ZnO nanoparticles (NPs) were synthesized by co-precipitation technique. Structural and optical properties of prepared ZnO NPs were characterized by X-ray diffraction, SEM technique, UV–visible, and PL spectroscopy. The powder sample was annealed at 600 °C. XRD analysis confirmed that the ZnO NPs possess the formation of a wurtzite-type hexagonal crystal structure. The average crystallite size of ZnO NPs was found to be 43 nm. SEM analysis revealed the rod-shaped ZnO NPs confirm the successful synthesis with uniform morphology and nanoscale dimension. The optical band gap (Eg) was calculated to be 2.76 eV from UV–visible spectroscopy, reflecting their semiconductor behavior. Photoluminescence (PL) spectroscopy technique was used to examine photonic emissions from the ZnO crystal lattice. The photoluminescence of ZnO NPs excited at 272 nm. The ZnO NPs showed a sharp emission peak at 543 nm in the green spectral region, which is attributed to defect levels corresponding to singly ionized oxygen vacancies within the ZnO matrix. The intensity of the green emission is correlated with the density of oxygen vacancies, indicating that the emission mechanism involves the transition between photo-excited holes and singly ionized oxygen vacancies. The observed PL characteristics concerned with the defect structure of the ZnO NPs and highlighted their potential applications in optoelectronic devices.