<p>Zinc oxide (ZnO) nanomaterials have attracted increasing attention for water purification owing to their photocatalytic, adsorptive, and antibacterial capabilities. Green synthesis using biological sources such as plants, fungi, and bacteria offers an eco-friendly and cost-effective alternative to conventional chemical methods, enabling control over nanoparticle size, shape, and surface functionality. Among biosynthetic routes, plant-based methods are particularly appealing due to their simplicity, scalability, and rich bioactive content. This review outlines various green synthesis strategies, key influencing factors (e.g., pH, temperature, species type), and their impact on ZnO’s physicochemical properties. Biosynthesized ZnO often exhibits a reduced bandgap due to organic incorporation or structural defects, enhancing photocatalytic efficiency. However, its intrinsic wide bandgap limits visible-light activation, necessitating material modifications such as elemental doping, heterojunction formation, and integration with carbon-based materials. In antibacterial applications, ZnO demonstrates strong ROS-mediated disinfection and membrane disruption, with enhanced effects observed in nanocomposites containing <i>Cassia fistula</i>, banana peels, and <i>Ailanthus altissima</i>. To further improve disinfection in low-oxygen environments, ZnO is combined with H₂O₂ or integrated with reduced graphene oxide (rGO) and silver nanoparticles, enabling controlled oxygen release and enhanced ROS generation. Despite this progress, most studies focus primarily on nanoparticle production and preliminary activity evaluations. Critical gaps remain in understanding pollutant degradation pathways, recyclability, long-term stability, and especially the recovery of ZnO nanomaterials after bacterial disinfection. Addressing these limitations is essential for advancing the practical implementation of green-synthesized ZnO in sustainable water treatment systems.</p> Graphical Abstract <p></p>