The Motuo fault (MTF) strikes along the Yarlung Zangbo suture zone on the eastern boundary of the Namche Barwa syntaxis. The movement pattern and Quaternary activity of the MTF remain unclear, which hampers efforts to undertake meaningful seismic hazard assessments near the southeastern part of the Tibetan plateau and to understand the tectonic evolution of the Namche Barwa syntaxis. In this study, the MTF is shown to feature left‐lateral strike‐slip movements with offset gullies and mountain ridges and appears to have ruptured during the late Pleistocene to Holocene, as evidenced from geological, paleoseismic, and radiocarbon dating investigations. Specifically, at least three surface‐rupturing paleoseismic events are revealed; two events occurred after 2606 B.P. and after 18.2 ka. Combining this information with previous Global Positioning System results in southeastern Tibet, we suggest that, as a boundary fault, the MTF regulates the movements of the Namche Barwa and Chayu blocks. The velocity difference between the two blocks advancing to the north is the main mechanism of left‐lateral strike‐slip motion along the MTF. The accumulation and release of shear stress between the two blocks have led to strong activity along the MTF, since the late Quaternary.