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In this study, a novel green tea/Mg-functionalized magnetic nano-adsorbent, denoted as GTE-MgO-Fe₃O₄ NPs, was developed and applied to the extraction of Methylene Blue (MB) from water-based solutions. The GTE-MgO-Fe₃O₄ NPs were synthesized by incorporating green tea extracts (GTE) and Mg species onto the surface of Fe₃O₄ nanoparticles using a hydrothermal method. Characterization analyses corroborated the successful functionalization of the Fe₃O₄ surface with GTE and Mg species, resulting in a superparamagnetic adsorbent equipped with abundant surface functional groups, which promoted MB adsorption and facilitated magnetic separation. Batch experiments revealed that different operating parameters had an impact on the adsorption behavior, such as adsorbent dosage, pH, coexisting ions, contact time, the initial MB concentration, and temperature. The investigations of adsorption kinetics and isotherms emphasized that the MB adsorption onto GTE-MgO-Fe₃O₄ NPs was an exothermic process dominated by chemisorption. The experimental adsorption capacity of GTE-MgO-Fe₃O₄ NPs for MB surpassed 174.93 mg g⁻¹, markedly superior to the performance of numerous other adsorbents. Ultimately, the utilized GTE-MgO-Fe₃O₄ NPs could be effectively regenerated through acid pickling, retaining over 76% of its original adsorption capacity after six adsorption–desorption cycles, which suggested that GTE-MgO-Fe₃O₄ NPs was a suitable adsorbent for eliminating MB from effluent.