The increasing scarcity of clean drinking water and pollution of freshwater sources as a result of industrialization have created a need for innovative wastewater treatment technologies. Nanomaterials have proven to be very efficient alternatives to traditional treatments because of their high surface area, reactivity, and catalytic activity. This chapter gives an in-depth overview of different nanomaterial types such as metal and metal oxide nanoparticles, carbon-based nanomaterials, polymeric nanomaterials, nanocomposites, zeolites, nanozymes, quantum dots, and nanofibers and their unique characteristics, action mechanisms, and wastewater treatment applications. The chapter further discusses their applications in practical applications such as adsorption, photocatalysis, membrane filtration, nanocatalysis, disinfection, electrochemical treatment, and magnetic separation. Although nanomaterials present enormous promise for effective, sustainable, and affordable water purification, the issues of scalability, environmental safety, and long-term sustainability need to be solved in order to see widespread practical applications.