Zinc oxide nanoparticles (ZnO-NPs) have gained attention due to their distinctive properties and applications. In the current study, we developed an eco-friendly, economical, and green route for the synthesis of ZnO-NPs using the aqueous extract of the brown marine alga Cystoseira crinita (Fucales; Sargassaceae). ZnO-NPs were characterized by UV-Vis spectroscopy, transmission electron microscope (TEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and dynamic light scattering (DLS). The antimicrobial and antioxidant activities were investigated. The UV absorption peaks at 268 nm and 360 nm confirm the formation of ZnO-NPs., and XRD patterns indicated the crystalline nature of ZnO-NPs. The TEM revealed that the size of ZnO-NPs was ranged between 23 to 200 nm corresponding to DLS, while the shape is multilayered rectangular particles. FTIR analysis revealed that phenolic compounds and proteins were the reducing and capping agents. ZnO-NPs showed antimicrobial activities against Bacillus cereus, Staphylococcus aureus, Escherichia coli, Salmonella typhi, Candida albicans, and Aspergillus niger, furthermore, showed moderate DPPH radical-scavenging activity.