Poly(ε-caprolactone) (PCL)-based materials have emerged as promising materials for water treatment due to their biodegradability, structural tunability, and compatibility with functional modifications. Beyond previous general PCL reviews, this work offers a focused assessment of PCL-based materials for water treatment, integrating fabrication routes, hierarchical structures, and treatment mechanisms into a coherent framework. A critical summary of recent progress in the design and application of PCL-based composites across four major treatment mechanisms (adsorption, membrane separation, photocatalytic degradation, and biodegradation) was provided, with an emphasis on underlying structure–property–performance relationships. The effects of fabrication strategies, including electrospinning, phase separation, freeze-drying, and 3D printing, on membrane morphology, surface functionality, and pollutant removal performance are systematically analysed. Strategies for performance enhancement are discussed in terms of polymer blending, nanofiller incorporation, and hierarchical structural design, highlighting how these approaches tune wettability, porosity, and interfacial interactions to enhance adsorption and separation efficiency. This review also examines multifunctional hybrid systems that couple photocatalysis with filtration or adsorption, along with the emerging use of PCL as a biodegradable carbon source and microbial carrier for denitrification. In addition, the review provides a comparative perspective that synthesises recent fabrication strategies and treatment pathways, clarifying their respective structural advantages and practical limitations. Despite notable advances, challenges remain in achieving long-term mechanical stability, recyclability, scalable fabrication, and consistent performance under real wastewater conditions. The review concludes with an outlook on integrating green fabrication, quantitative structure–performance correlations, and digital optimisation tools to accelerate the translation of PCL-based materials into practical, sustainable water treatment technologies.