A global high-resolution S wave velocity model RG5.5 is obtained for the upper 500 km of Earth's mantle using a 5¿¿5¿ equal-area block parameterization. The data set consists of some 18,000 seismograms associated with 971 events with magnitudes larger than 5.5. Fundamental modes (Love and Rayleigh waves) are used with periods from 75 to 250 s. The horizontal resolution length is around 1000 km, and the vertical resolution varies with depth from 60 to 250 km. Model RG5.5 has many features consistent with previous three-dimensional global and local seismic studies, but many new features are found. The S waves under mid-ocean ridges have broad slow velocity and have very slow velocity in the upper 100 km below the surface. The minimum velocity is at depths near 50 km or shallower. The lateral extent of the slow velocity region across ridges increases with spreading rate. The S wave velocities under rides are strongly correlated with spreading rates at shallow depth, but the correlation decreases with depth and almost disappears at 100 km. The slow velocities shift off the current spreading positions below 100 km depth under the Mid-Atlantic Ridge and may record past positions of the ridge and/or be related to hotspots near the ridge. Some major hotspots are associated with slow-velocity anomalies with magnitudes of about 1--2% slower than the global average and with lateral dimension larger than 1000 km at depths between 100 and 200 km. Differences in the upwelling structure between ridges and hotspots are indicated. The S wave velocity structures may suggest an active mechanism for the East African Rift Valley and a plate extension mechanism for the Baikal Rift Valley. ¿ American Geophysical Union 1993