The X-ray crystal structure of bis((S)-5,5-dimethylthiazolidine-4-carboxylic acid) protium chloride hydrate, I, has been determined. Crystals are orthorhombic, P212121, with cell dimensions a = 6.463(2), b = 11.832(3), c = 23.966(6) Å, and Z = 4. The structure was solved by standard methods and refined to R = 0.073, Rw = 0.064 for 2969 independent reflections. Compound I is composed of two zwitterion molecules bridged through a proton which causes a very short hydrogen bond, O … O′ = 2.450 Å. The proton is located roughly halfway between the two carboxylate oxygen atoms, but is not on a special crystallographic position. Characteristic features of the infrared and Raman spectra, mass spectra, and 1H and 13C nmr spectra are discussed.