Oceanic hotspots are generally accepted to be the manifestations of plumes of hot, upwelling mantle material(1,2), but the nature of such flows remains enigmatic. Iceland, for example, is one of the most thoroughly investigated hotspots, yet previous seismological(3-5) and geodynamic(6-12) studies have been unable to constrain the width or temperature of the plume. Here we report the results of a regional broadband seismic experiment undertaken to determine the three-dimensional velocity structure of the upper mantle beneath Iceland using relative travel times of body waves from teleseismic earthquakes. Inversion solutions of the data show a cylindrical zone of low P- and S-wave velocities that extends from 100 km to at least 400 km depth beneath central Iceland. The radius of the low-velocity anomaly is about 150 km, and its magnitude is approximately 2% for P waves and 4% for S waves, indicating that Iceland is underlain by a hot, narrow plume of upwelling mantle.