We find, by means of a deep inelastic neutron scattering experiment, a significant excess of proton mean kinetic energy hEki in supercooled water, compared with that measured in stable liquid and solid phases. The measured values of hEki at moderate degrees of supercooling do not fit the predicted linear increase with temperature observed for the water stable phases. This anomalous behavior is confirmed by the shape of the measured momentum distribution, thus supporting a likely occurrence of ground-state quantum delocalization of a proton between the O atoms of two neighboring molecules. These results strongly suggest a transition from a single-well to a double-well potential felt by the delocalized proton, with a reduced first neighbor O-O distance, in the supercooled state, as compared to ambient condition. The anomalous properties of water have attracted great attention from the scientific community for a long time and are still a topic of intense research. These remarkable properties, most pronounced in the supercooled metastable state [1], can be ascribed to water’s unique structure, consisting of a random and fluctuating three-dimensional network of hydrogen bonds [2]. Despite the combined effort of powerful molecular dynamics simulation and novel experimental methods, a complete description of water is still missing. Therefore, this apparently simple liquid still represents a challenging puzzle [3‐5]. These issues have received great interest in recent years thanks to possibilities opened by novel experimental techniques, detailed theoretical predictions, and computer simulation methods. In particular, the development of pulsed neutron sources has allowed the remarkable advance of the deep inelastic neutron scattering (DINS) technique [6,7]. DINS is based on measurements at high energy, @!, and high momentum, @q, transfers, thus providing a probe of both the short-time (t � 10 � 15 s) dynamics and local (r � 1 � A) environment of the atoms in materials [6,7]. The high energy and momentum transfers achieved allow us to describe the scattering process within the framework of the impulse approximation (IA) [7‐9]. The scattering cross section is then expressed in terms of the single particle momentum