The conversion of agricultural waste into functional nanomaterials offers a sustainable pathway to address environmental challenges while promoting circular economy principles. This chapter explores recent advances in the valorization of crop residues, fruit peels, husks, shells, and other agro-wastes into carbon-based, metal/metal oxide, biopolymer, and hybrid nanostructures. Green synthesis strategies, including plant extract-mediated, sol–gel, hydrothermal, and precipitation methods, are highlighted for their low environmental footprint. Applications in water treatment, air purification, soil remediation, and energy–environment synergies are critically reviewed, demonstrating the versatility of these materials in pollutant removal, catalytic degradation, nutrient recovery, and CO2 capture. Challenges such as feedstock variability, scalability, material stability, and economic feasibility are discussed alongside future perspectives for industrial integration. By linking waste management with nanotechnology innovation, this work underscores the potential of agricultural residues as renewable resources for next-generation environmental solutions.