半纤维素(Hemicellulose)是与纤维素共同存在于大多数植物细胞壁的一类杂聚多糖（矩阵型多糖，如阿拉伯木聚糖）。纤维素是结晶的、强壮的、抗水解，聚合度为7000-15000。半纤维素是随机的、非结晶结构；强度低；在稀酸稀碱条件下或多种半纤维素酶作用下易于水解，聚合度为500-3000。

半纤维素包括木聚糖、聚半乳糖葡萄糖甘露糖、聚阿拉伯糖半乳糖、聚葡萄糖甘露糖等。

粗制的半纤维素可将其分为一个中性组分（半纤维素A）和一个酸性组分（半纤维素B）。 半纤维素是由几种不同类型的单糖构成的异质多聚体，这些糖是五碳糖和六碳糖，包括木糖、阿拉伯糖、甘露糖、半乳糖、鼠李糖等。木聚糖在木质组织中占总量的50%。

A hemicellulose (also known as polyose) is any of several heteropolymers (matrix polysaccharides), such as arabinoxylans, present along with cellulose in almost all plant cell walls. While cellulose is crystalline, strong, and resistant to hydrolysis, hemicellulose has a random, amorphous structure with little strength. It is easily hydrolyzed by dilute acid or base as well as myriad hemicellulase enzymes.