Background/aim: Aqueous phytochemical extract of Spirogyra (Algae) was used in the present work as a reducing and capping agent to synthesize zinc oxide nanoparticles (NPs). Methodology and results: The phytochemical synthesis of nanoparticles was confirmed using UV-Vis Spectrophotometer, showing an absorption peak at the 280nm wavelength range. Fourier transform infrared spectroscopy (FTIR) evaluated the role of active phytochemicals derived from Spirogyra extract. Scanning electron microscopy (SEM) identified irregular and homogenous morphology, and the XRD identified the crystalline structure of ZnO nanoparticles with an average size of 52.96 nm. EDX analysis identified the elemental composition of ZnO nanoparticles. Furthermore, the biological activities of the phytochemically synthesized NPs revealed an excellent therapeutic potential. The synergistic bactericidal effect of the NPs and antibiotics was assessed against selected pathogenic bacterial strains. The highest inhibition activity was observed against K. pneumonia and B. subtilis with an inhibition zone of 30± 0.21 and 30± 0.26, respectively. DPPH (2, 2- diphenyl-1 picryl-hydrazine) assay demonstrated ZnO NPs' free radical scavenging ability (81.10±0.26) at 100 µg/ml. The biocompatible ZnO nanoparticles demonstrated outstanding antileishmanial efficacy against amastigotes and promastigotes (79% ±0.65 and 80% ±0.155, respectively) compared to the standard treatment of Amphotericin B (88 and 94% at 100µg/ml). Biogenic Zinc oxide nanoparticles were used as a catalyst for removing organic dyes, methylene blue and Methyl red, with an efficiency of 84% and 95%, respectively. Conclusion: The finding of this study suggests that spirogyra extract-mediated synthesis of ZnO nanoparticles is a promising approach towards sustainable and eco-friendly nanotechnology and may be further evaluated for in-situ and commercial applications.