Abstract: The thio-functionalized nano-Fe₃O₄-polymer magnetic composite material (SH-nFe₃O₄-polymer) was synthesized via suspension polymerization. The SH-nFe₃O₄-polymer was characterized by TGA, EA, AAS, XRD, FTIR, TEM and VSM. The application for its adsorption properties on the methylene blue (MB) from water was investigated. The results show that the SH-nFe₃O₄-polymer has an average size of 250-300 nm, with the saturation magnetization intensity of 5.88 emu/g. The adsorption capacities were investigated by adsorbing the MB from aqueous solutions, which demonstrate an excellent adsorption capacity with the maximum adsorption capacity at 476.2 mg/g, much higher than those of the tetraethylenepentamine-functionalized nano-Fe₃O₄-polymer magnetic composite (TEPA-nFe₃O₄-polymer, 30.6 mg/g) and the thiol-functionalized polymer free of magnetic core (SH-polymer, 74.6 mg/g). The isothermal adsorption curve mainly obeys the Langmuir mode. The adsorption thermodynamic studies suggest that the adsorption processes of the MB by the SH-nFe₃O₄-polymer are endothermic, entropy favored, and spontaneous in nature. The adsorption kinetic studies show that the adsorption processes reach the equilibrium within 10 min and the kinetic data are well fitted to the pseudo-second-order model. The activation energy for the MB removal is found to be 9.53 kJ/mol. The MB in aqueous solution can be effectively removed by the SH-n Fe₃O₄-polymer. The primary mechanism studies reveal that the adsorption processes may be related to the electrostatic attraction, π-π and hydrophobic interactions between the MB and the SH-nFe₃O₄-polymer. The existing of the magnetic core might be favorable to the mass transfer thus accelerating the adsorption process.