Pentosans are polymers composed of pentoses. In contrast to cellulose, which is composed of hexose (glucose) monomers, pentosans are derived from five-carbon sugars such as xylose. Pentosan-rich biomass is the precursor to furfural.

The pentosan content has been determined for many natural materials:

Pentosans can act as heparinoids, glycosaminoglycans which are derivatives of heparin.

They can have an influence on bread quality.