Water is a precious resource required for the sustenance of almost all life forms present on the Earth. The increase in global human population, drastic changes in global climate, fast-growing industrialization, and the presence of emerging contaminants are leading to water scarcity and pollution. Conventional water treatment technologies are inefficient in removing emerging contaminants and unable to generate potable high-quality water. Recently, the applications of polymer-based nanocomposites in water remediation are gaining attention due to their advantages such as low costs, good surface area, high stability, processability, and recyclability. These nanocomposites are usually synthesized by incorporating nanomaterials (such as metal oxide nanoparticles, carbon nanotubes, graphene, carbon dots, and nano clays) or nanoscale fillers into the polymer matrix. These nanocomposites can effectively work as adsorbents and photocatalyst materials for full-scale water treatment to remove heavy metals, organic dyes, pesticides, pharmaceutical drugs, and biological contaminants (bacteria, viruses, and fungi). The need for advances in nanotechnology for sustainable water management can be studied with the aid of these nanocomposites. This chapter focuses on the recent developments in the use of polymeric nanocomposites for water treatment and mechanisms including adsorption and photodegradation.