纤维素是自然界中分布最广、含量最多的一种多糖，是组成植物细胞壁的主要成分。棉花、亚麻、苎麻和黄麻部含有大量优质的纤维素。木头中纤维素含量约为50％，棉花的纤维素含量更高，达90%。木材中的纤维素则常与半纤维素和木质素共同存在。

纤维素是一种复杂的多糖，有8000至10000个葡萄糖残基通过β-1,4-糖苷键连接而成。天然纤维素为无味的白色丝状物。纤维素不溶于水、稀酸、稀碱和有机溶剂，但在加热的条件下会被酸水解，主要的生物学功能是构成植物的支持组织。

Cellulose is an organic compound with the formula (C
6H
10O
5)
n, a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell wall of green plants, many forms of algae and the oomycetes. Some species of bacteria secrete it to form biofilms. Cellulose is the most abundant organic polymer on Earth. The cellulose content of cotton fiber is 90%, that of wood is 40–50% and that of dried hemp is approximately 57%.