A glucan is any of a class of complex sugars (polysaccharides) that serve a variety of functions in living cells. For instance, 1,3-Β-D-glucan gives the necessary stiffness to the cell walls of microbes such as algae, bacteria and fungi. Some vitamin and herb manufacturers are also touting 1,3-Β-D-glucan as a supplement to boost one's immune system, but this use has not been validated in clinical trials.

Glucans are composed of D-glucose monomers linked together by glycosidic bonds. Dextran, laminarin, and lichenin are other examples of glucans.