ZnO nanoparticles (NPs) synthesized by using an aqueous extract of Melia azedarach seed are reported in this paper. The structural, morphological, compositional, and optical properties of the NPs were evaluated. A hexagonal phase with spherical morphology and an average crystallite size of 57.74 nm was obtained. Optical studies indicated a maximum absorption peak around 296 nm and bandgap energy of 3.88 eV. Cytotoxicity assay of both the plant extract and the as-prepared ZnO nanoparticles against the human hepatoma (HepG2) cell line showed that the M. azedarach seed-mediated ZnO nanoparticles were less toxic compared to ZnO synthesized by the conventional method, while the plant itself showed dose-dependent low toxicity. ZnO nanoparticles by the green route also exhibited good antibacterial activity, with higher efficiency for Gram-negative bacteria than Gram-positive bacteria.