Secretin is a hormone that controls the secretions into the duodenum, and also separately, water homeostasis throughout the body. Acting as an anti-Müllerian hormone it is produced in the S cells of the duodenum in the crypts of Lieberkühn. Its effect is to regulate the pH of the duodenal contents via the control of gastric acid secretion and buffering with bicarbonate. It is notable for being the first hormone to be identified. In humans, the secretin peptide is encoded by the SCT gene. It has recently been discovered to play a role in osmoregulation in the hypothalamus, pituitary, and kidney.

Secretin is a linear peptide hormone, which is composed of 27 amino acids and has a molecular weight of 3055. A helix is formed in the amino acids between positions 5 and 13.