Hyaluronic acid (HA; conjugate base hyaluronate), also called hyaluronan, is an anionic, nonsulfated glycosaminoglycandistributed widely throughout connective, epithelial, and neural tissues. It is unique among glycosaminoglycans in that it is nonsulfated, forms in the plasma membrane instead of the Golgi apparatus, and can be very large, with its molecular weightoften reaching into the millions. One of the chief components of the extracellular matrix, hyaluronan contributes significantly to cell proliferation and migration, and may also be involved in the progression of some malignant tumors.

The average 70 kg (154 lb) person has roughly 15 grams of hyaluronan in the body, one-third of which is turned over (degraded and synthesized) every day. Hyaluronic acid is also a component of the group A streptococcal extracellular capsule, and is believed to play a role in virulence.

Hyaluronic acid has been FDA approved to treat osteoarthritis of the knee via injecting it into the joint;Hyaluronic acid is a common ingredient in skin-care products. Hyaluronic acid is used as a dermal filler in cosmetic surgery. It is typically injected using either a classic sharp hypodermic needle or a micro-cannula. Complications include the severing of nerves and vessels, pain and bruising. In some cases hyaluronic acid fillers result in a granulomatousforeign body reaction.