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Differential arrival times of P-to-S conversions at 410 and 660 km depth, measured from radial receiver functions, are used to determine the thickness of the mantle transition zone beneath the Galápagos hotspot. For an area of approximately 700 km² surrounding the Galápagos archipelago, P660s−P410s times are not significantly different from those for the Pacific Basin. In contrast, a subset of the Galápagos data yields differential times indicating thinning of the mantle transition zone by 18±8 km within an area approximately 100 km in radius centered about 40 km southwest of the center of the island of Fernandina. This anomaly is consistent with an excess temperature of 130±60 K within this volume of the transition zone, comparable to that inferred beneath the Iceland and Society hotspots. The most straightforward interpretation of this anomaly is that a mantle plume upwelling from depths greater than 410 km …