The Yellowstone hot spot has recently been shown to be a plume that extends into the transition zone. At roughly 60-120 km depth, the plume material rising beneath Yellowstone Park is sheared SW by North America Plate motion, producing a profound low velocity layer emplaced beneath the thin lithosphere. To constrain the absolute seismic velocity of the plate-sheared plume layer, fundamental mode Rayleigh wave observations have been inverted for phase velocity using the two plane wave technique. The resulting phase velocity models are inverted with Moho-converted P to S arrival times to better constrain crustal thickness and absolute S wave velocity structure to ∼120 km depth. A regionalized S wave velocity model has an extremely low velocity minimum of 3.8 ± 0.1 km/s at 80 km depth beneath the hot spot track. Nonregionalized 3-D velocity models find a velocity minimum of 3.9 km/s beneath the hot spot track. Below 120 km depth, our resolution diminishes such that the lateral spreading of the plume track is not resolved. The volume of the low velocity plume layer is small and the estimated buoyancy flux for the Yellowstone plume is <0.1 Mg/s which contrasts with the ∼9 Mg/s value for Hawaii. In addition, a notable region of thick crust and high lower crustal velocities is found around Billings, Montana, consistent with previous refraction and receiver function studies that interpret this as evidence for a massive Precambrian underplating event.