<p>Chitosan, the deacetylated derivative of chitin, has emerged as a versatile, renewable, and biodegradable polymer for advanced sustainable materials. Its polyglucosamine backbone, bearing reactive amino and hydroxyl groups, enables tunable solubility, charge density, and molecular interactions, allowing precise structural modification for targeted performance. This review highlights the structure–function relationships that guide chitosan’s application across protective coatings, food packaging, environmental purification, agriculture, and additive manufacturing. In coatings, crosslinking and inhibitor-loaded architectures provide durable and self-healing corrosion protection. In food packaging, multilayer films and electrospun nanofibers deliver controlled antimicrobial release and real-time spoilage sensing. Chitosan–graphene, MOF, and metal oxide hybrids offer high-capacity adsorption of pollutants and photocatalytic degradation for water purification. In agriculture, chitosan-based biostimulants and controlled-release fertilisers enhance nutrient efficiency and stress tolerance. Meanwhile, photocurable and nanocomposite bioinks expand chitosan’s role in 3D bioprinting and sustainable electronics. Remaining challenges include scalability, stability in humid environments, and alignment with industrial processing. Future opportunities lie in green derivatisation routes, circular bioeconomy sourcing, and integrated multifunctional system design. Collectively, chitosan represents a platform polymer that can advance environmentally responsible material solutions.</p> Graphical Abstract <p></p>