Copper (II) oxide nanoparticles (CuO-NPs) are gaining significant attention due to their unique properties and applications in various fields, including photocatalysis, environmental remediation, and sensors. The biosynthesis of CuO-NPs offers an eco-friendly and cost-effective approach compared to traditional chemical synthesis methods. Neem leaf extract, known for its reducing and stabilizing properties, serves as an ideal candidate for nanoparticle synthesis. This study aims to biosynthesize CuO-NPs using copper acetate monohydrate as a precursor and neem leaf (Azadirachta indica) extract as a reducing and stabilizing agent, followed by a thorough characterization of the synthesized nanoparticles. CuO-NPs were synthesized by mixing copper acetate monohydrate with neem leaf extract, followed by annealing at 400°C for one hour. The synthesized nanoparticles were characterized using several techniques: X-ray diffraction (XRD) for structural analysis, Fourier-transform infrared (FTIR) spectroscopy for functional group analysis, UV-Visible spectroscopy for optical properties, and atomic absorption spectroscopy (AAS) to determine copper concentration. XRD analysis confirmed the formation of CuO nanoparticles with a monoclinic crystal structure, with crystallite sizes of 49.62 nm for sample A and 55.95 nm for sample B. FTIR spectra exhibited peaks at 516.92 cm-1, 518.85 cm-1, and 617.22 cm-1, indicating successful nanoparticle formation. AAS analysis revealed copper concentrations of 0.44 mg/L and 0.40 mg/L for samples A and B, respectively. The UV-Visible spectra indicated direct energy band gaps of 1.75 eV for sample A and 2.25 eV for sample B. The findings highlight the successful synthesis of CuO-NPs with desirable structural and optical properties. The energy band gaps suggest potential for photocatalytic applications, and the presence of functional groups confirmed nanoparticle formation. This study successfully demonstrates the biosynthesis of CuO-NPs using neem leaf extract. The synthesized nanoparticles show promising structural and optical characteristics, indicating their potential use in environmental and energy applications. The study presents an eco-friendly method for CuO-NP synthesis using neem leaf extract, highlighting its applicability for green synthesis and potential photocatalytic applications.