Melanocyte-stimulating hormone (MSH) is a peptide hormone produced by cells in the intermediate lobe of the pituitary gland. It stimulates the production and release of melanin (melanogenesis) by melanocytes in skin and hair.

In some animals (such as the clawed toad Xenopus laevis) production of MSH is increased when the animal is in a dark location. This causes pigment to be released from cells in the toad's skin, making it become darker, and harder for predators to spot.

An increase in MSH will cause a darkening in humans too. Melanocyte-stimulating hormone increases in humans during pregnancy. This, along with increased estrogens, causes increased pigmentation in pregnant women. In Addison's disease high levels of adrenocorticotropic hormone (ACTH), a compound similar to MSH causes an abnormal darkening.

Different levels of MSH are not the major cause of racial variation in skin colour. In many red headed people, and other people who do not tan well, there are variations in their hormone receptors, causing them to not respond to MSH in the blood.

Melanocyte-stimulating hormone belongs to a group called the melanocortins. This group includes ACTH, alpha-MSH, beta-MSH and gamma-MSH. Alpha-MSH is the most important in pigmentation.

A synthetic analog of MSH, called Melanotan is being developed by an Australian company. There are two versions of the analog (Melanotan I and Melanotan II); inexplicably, Melanotan II causes erections in some male test subjects.