Wastewater serves as a significant issue today, and wastewater treatment technology continuously advances to address the growing need for potable water. The lack of pure water around the world poses major threats to the survival of all living creatures. Advanced water/wastewater treatment techniques such as filtration and ion exchange separation are required to remove biodegradable or non-biodegradable harmful and hazardous contaminants from water. Polymer nanocomposites (PNCs) can help overcome significant issues in cleaning wastewater. Thanks to their exceptional features, such as large surface and increased reactivity, in addition to this customisable functionality, PNCs are developing as efficient and adaptable materials for eliminating a wide variety of contaminants. This chapter investigates the many forms of PNCs, inorganic nanoparticle-based, carbon-based and biopolymer-based composites and their distinct roles in wastewater purification. Adsorption, catalytic degradation, membrane filtration and antimicrobial activity are all thoroughly examined to demonstrate how these materials successfully target contaminants such as heavy metals, industrial colours and biological pollutants. The chapter also discusses advanced PNC applications, such as oil/water separation and desalination. Advantages, challenges and environmental concerns are also discussed, providing a holistic view of the scalability, economic aspects and sustainability of PNCs in wastewater treatment. Finally, new research directions are suggested to aid in the development of more environmentally acceptable and commercially viable polymer nanocomposite solutions for environmental applications.