C-peptide is a peptide which is made when proinsulin is split into insulin and C-peptide. They split when proinsulin is released from the pancreas into the blood in response to a rise in serum glucose - one C-peptide for each insulin molecule.

C-peptide is the abbreviation for "connecting peptide", although its name was probably also inspired by the fact that insulin is also composed of an "A" chain and a "B" chain. C-peptide was discovered in 1967. It should not to be confused with c-reactive protein or Protein C. The first documented use of the C-peptide test was in 1972.