Volcanic hotspot islands are thought to be surface manifestations of mantle plumes that rise from the core–mantle bound- ary. When mantle plumes approach the surface, their mostly vertical rise must be deflected into near-horizontal flow beneath tectonic plates. This creates an opportunity to constrain their dynamics and their interactions with lithospheric plates and mid-ocean ridges. Seafloor observations have been used to propose that a focused flow in the asthenosphere transports plume heat to the nearest mid-ocean ridge, where it efficiently dissipates through formation of lithosphere. Here we present imaging results from a seismological survey of a proposed plume-to-ridge flow channel between the Réunion hotspot and the Central Indian Ridge. Rayleigh-wave tomography and shear-wave splitting confirm the presence of a channelized flow of shallow asthe- nosphere, eastward from the hotspot to the spreading ridge. At a larger scale, a deeper reservoir of hot asthenosphere fills vast tracts of the Indian Ocean basin east and north of Réunion Island. Its flows, decoupled from overlying lithospheres, are also directed towards the Central Indian Ridge but extend well beyond, tapped but not significantly depleted by the spreading ridge. Based on seismic and geochemical observations, we suggest that this hidden heat reservoir is generated and driven by the mantle plume, which buffers more heat near the surface than expected.