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The X‐discontinuity, which appears at the depth of approximately 300 km, is an important seismic interface with positive velocity contrasts in the upper mantle. Detecting its presence and topography can be useful to understand phase transformations of relevant mantle minerals under the high‐temperature and high‐pressure circumstance of the Earth's interior. In this study, we detect the X‐discontinuity beneath the Ryukyu subduction zone using five intermediate‐depth events recorded by the dense Alaska Regional Network (AK). The X‐discontinuity is successfully revealed from the robust slant stacking of the secondary down‐going and converting SdP phases. From the depth distribution of conversion points, we find that the X‐discontinuity's depth ranges between 269 km and 313 km, with an average depth of 295 km. All the conversion points are located beneath the down‐dipping side of the Philippine Sea slab. From energy comparisons in vespagrams for observed and synthetic seismograms, the strong converted energy is more likely from a thin high‐velocity layer, and the S‐wave velocity jumps across the X‐discontinuity are up to 5% to 8% with an average of 6.0%. According to previous petrological and seismological studies, the X‐discontinuity we detected can be interpreted as the phase transformation of coesite to stishovite in eclogitic materials within the oceanic crust.