In situ X‐ray diffraction study of post‐spinel transformation in hydrous peridotite (2 wt.% H2O) indicates that the phase boundary is shifted to higher pressures by 0.6 GPa relative to anhydrous peridotite at 1473 K, whereas, it shows no obvious shift at high temperature around 1873 K. A linear equation for the boundary is P (GPa) = −0.002 T (K) + 26.3, which is applicable for temperatures below 1800 K. The present data shows that a significant part of the depressions seen in the 660‐km seismic discontinuity may be affected by existence of water. It is also well resolved that the olivine‐wadsleyite phase transformation corresponding to the 410‐km seismic discontinuity is shifted to lower pressures by 1–2 GPa by the addition of water. Thus, the topography of seismic discontinuities in the transition zone associated with slabs can be attributed not only to cold subduction but also wet subduction.