Pepsin is an enzyme that catalyzes the breakdown of proteins (i.e. it is a protease) to polypeptides in the vertebrate stomach. It is one of the main components, along with hydrochloric acid, of gastric juice. It is secreted as an inactive precursor called pepsinogen.

Pepsin was the first animal enzyme to be discovered, by Theodor Schwann in 1836. Its name comes from the Greek word pepsis, meaning digestion.