In this study, Fe3O4/montmorillonite was synthesized as a heterogeneous Fenton catalyst for the removal of Reactive Orange 16 (RO16) from aqueous solutions based on chemical oxygen demand (COD). System parameters such as H2O2 concentration, catalyst dose, pH, and reaction time were numerically optimized based on Central Composite Design (CCD). The catalyst was characterized with X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM), dynamic light scattering (DLS), ζ potential, and Brunauer-Emmett-Teller (BET). The adsorption process contributed to the removal of RO16 but the heterogeneous Fenton process had a higher share and occurred faster than the adsorption process. Optimum conditions were determined as catalyst dosage:1.83 (g/L), H2O2 concentration:77.98 (mM), pH:3, and reaction time:60 min. Under these conditions, COD removal efficiency estimated from the model was 84.82% and found experimentally was 85.90%. Successful removal of RO16 from aqueous environments is possible with a heterogeneous Fenton process using Fe3O4/MMT.