Sustainable provision of safe, clean and adequate water supply is a global challenge. Water treatment and desalination technologies remain chemically and energy intensive, ineffective in removing key trace contaminants, and poorly suited to deployment in decentralized (distributed) water treatment systems globally. Several recent efforts have sought to leverage the reactive and tunable properties of nanomaterials to address these technological shortcomings. This Review assesses the potential applications of nanomaterials in advancing sustainable water treatment systems and proposes ways to evaluate the environmental risks and social acceptance of nanotechnology-enabled water treatment processes. Future areas of research necessary to realize safe deployment of promising nanomaterial applications are also identified.