Mars Odyssey Gamma-Ray Spectrometer (GRS) neutron spectrometer data are analyzed to determine the concentration and boundary of buried water ice near the south pole. The measurements are consistent with a circumpolar layer of average ice concentration from 35 to 100% by weight and superposed dust with thickness density product 30 to 40 g/cm2. The region of buried water ice extends from near the south pole to latitudes 48S to 58S. The equatorward boundary of the ice-rich region compares favorably with the boundary of an ice-rich dust mantle inferred from Mars Orbiter Laser Altimeter data and the locations of dissected terrain inferred from Mars Orbiter Camera images. The ice-rich mantle is identified as the source of enhanced hydrogen sensed by the neutron spectrometer.