Long-lived ¹³⁷Cs/¹³⁴Cs, ⁶⁰Co, and ⁶³Ni with high-energy β/γ emissions are primary toxic fission/activation products in nuclear-contaminated water, causing irreversible damage to the ecological environment and human health. Rapid remediation of these radionuclides emerges as an urgent task, while the deficiencies in adsorption kinetics, selectivity and acid resistance hinder the material innovation for practical applications. Herein, we have developed a hybrid cadmium/tin selenide adsorbent K_3.4(CH₃NH₃)_0.45(NH₄)_0.15Cd₂Sn₃Se₁₀·3.4H₂O (CdSnSe-2K) for the enrichment and sequestration of Cs⁺, Co²⁺, and Ni²⁺ ions. CdSnSe-2K features a unique open-framework constructed from the interlinkage of T2-{M₄Se₁₀} (M = Sn^IV/Cd^II) supertetrahedral clusters and M bridges, accelerating the ion transfer in the tunnel system and thus affording quick kinetics (k₂ = 9.706, 0.182, 0.099 g mg⁻¹ min⁻¹ for Cs⁺, Co²⁺, Ni²⁺). Remarkably, the removal rate R^Cs increases to 98.54% in just 2 min and 99.57% in 5 min (equilibrium), which is responsible for an ultra-high distribution coefficient K_d of 2.31 × 10⁵ mL g⁻¹. The soft-Lewis basic Se component endows CdSnSe-2K with a fantastic adsorption selectivity for Cs⁺, Co²⁺, and Ni²⁺ against various coexisting cations and anions in complex actual water environments even including the highly salty sea water medium. CdSnSe-2K exhibits a fantastic acid/base tolerance over pH = 2–13, and the maximum adsorption capacities q^Cs_m, q^Co_m and q^Ni_m under neutral condition are 218.08 ± 80.08, 43.95 ± 16.57 and 42.53 ± 16.50 mg g⁻¹, respectively. In addition, CdSnSe-2K single crystals were also employed as the stationary phase for the fabrication of an ion exchange column, which shows an elimination efficiency above 99% in the long-term filtration for Cs⁺ (8200 BVs), Co²⁺ (4000 BVs) and Ni²⁺ (4000 BVs). This work not only highlights CdSnSe-2K as a promising Cs⁺, Co²⁺, and Ni²⁺ scavenger in radionuclide remediation, but also provides a supertetrahedral cluster-based construction strategy for open-framework selenide materials with enhanced adsorption kinetics, selectivity, and acid resistance.