<p>The increasing global demand for cost-effective and environmentally sustainable water treatment technologies has catalysed research into the utilization of industrial by-products such as fly ash (FA), a residue generated from fuel combustion. Owing to its high porosity, substantial surface area, and distinctive chemical composition, FA exhibits considerable efficacy in the removal of various waterborne contaminants, including heavy metals, organic compounds, and pathogenic microorganisms. Moreover, its alkaline constituents contribute to pH regulation, while the presence of silica and titanium dioxide enhances filtration performance and facilitates photocatalytic degradation of pollutants under ultraviolet (UV) irradiation. The multifunctional nature of FA supports its integration into diverse water purification technologies, including emerging methods such as capacitive deionization. Given that global FA production exceeds one billion tonnes annually, the valorization of this abundant waste material presents a dual benefit—mitigating environmental burdens associated with waste disposal and providing a sustainable resource for water treatment applications. This review synthesizes recent advancements in FA-based water treatment strategies, with a focus on its functional mechanisms, technological versatility, and contributions to sustainable environmental management.</p> Graphical Abstract <p></p>