Gaboxadol hydrochloride, also known as THIP hydrochloride (systematic name: 3‐hydroxy‐4,5,6,7‐tetrahydro‐1,2‐oxazolo[5,4‐c]pyridin‐6‐ium chloride), C6H9N2O2+·Cl−, exists as two enantiotropically related polymorphs. Transformation between the polymorphs occurs in a single‐crystal‐to‐single‐crystal manner at 221 K, and the enthalpy of transformation from the high‐temperature form to the low‐temperature form is −0.7 kJ mol−1. Single‐crystal structures have been determined at 298 and 220 K. At 298 K, the structure is triclinic (space group P), with two formula units in the crystallographic asymmetric unit. At 220 K, the structure is monoclinic (space group I2/a), with one formula unit in the asymmetric unit. The structures contain identical hydrogen‐bonded layers and the transformation between the polymorphs corresponds to a shift of adjacent layers relative to each other. The transformation is shown to be reversible by differential scanning calorimetry and variable‐temperature powder X‐ray diffraction.