Water contaminants, including pharmaceuticals, endocrine-disrupting compounds, pesticides, per- and polyfluoroalkyl substances, dyes, heavy metals, microplastics, and pathogenic microorganisms, are increasingly detected in aquatic environments and pose significant challenges to conventional water treatment technologies due to their persistence, toxicity, and complex behavior. Many existing treatment processes exhibit limited efficiency in completely removing these contaminants, creating an urgent need for more advanced and sustainable remediation strategies. This narrative review examines recent advances in smart nanoadsorbents and advanced nanomaterials for water purification and contaminant removal. Various classes of nanomaterials, including metal and metal oxide nanoparticles, carbon-based nanomaterials, metal–organic frameworks, covalent organic frameworks, nanocellulose-based materials, polymeric nanomaterials, nanostructured biochars, and nanostructured membranes, are discussed in terms of their structure, properties, and water treatment applications. Particular emphasis is placed on stimulus-responsive nanoadsorbents capable of responding to environmental triggers such as pH, temperature, light, and magnetic fields. Major removal mechanisms, including adsorption, photocatalytic degradation, membrane-based separation, electrochemical processes, antimicrobial inactivation, and redox reactions, are critically evaluated. The reviewed studies demonstrate that multifunctional and hybrid nanomaterial systems generally outperform single-component nanomaterials by providing enhanced removal efficiency, improved selectivity, greater stability, and superior reusability. Recent pilot-scale investigations and real-world applications further highlight the potential of these materials for practical water and wastewater treatment. Nevertheless, challenges related to production costs, scalability, regeneration, environmental safety, potential ecotoxicity, and regulatory acceptance continue to limit their widespread implementation. Overall, smart nanoadsorbents and advanced nanomaterials represent promising platforms for next-generation water treatment technologies; however, overcoming technical, economic, and regulatory barriers remains essential for their successful large-scale deployment.