Smart nanomaterials—nanoscale systems that respond to external stimuli such as temperature, pH, light, or magnetic fields—offer transformative potential across energy, environmental, and biomedical fields. This chapter focuses recent advances in the design, synthesis, and application of these responsive materials. In energy systems, smart nanomaterials enable self-regulating solar cells, high-performance batteries, and thermally adaptive coatings. Environmental applications include responsive sensors, photocatalysts, and regenerable adsorbents for pollution control and water treatment. In medicine, they facilitate targeted drug delivery, real-time diagnostics, and adaptive biomaterials for tissue engineering. The integration of stimuli-responsiveness at the nanoscale provides improved functionality, precision, and sustainability. Key challenges in safety, scalability, and material stability are also discussed, along with emerging trends in AI-guided material design and biodegradable smart systems. This chapter also highlights the interdisciplinary potential of smart nanomaterials to address global challenges through intelligent, adaptive technologies.