In this green synthesis approach for copper oxide (CuO) nanoparticles (NPs), the reducing agent obtained from Camellia sinensis (Tea) extract was used. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), ultraviolet visible (UV-Vis) spectroscopy, field emission scanning electron microscope (FESEM) with energy-dispersive X-ray (EDX), high resolution transmission electron microscope (HRTEM) and Selected Area Electron Diffraction (SAED) techniques were used for as-prepared sample characterization. From XRD result, the crystallite size estimated by Scherrer method was found to be 35 nm. Morphological results reveals that the CuO NPs possesses small particle size with the range of 40–45 nm, led to high surface area, which enhances the absorption of more number of dye molecules, which results in high degradation efficiency. Present work reveals that degradation of malachite green (MG) dye in aqueous solutions under sonocatalytic treatment. CuO NPs exhibit excellent degradation efficiency against MG dye (95 % removal in 70 min). Moreover, the rate constant for the degradation efficiency was investigated with pseudo- first and second order kinetics. Moreover, CuO NPs possesses potential antibacterial activity against human pathogens such as Gram-positive (Staphylococcus pyogenes) and Gram-negative (Pseudomonas aeruginosa). Display omitted •Copper Oxide nanoparticles (CuO NPs) were synthesized by facile green method.•Green synthesized CuO NPs was tested against the degradation of malachite green dye under sonocatalytic treatment.•The comparisons of different dye concentrations on three different catalyst dosage levels were studied.•CuO NPs possesses magnificent degradation efficiency for MG dye in 70 min sonication.•Moreover, the CuO NPs possesses potential antibacterial activity against human pathogens.