<p>Radioactive nickel (⁶³Ni) can cause irreversible damage to the ecosystem and human safety. However, the highly selective removal of ⁶³Ni²⁺ from complex nuclear wastewaters remains a critical challenge. Herein, a strategy is proposed for the highly efficient removal of Ni²⁺ through a synergistic mechanism of ion exchange and chelation of polyamine, with an ethylenediamine-functionalized layered thiostannate (Hen)₂Sn₃S₇·(en)_1.5·3H₂O (en = ethylenediamine; denoted as EN-SnS) developed. Both protonated and unprotonated ethylenediamines are intercalated into the interlayer spaces and hexagonal windows. EN-SnS exhibits superior adsorption performance for Ni²⁺ with rapid adsorption kinetics (equilibrium 15 min), high adsorption capacity (41.02 mg g⁻¹), and wide pH tolerance (3.09–10.84). At the optimal pH of 5.00, the material achieves a maximum <i>K</i><Stack> _d ^Ni </Stack> of 1.39 × 10⁵ mL g⁻¹, with the Ni²⁺ concentration decreasing sharply from 6.25 ppm to 40 ppb. EN-SnS exhibits high affinity for Ni²⁺, with affinity order Ni²⁺ &gt; Co²⁺ &gt; Eu³⁺ &gt; Cd²⁺ &gt; Ba²⁺ &gt; Sr²⁺ &gt; Cs⁺. Notably, remarkable efficiency is also demonstrated in removing Ni²⁺ from complex wastewaters, even for seawater (removal rate <i>R</i>^Ni of 99.6%). This work identifies a synergistic mechanism of ion exchange and chelation as a potent construction strategy, aiming for the selective elimination of radioactive metal ions.</p>