The demand for environmentally friendly and sustainable materials has raised awareness of bio-reinforced nanocomposites. Using the green synthesis technique and Mentha pulegium extract as a reducing agent, a novel CuO/Fe₂O₃ nanocomposite was developed. The structural and optical properties of CuO/Fe₂O₃ nanocomposite were investigated using X-ray diffraction (XRD), UV-visible, and Fourier-transform infrared (FTIR) spectroscopies. The obtained CuO/Fe₂O₃ nanocomposite exhibits absorption at 290 nm with a direct bandgap ranging from 4.73 to 5.06 eV and an indirect bandgap of 2.60 to 2.90 eV, according to UV-visible spectra. FTIR analysis exhibits appearance three peaks at 566, 485, and 467 cm⁻¹ attributed to the CuO NPs, Fe₂O₃ NPs, and CuO/Fe₂O₃ nanocomposites binding of iron and copper(II) chloride. The produced nanoparticles formed a hexagonal and monoclinic crystal structure with a crystallite size of approximately 22.93 nm, according to X-ray diffraction (XRD) research results. By using visible and UV-vis spectroscopy, the catalytic activity of produced CuO/Fe₂O₃ in the breakdown of methylene blue (MB) dye was investigated, and the results were compared to the catalytic activity of CuO and Fe₂O₃ NPs. The decolorization ratios of dye for CuO/Fe₂O₃ nanocomposite were 99.52% within 120 min, while CuO and Fe₂O₃ NPs reduced by 62.15% and 74.83%, respectively. The present research highlights the importance of metallic nanocomposite produced by green synthesis for industrial applications.