Tsunamis triggered by powerful earthquakes cause extensive damage and loss of life within many regions of the World Ocean. Although coastal inundation from major tsunamis is becoming increasingly well understood, we know little about the broader aspects of such events on distal marine systems. Here we use time series from moored oceanic sensors to show that the Tohoku tsunami generated by the magnitude 9.0 earthquake off eastern Japan in March 2011 caused days of surge-like currents and turbulent mixing in the inner basin of an anoxic Canadian fjord located over 7000 km from the seafloor rupture zone. Mixing, combined with the inflow of more oxygen-rich water from the adjoining outer basin, led to abrupt changes in the hydrodynamics, bottom sedimentation, and zooplankton behavior in the basin. These findings help define mechanisms by which major transoceanic tsunamis can significantly alter coastal marine environments located far from the source area.