Proteoglycans are protein that are covalently bonded at multiple sites along the protein chain to a class of polysaccharides, known as glycosaminoglycans. Proteoglycans are a significant component in the extracellular matrix. The major functions of proteoglycans depend on the glycosaminoglycan component of the molecule.

All cellular processes that involve molecular interactions at the cell surface, such as cell–matrix, cell–cell and ligand–receptor interactions, involve proteoglycans.