Iceland, one of the most thoroughly investigated hotspots(1-3), is generally accepted to be the manifestation of an upwelling mantle plume(4). Yet whether the plume originates from the lower mantle or from a convective instability at a thermal boundary layer between the upper and lower mantle near 660 km depth(5.6) remains unconstrained. Tomographic inversions of body-wave delay times show that low seismic velocities extend to at least 400 km depth beneath central Iceland(7,8), but cannot resolve structure at greater depth. Here we report lateral variations in the depths of compressional-to-shear wave conversions at the two seismic discontinuities marking the top and bottom of the mantle transition zone beneath Iceland. We find that the transition zone is 20 km thinner than in the average Earth(9) beneath central and southern Iceland, but is of normal thickness beneath surrounding areas, a result indicative of a hot and narrow plume originating from the lower mantle.