In this study, green synthesis, characterization and antibacterial activity of zinc oxide nanoparticles were investigated. The zinc oxide nanoparticles were successfully synthesized a fast, convenient, cost effective, and environmentally friendly method by biologically reducing zinc nitrate hexahydrate (Zn (NO3)26H2O) solution with Justicia Schimperiana leaf extract under optimum conditions. The presence of active flavonoids, carbohydrates, phenolic groups, alkaloids, terpenoids and tannins in the Justicia Schimperiana extract was, identified using qualitative screening methods (observing the colour changes) and FT-IR Spectroscopy. The formation of ZnO NPs was visually indicated by the color changes from colorless to yellow color. Biosynthesized nanoparticles were further characterized by UV-Visible, FT-IR, and XRD spectroscopies. The reduction process was simple and easy to manage, monitored by UV-Visible spectroscopy which showed surface plasmon resonance (SPR) of the ZnO NPs at 367 nm, confirming their formation. X-ray diffraction was employed to investigate the crystal structure revealing an average particle size of approximately 12.67 nm calculated using the line width of the plane and refraction peak in Scherer‟s equation. Green synthesized zinc oxide nanoparticles were evaluated for antibacterial activities against both gram positive and gram negative bacteria. Nanoparticles exhibited maximum zone of inhibition against S.pyogene (18.5 mm) while least activity was observed against K.pneumonia.