Trypsin is an enzyme that cleaves proteins at specific positions. It cuts at R-X and K-X bonds, unless X is P (see amino acids for the codes). Many trypsin preparations contain some chymotrypsin activity. The pH optimum for trypsin activity is pH 7-9, and it is permanently inactivated at pH > 11. Trypsin retains activity in 0.1% SDS, 1 M guanidine HCl, and 30% ethanol. The autocatalytic activity of trypsin can be slowed down with 20 mM calcium.

Together with pepsin and chymotrypsin, trypsin is one of the three principal digestive proteinases. It is produced by the pancreas, and works in the small intestine, where it degrades proteins to polypeptides and amino acids.