甲壳素（英语：Chitin，IPA:英语发音：/ˈkaɪtɨn/ KY-tin，日语音译为キチン质），分子结构「C₈H₁₃O₅N)_n」，又名「几丁质」、「几丁聚糖」、「几丁寡糖」、「甲壳质」或「壳多糖」，是一种含氮的多糖类物质，为虾、蟹、昆虫等甲壳的重要成分，化学名为8-(1,4)-2-乙酰氨基-2-脱氧-D-葡聚糖，也称为聚（N-乙酰基-D-葡糖胺）。

几丁质为为长链状聚合物，是由约8000个葡萄糖的衍生物，N-乙酰葡糖胺作为单体聚合而成，是自然界的一种半透明而坚固的材料，常见于真菌的细胞壁和节肢动物（如虾、蟹）或昆虫的外骨骼。几丁质与属多糖的纤维素类似，都会构成奈米纤维或细毛状的晶体结构。在实际功能上，则近于构成皮肤的角蛋白，因为具有这些特性，几丁质在医学和工业上具有实用价值。

Chitin (C₈H₁₃O₅N)_n (/ˈkaɪtᵻn/ KY-tin) is a long-chain polymer of an N-acetylglucosamine, a derivative of glucose, and is found in many places throughout the natural world. It is a characteristic component of the cell walls of fungi, the exoskeletons of arthropods such as crustaceans (e.g., crabs, lobsters and shrimps) and insects, the radulae of molluscs, and the beaks and internal shells of cephalopods, including squid and octopuses and on the scales and other soft tissues of fish and lissamphibians. The structure of chitin is comparable to the polysaccharide cellulose, forming crystalline nanofibrils or whiskers. In terms of function, it may be compared to the protein keratin. Chitin has proved versatile for several medical, industrial and biotechnological purposes.