<p>Chitin, chitosan, and nano-chitosan are renewable biopolymers recognized for their remarkable physicochemical and biological adaptability. Their natural abundance, biocompatibility, and tunable molecular structure make them pivotal materials for advancing sustainable technologies. This review explores their hierarchical structural evolution, functional diversity, and broad application spectrum. The interplay between acetylation and deacetylation governs solubility, crystallinity, and reactivity, while advanced characterization techniques such as FTIR, NMR, XRD, SEM, TEM, TGA, and DSC reveal their intricate structure–property relationships. The amino and hydroxyl groups provide active sites for metal coordination, enzyme immobilization, and catalytic conversion, while humidity-driven proton transport supports their role as bio-derived electrolytes in energy systems. Nano-chitosan, characterized by its high surface area and enhanced reactivity, demonstrates exceptional adsorption, catalytic, and biological performance. When integrated with graphene, metal oxides, or bioactive fillers, it forms composites with superior mechanical, electrical, and barrier properties. Collectively, these materials represent a versatile and sustainable platform for next-generation applications spanning food preservation, environmental remediation, biomedical engineering, and flexible electronic devices.</p> Graphical Abstract <p></p>