ ions is achieved by a layered thiostannate, namely [(EtNH 3 ) 1.68 (Et 2 NH 2 ) 0.32 ]Sn 3 S 7 •0.68H 2 O (FJSM-SnS-4), which exhibits excellent acid−base and β and γ irradiation resistances. FJSM-SnS-4 possesses high maximum adsorption capacities and rapid kinetics for Cs + and Sr 2+ ions under neutral conditions. It can even capture Cs + ions under acidic conditions (e.g., pH 1.6 and 0.4). It is interesting that FJSM-SnS-4 has excellent selectivity for Sr 2+ ions in neutral environments, whereas it prefers capturing Cs + from 1 M HCl solutions with coexisting K + , Na + , Ca 2+ , Mg 2+ , and Sr 2+ ions. The Cs + and Sr 2+ adsorption mechanism has been revealed at the molecular level by single-crystal structural analysis coupled with XPS, EXAFS, and EA characterization, that is, ion exchange between Cs + and Sr 2+ ions and protonated organic amines in interlayer space owing to the flexibility of the current layered thiostannate framework and the strong affinity of the soft basic S 2− of the framework for Cs + and Sr 2+ ions.