The March 2011 Tohoku-oki earthquake was only the second giant (moment magnitude M-w >= 9.0) earthquake to occur in the last 50 years and is the most recent to be recorded using modern geophysical techniques. Available data place high-resolution constraints on the kinematics of earthquake rupture(1), which have challenged prior knowledge about how much a fault can slip in a single earthquake and the seismic potential of a partially coupled megathrust interface(2). But it is not clear what physical or structural characteristics controlled either the rupture extent or the amplitude of slip in this earthquake. Here we use residual topography and gravity anomalies to constrain the geological structure of the overthrusting (upper) plate offshore northeast Japan. These data reveal an abrupt southwest-northeast-striking boundary in upperplate structure, across which gravity modelling indicates a south-tonorth increase in the density of rocks overlying the megathrust of 150-200 kilograms per cubic metre. We suggest that this boundary represents the offshore continuation of the Median Tectonic Line, which onshore juxtaposes geological terranes composed of granite batholiths (in the north) and accretionary complexes (in the south) 3. The megathrust north of the Median Tectonic Line is interseismically locked(2), has a history of large earthquakes (18 with M-w >= 7 since 1896) and produced peak slip exceeding 40 metres in the Tohoku-oki earthquake(1). In contrast, the megathrust south of this boundary has higher rates of interseismic creep(2), has not generated an earthquake with M-J > 7 (local magnitude estimated by the Japan Meteorological Agency) since 1923, and experienced relatively minor (if any) co-seismic slip in 2011(1). We propose that the structure and frictional properties of the overthrusting plate control megathrust coupling and seismogenic behaviour in northeast Japan.