Mag@silica-Ag composite has a high sorption ability for I⁻ in aqueous solution due to its high surface area and strong affinity for the studied anion. The material adsorbed I⁻ rapidly during the initial contact time (in 45 min, η = 80%) and reached adsorption equilibrium after 2 h. Moreover, mag@silica-Ag proved to selectively remove I⁻ from a mixture of Cl⁻, NO₃⁻ and I⁻. The adsorption behavior fitted the Langmuir isotherm perfectly and the pseudo-second-order kinetic model. Based on the Langmuir isotherm, the maximum adsorption capacity of mag@silica-Ag was 0.82 mmol/g, which is significantly higher than previously developed adsorbents. This study introduces a practical application of a high-capacity adsorbent in removing radioactive I⁻ from wastewaters.