3D-printed polymer nanocomposites are transforming water treatment technology. Materials of this kind allow sophisticated water filtration systems to be designed and built with remarkable accuracy using additive manufacturing. Nanomaterials in polymer matrices give these composites outstanding mechanical, thermal, and chemical capabilities, making them efficient removers of water contaminants. Nanoscale additions, including metal oxides, carbon-based compounds, and organic–inorganic hybrids, improve the adsorption, catalytic, and antibacterial capabilities of the filtration system. This chapter covers the latest 3D-printed polymer nanocomposites for water treatment, including design, fabrication, and functionalization. It explores how nanoparticles improve 3D-printed structures for water treatment applications such as filtration, adsorption, and photocatalytic degradation. Material compatibility, structural integrity, and cost-effectiveness issues have been addressed when scaling these technologies for real-world applications. The recent case studies showing how these modern water treatment methods are implemented and benefit the environment have been discussed. Finally, the potential of 3D-printed polymer nanocomposites to revolutionize water purification and develop more sustainable and efficient systems has been explored.