Magnetic Mg/Al layered double hydroxides (LDH) with three cationic ratios (Mg/Al = 2:1, 3:1, and 4:1) were successfully synthesized and utilized for the first time in an iodide adsorption study. The effects of the Mg/Al ratio of LDH on iodide adsorption were investigated, and physicochemical properties of synthetic LDHs depending on Mg/Al ratio were confirmed by XRD, TEM, ICP-OES, VSM, Zeta-potential, and BET analyses. The ferrimagnetic property was well preserved even after a coating of LDH on magnetite irrespective of the Mg/Al ratio. Among the three Mg/Al ratios, the calcined Fe3O4@4:1 Mg/Al LDH exhibited excellent performance for iodide removal with 105.04 mg/g of the maximum iodide adsorption capacity due to its wide interlayer spacing and largest BET surface area. In the presence of competing carbonate anions, the Fe3O4@4:1 LDH showed removal rate of >80% at a dosage of over 3 g/L solid to liquid ratio. The recyclability test of Fe3O4@4:1 LDH showed that the removal performance for iodide is maintained at >80% even during the first to the fourth cycles. These results demonstrated that the magnetic Mg/Al LDH adsorbent can be effectively utilized for remediation of radioactive iodide anions with high efficiency and economics.