The amino acid serine has long been known to form a protonated “magic-number” cluster containing eight monomer units that shows an unusually high abundance in mass spectra and has a remarkable homochiral preference. Despite many experimental and theoretical studies, there is no consensus on a Ser8H+ structure that is in agreement with all experimental observations. Here, we present the structure of Ser8H+ determined by a combination of infrared spectroscopy and ab initio molecular dynamics simulations. The three-dimensional structure that we determine is ∼25 kcal mol–1 more stable than the previous most stable published structure and explains both the homochiral preference and the experimentally observed facile replacement of two serine units.