Mucilage is a thick, gluey substance produced by most plants and some microorganisms. It is a polar glycoprotein and an exopolysaccharide.

It occurs in various parts of nearly all classes of plant, usually in relatively small percentages, and is frequently associated with other substances, such as tannins and alkaloids.

Mucilage in plants is thought to aid in water storage and seed germination, and to act as a membrane thickener and food reserve. Among the richest sources are cacti (and other succulents) and flax seeds.

Mucilage has a unique purpose in some carnivorous plants. The plant genera Drosera (Sundews), Pinguicula, and others have leaves studded with mucilage-secreting glands, and use a "flypaper trap" to capture insects.

Exopolysaccharides are the most stabilising factor for microaggregates and are widely distributed in soils. Therefore exopolysaccharide-producing "soil algae" play a vital role in the ecology of the world's soils. The substance covers the outside of, for example, unicellular or filamentous green algae and cyanobacteria. Amongst the green algae especially, the group Volvocales are known to produce exopolysaccharides in a certain part of their life cycle.