In this work, FeM composites consisting of montmorillonite and variable amounts of Fe₃O₄ were successfully synthesized via a facile co-precipitation process. They were characterized using X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscope (FESEM), energy dispersive X-ray spectroscopy (EDX), transmission electron microscope (TEM), N₂ adsorption-desorption and Fourier transform infrared spectroscopy (FT-IR) techniques to explain the effect of Fe₃O₄ content on the physicochemical properties of the Fe₃O₄-montmorillonite (FeM) composites. The FeM composites were subsequently used as heterogeneous Fenton catalysts to activate green oxidant (H₂O₂) for the subsequent degradation of ofloxacin (OFL) antibiotic. The efficiency of the FeM composites was studied by varying various parameters of Fe₃O₄ loading on montmorillonite, catalyst dosage, initial solution pH, initial OFL concentration, different oxidants, H₂O₂ dosage, reaction temperature, inorganic salts and solar irradiation. Under the conditions of 0.75 g/L FeM-10, 5 mL/L H₂O₂, and natural pH, almost 81 % of 50 mg/L of OFL was degraded within 120 min in the dark, while total organic carbon (TOC) reduction was about 56 %. Moreover, the FeM-10 composite maintained high efficiency and was stable even after four continuous cycles, making it a promising candidate in real wastewater remediation. Results from such study have confirmed that synergetic effect between Fe₃O₄- montmorillonite and H₂O₂ is necessary, in order to achieve higher efficiency.