Copper nanoparticles have garnered significant attention due to their diverse applications across various fields. Green synthesis of nanoparticles offers a sustainable, cost-effective alternative to traditional methods, supporting the global shift towards greener practices in nanotechnology. This study presents the first green synthesis of copper nanoparticles using an aqueous leaf extract from Cucurbita maxima, a globally available, edible, and inexpensive plant. Nanoparticles formation was indicated by a colour change from brown to green, further confirmed by an absorption peak at 320 nm in UV–visible spectrophotometric analysis. Field emission scanning electron microscopy revealed nanoparticle sizes ranging from 50 to 60 nm. The Fourier-transform infrared spectroscopy analysis identified peaks at 3380 cm−1, 1640 cm−1, 1240 cm−1, and 680 cm−1, indicating functional groups responsible for Cu2+ reduction. Zeta potential analysis confirmed the nanoparticles were negatively charged and stable due to electrostatic repulsions. Antibacterial studies showed a concentration-dependent response, with 100 μg/mL CuNPs yielded zones of 9.0 ± 1.0 mm (E. coli) and 7.5 ± 1.0 mm (S. aureus); at 200 μg/mL, inhibition increased to 17.0 ± 2.0 mm and 15.6 ± 2.07 mm, respectively. In vitro anticancer studies demonstrated that copper nanoparticles exhibited significant cytotoxicity against MCF-7 and MDA-MB-468 breast cancer cells, with minimal impact on non-tumorigenic breast epithelial cells. Furthermore, molecular docking analysis revealed strong binding interactions of copper nanoparticles with epidermal and vascular endothelial growth factor receptors, suggesting interference with downstream signalling and cancer progression. This research focuses on the green synthesis of copper nanoparticles using Cucurbita maxima, showcasing their significant antimicrobial and anti-cancerous activity with strong biomedical potential.