Small-scale convection beneath the oceanic plates has been invoked to explain off-axis non-plume volcanism, departure from simple seafloor depth-age relationships, and intraplate gravity lineations. We deployed thirty broadband OBS stations on ~40 Ma seafloor in the equatorial Pacific, in a region notable for gravity anomalies measured by satellite altimetry elongated in the direction of plate motion. P-wave teleseismic tomography reveals alternating upper mantle velocity anomalies on the order of ±2%, oriented parallel to the gravity lineations. These features, which correspond to ~300-500 ˚K lateral temperature contrast, and possible hydrous or carbonatitic partial melt, are strongest between 150 and 260 km depth, indicating rapid vertical motions through a low-viscosity asthenospheric channel. Coherence and admittance analysis using new multibeam bathymetry soundings substantiates the presence of asthenospheric density variation, and forward modelling predicts gravity anomalies that qualitatively match observed lineations. This study provides observational support for small-scale convective rolls beneath the oceanic plates.