While the northward indentation of the Indian into Eurasian plates has been intensively investigated, its oblique subduction beneath the Indochina Peninsula (ICP) and the role it played on mantle structure and dynamics remain enigmatic. In this first regional-scale receiver function study of the mantle transition zone (MTZ) discontinuities beneath the ICP and its surrounding areas, we stack ~12,000 receiver functions recorded at 33 stations using a non-plane wave common-conversion-point stacking technique. Systematic spatial variations of MTZ thickness with departures between -21 and +24 km from the globally averaged value are revealed, providing independent evidence for the presence of slab segments in the MTZ beneath the central and a slab window beneath the western ICP. The results also support the existence of broad mantle upwelling adjacent to the eastern edge of the slab segments, which might be responsible for the widespread Cenozoic volcanisms and pervasively observed upper mantle low velocities in the area.