Quantum tunneling in chemistry is often attributed to the processes at low or near room temperatures when the rate of thermal reactions becomes far less than the rate of quantum tunneling. However, in some rapid processes, quantum tunneling can be observed even at high temperatures. Herein, we report the experimental evidence for anomalous H/D kinetic isotope effect (KIE) during sonochemical dissociation of water molecule driven by 20 kHz power ultrasound measured in H 2 O/D 2 O mixtures saturated with Ar or Xe. Hydrogen released during ultrasonic treatment is enriched by light isotope. The observed H/D KIE (α=2.15-1.50) is much larger than what is calculated assuming a classical KIE for T g =5000 K (α=1.15) obtained from the sonoluminescence spectra in H 2 O and D 2 O. Furthermore, the α values sharply decrease with increasing of H 2 O content in H 2 O/D 2 O mixtures reaching a steady-state value close to α=1.50, which also cannot be explained by O-H/O-D zero-point energy difference. We suggest that these results can be understood in terms of quantum electron tunneling occurring in nonequilibrium picosecond plasma produced at the last stage of cavitation bubble collapse. Thermal homolytic splitting of water molecule is inhibited by extremely short lifetime of such plasma. On the contrary, immensely short traversal time for electron tunneling in water allows H 2 O dissociation by quantum tunneling mechanism.