Fermi resonance was observed between the OH stretch and the overtone of the OH bending modes of HDO molecules contaminated in phase VII of D2O ice over the pressure range from 17 to 30 gigapascals. An anharmonic coupling constant, which is related to the potential energy surface on which hydrogen-bonded protons oscillate, was found to range around 50 wave numbers through the resonant pressure range. Its experimentally obtained magnitude and pressure-insensitive behavior will be useful for theoretical studies of the potential energy surface and hence of the nature of hydrogen bonding in ice.