Gastrin is a linear peptide hormone produced by G cells of the duodenum and in the pyloric antrum of the stomach. It is secreted into the bloodstream. In humans, gastrin is a hormone that stimulates secretion of gastric acid by the parietal cells of the stomach. Gastrin is found primarily in three forms: gastrin-34 ("big gastrin"), gastrin-17 ("little gastrin"), gastrin-14 ("minigastrin").

Gastrin is released in response to certain stimuli. These include: stomach distension, vagal stimulation (mediated by the neurocrine bombesin, or GRP in the human), the presence of partially digested proteins especially amino acids, hypercalcemia. Gastrin release is inhibited by: The presence of acid (primarily the secreted HCl) in the stomach (a case of negative feedback) and somatostatin also inhibits the release of gastrin, along with secretin, GIP, VIP, glucagon and calcitonin.

The presence of gastrin stimulates parietal cells of the stomach to secrete hydrochloric acid (HCl)/gastric acid. This is done either directly or indirectly. Directly it binds onto CCK2/gastrin receptors on parietal cells stimulating them to secrete H+ ions. Indirectly it binds onto CCK2/gastrin receptors on ECL cells in the stomach, which then responds by releasing histamine, which in turn acts in a paracrine manner on parietal cells stimulating them to secrete H+ ions. This is the major stimulus for acid secretion by ECL cells.

It also causes chief cells to secrete pepsinogen, the zymogen (inactive) form of the digestive enzyme pepsin. Pepsinogen is converted to pepsin in a low pH environment, and the HCl provides a suitable environment for its activity. It can also increase antral muscle mobility and trophic effect on GI tract and causes promotion of contraction of circular muscle of the stomach.  Gastrin has also been shown to induce production of pancreatic enzymes by acinar cells. It increases gastric blood flow.