P-wave velocity images are determined under the Baikal rift zone in Siberia by using teleseismic tomography. Our results show prominent low-velocity anomalies in the upper mantle under the Baikal rift zone and high-velocity anomalies in the lithosphere under the Siberian craton. The low-velocity anomalies are interpreted as a mantle upwelling (plume) which has played an important role in the initiation and evolution of the Baikal rift zone. The rift formation may be also controlled by other factors such as older (prerift) linear lithosphere structures favorably positioned relative to the upwelling and favorable orientation of the far-field forces caused by the India–Asia collision.