Nanotechnology has achieved great attention due to its impressive performance especially engineered nanoparticles (ENPs). Copper-based nanoparticles offer favorable development in the fabrication of agrochemicals including fertilizers and pesticides in the field of agriculture. However, their toxic impact on melon plants (Cucumis melo) still needs to be investigated. Therefore, the aim of the current work was performed to focus on the toxic impact of Cu oxide nanoparticles (CuONPs) in hydroponically grown Cucumis melo. Our results demonstrated that CuONPs with 75, 150, and 225 mg/L significantly (P<0.005) suppressed the growth rate and badly affect physiological and biochemical activities in melon seedlings. Also, results revealed remarkable phenotypical changes besides significantly reduced fresh biomass and decreased levels of total chlorophyll contents in a dose-dependent manner. Atomic absorption spectroscopy (ASS) analysis exhibited that C. melo treated with CuONPs accumulates NPs in the shoot. Moreover, exposure to higher CuONPs (75-225mg/L) significantly increased the reactive oxygen species (ROS) accumulation, malondialdehyde (MDA), and hydrogen peroxide (H2O2) level in the shoot and induced toxicity in melon root with an increase in electrolyte leakage. Furthermore, antioxidant enzyme peroxidase (POD) and superoxide dismutase (SOD) activity in the shoot significantly increased under exposure to higher CuONPs. Exposure to higher concentrations of CuONPs (225 mg/L) significantly deformed the stomatal aperture. Furthermore, reducing the number and abnormal size of palisade mesophyll and spongy mesophyll cells were investigated especially at high doses of CuONPs. Overall, our current work demonstrates that CuONPs of 10-40 nm size provide direct evidence for a toxic effect in C. melo seedlings. Our findings were expected to inspire the safe production of NPs and agrifood security. Thus, CuONPs prepared from toxic route and its bioaccumulation into our food chain through crop plants possess a serious threat to the ecological system.