Numerous geoscientific investigations have been conducted on the southeastern Tibetan Plateau and adjacent areas for understanding crustal and mantle deformation associated with the indentation of the Indian Plate into Eurasia. A number of key issues, such as the causes of a sudden change of fast polarization orientations from N-S to almost E-W at approximately 26°N revealed by shear wave splitting (SWS) studies, and the geodynamic implications of the transition still remain enigmatic, partially due to the lack of sufficient SWS measurements on the Indochina Peninsula. Here we employ the SWS technique to systematically illuminate upper mantle anisotropy beneath the Indochina Peninsula with an unprecedented data coverage. The resulting 409 SWS measurements from 29 stations show that upper mantle anisotropy beneath the vast majority of the study area is characterized by dominantly E-W fast orientations which are nearly orthogonal to the strike of most of the major tectonic features in the study area, ruling out significant lithospheric contributions to the observed anisotropy. This observation, when combined with results from seismic tomography, numerical modeling, surface movement, and focal mechanism investigations, suggests that the observed azimuthal anisotropy is mostly the consequence of absolute plate motion or the westward rollback of the oceanic Indian slab. The flow system induced by the rollback or absolute plate motion may experience regional alteration from mantle upwelling along the eastern edge of the slab and through a previously detected slab window, leading to local variations in the observed splitting parameters.