Secretin is a hormone produced in the S cells of the duodenum in response to low duodenum pH and fatty acids in the duodenum to stimulate the secretion of bicarbonate from bicarbonate producing organs(liver, pancreas, Brunner's glands) when the pH drops below a set value. This helps neutralize the gastric acid entering the duodenum from the stomach.

It also inhibits acid secretion from the stomach by reducing gastrin release from the G cells of the stomach.

It contains 27 amino acids and is homologus to glucagon.

Secretin was the first substance which was identified to cause a physiological effect in the body after being transported via the blood. The term hormone was coined by Bayliss and Starling to categorise such chemical messengers.