Insulin (from Latin insula, "island", as it is produced in the Islets of Langerhans in the pancreas) is a polypeptide hormone that regulates carbohydrate metabolism. Apart from being the primary effector in carbohydrate homeostasis, it has effects on fat metabolism. It can change the liver's ability to release fat stores. Insulin's concentration (more or less, presence or absence) has extremely widespread effects throughout the body.

Insulin is used medically in some forms of diabetes mellitus. Patients with type 1 diabetes mellitus depend on exogenous insulin (commonly injected subcutaneously) for their survival because of an absolute deficiency of the hormone; patients with type 2 diabetes mellitus have either relatively low insulin production or insulin resistance or both, and a non-trivial fraction of type 2 diabetics eventually require insulin administration when other medications become inadequate in controlling blood glucose levels.

Insulin has a molecular weight of 5.808 kDa. It has the empirical formula C₂₅₇ H₃₈₃ N₆₅ O₇₇ S₆.