Cellulose, the most abundant natural polymer, has garnered significant attention as a catalyst and a catalytic support in sustainable chemical processes. Its unique structure, high surface area, and tunable functional groups, such as hydroxyl moieties, make it an ideal candidate for facilitating various catalytic reactions. The functionalisation of cellulose enhances its catalytic properties, enabling acid–base, redox, and enzymatic reactions. Additionally, its biodegradability and renewability align with green chemistry principles, providing an eco-friendly alternative to traditional catalysts. Cellulose has emerged as a promising material for catalysis in water treatment. Its abundance, eco-friendliness, and ease of functionalisation make it an attractive alternative to conventional catalysts in addressing water pollution challenges. By incorporating functional groups or immobilising active species on its surface, cellulose can facilitate advanced oxidation processes, adsorption, and photocatalytic degradation of contaminants. Modified cellulose-based catalysts have demonstrated efficiency in removing organic pollutants, heavy metals, and microbial pathogens from water. Available as colloidal solutions, films, and hydrogels, nanocellulose is effective in removing contaminants like heavy metals, dyes, and pharmaceuticals. Cellulose–ZnO catalysts offer the dual advantage of high dye removal efficiency and environmental sustainability. Cellulose enhances the adsorption of the dye molecules, facilitating closer interaction with the reactive sites on ZnO. This study shows its successful photocatalyst degradation of Crystal Violet dye and reusability and the impact of factors like the pH and catalyst morphology. Cellulose–ZnO materials exhibit favourable structural properties, characterised using Fourier transform infrared and scanning electronic microscopy, for catalytic applications; these materials present a significant step toward achieving cleaner water and promoting environmental sustainability.