Copper (Cu) is an essential trace element that plays a vital role in various physiological processes in plants. Due to its significance, copper oxide (CuO) nanoparticles (NPs) are increasingly being incorporated into fertilizers as an alternative to traditional forms. However, there is a limited understanding of how CuO NP spraying and light conditions affect plants, particularly leafy vegetables grown hydroponically indoors. This study aimed to evaluate the impact of different LED lighting spectra and CuO NP spraying on the growth and antioxidant potential of Swiss chard (Beta vulgaris var. cicla, ‘Barese’). The plants were cultivated in hydroponic systems using two types of lighting: a red and blue spectrum (R90%:B10%) and a white, red, and blue spectrum (W65%:R30%:B5%), with an intensity of 250 ± 5 μmol m^-2 s^-1. The plants were sprayed with an aqueous suspension of CuO NPs at a concentration of 30 ppm (size 40 nm). Various growth parameters—such as fresh weight, dry weight, and leaf area—were measured, along with antioxidant indices. The results indicated that CuO NPs positively influenced plant growth under white, red, and blue (WRB) illumination, but had no effect under red and blue (RB) illumination. Cu accumulation was higher under RB illumination, yet it remained within safe limits. In terms of antioxidant capacity, RB illumination enhanced DPPH, ABTS, and FRAP values compared to WRB; however, significant effects of CuO NPs were only observed for DPPH (with RB) and FRAP (with WRB). CuO NPs did not significantly impact the total phenolic content, although this content was higher under WRB illumination. This study found varying effects of CuO NPs on antioxidant enzymes: ascorbate peroxidase, monodehydroascorbate reductase (MDHAR), and superoxide dismutase increased under RB illumination, while glutathione reductase, MDHAR, dehydroascorbate reductase, and catalase increased under WRB. In conclusion, the findings suggest that CuO NPs positively influence the growth of Swiss chard and enhance its antioxidant system, particularly under WRB LED illumination.