Chitin is a polysaccharide that is a primary component of cell walls in fungi. It is a β(1,4)-linked homopolymer of N-acetylglucosamine. Chitin provides important structural stability to fungal cell walls and is often embedded deeply within the cell wall structure, anchoring other components at the cell surface. The organization of the cell wall layers allows the fungal cell to effectively monitor and interact with the external environment.

Chitin and its fragments, chitin oligosaccharides, act as signaling molecules in fungi and are involved in the regulation of various biological processes. Chitin-directed organization of the cell wall layers allows the fungal cell to effectively monitor and interact with the external environment.

Chitin is also found in the exoskeletons of arthropods such as crustaceans and insects, the radulae, cephalopod beaks and gladii of mollusks, and in some nematodes and diatoms. It is also synthesized by at least some fish and lissamphibians.

Chitin has many potential applications, including in agriculture as a good inducer of plant defense mechanisms for controlling diseases and as a soil fertilizer or conditioner to improve fertility and plant resilience that may enhance crop yields. It has also been exploited for many biomedical applications, including targeted drug delivery and vaccine development.