The hot thermal boundary layer produced by heat transport from the Earth＇s core to the base of the mantle is thought to contain strong horizontal shear flows and to nucleate instabilities in which hot material rises into the convecting mantle as thermal plumes(1-3). A recent study(4,5) proposes that the Hawaiian plume is deflected by mantle convection and, in the lowermost mantle, is located to the southeast of its surface manifestation. Here we present seismic data that densely sample, with core-reflected shear waves, a region beneath the central Pacific Ocean which includes the predicted location of the deflected root of the Hawaiian hotspot. Our mapping of the structure in this region of the lowermost mantle reveals strong lateral gradients in shear-wave velocity and anisotropic shear-wave polarization direction over distances of only several hundred kilometres. We interpret these gradients as being indicative of small-scale dynamical structure in the thermal boundary layer, where vertical flow into the Hawaiian plume at its root is accompanied by horizontal flow towards the plume.