Slab subduction imaged by seismic tomography implies extensive mantle convection and circulation in the Earth’s interior. Widespread subduction of the western Pacific plate beneath the eastern Eurasian plate has left peculiar geochemical and geophysical signatures in the upper mantle of East Asia, which allow to investigate the extent of thermochemical interactions between the subducting plates and the surrounding mantle. Here we provide refined images of the stagnant Pacific slab and the mantle transition zone beneath northeast Asia. We applied teleseismic traveltime tomography and 3-D body-wave waveform modeling by using high-quality datasets from dense seismic arrays including in the Korean Peninsula and southwestern Japan, and found a clear signature of fragmentation of the stagnant Pacific slab. We imaged a localized low-velocity anomaly with high Vp/Vs ratio at 410-km depth above the slab gap, which was interpreted to be partial melting. The segmented Pacific slab reflects a rapid change in boundaries between the Pacific plate, Eurasian plate, and Philippine Sea plate during the Early to mid-Miocene. The slab fragmentation likely has facilitated localized mantle convection in the upper mantle and the mantle transition zone around the slab segments. Our images provide evidence of dynamic interplay between the evolving plate configurations, subducting slabs, and the surrounding mantle.