Aggrecan, or large aggregating proteoglycan, is a proteoglycan, or a protein modified with carbohydrates; the human form of the protein is 2316 amino acids long and can be expressed in multiple isoforms due to alternative splicing. Along with Type-II collagen, aggrecan forms a major structural component of cartilage, particularly articular cartilage.

Aggrecan consists of two globular structural domains at the N-terminal end and one globular domain at the C-terminal end, separated by a large domain heavily modified with glycosaminoglycans. The two main modifier moieties are themselves arranged into distinct regions, a chondroitin sulfate and a keratan sulfate region.