No great earthquakes have been historically documented at the Izu-Bonin Trench, where subduction is believed to occur largely by aseismic slip, although the details are poorly understood. We deployed an array of ocean bottom pressure gauges here for a year from May 2015. The array recorded the coseismic seafloor uplift/subsidence and tsunamis generated by the nearby Mw6.0 thrust earthquake. In association with this event, we detected two much larger aseismic slip events with rise times around 1 h. The total moment of these two aseismic events was 17 times larger than that of the mainshock. Such aseismic, yet still rapid, slip can be interpreted as one amid the transitional regime. This regime is expected to host slow slip events near its boundary with the stable sliding regime and, possibly, tsunami earthquakes and very low frequency earthquakes near the boundary with the unstable seismic slip regime. Slip in the transitional regime may be a prevalent mode of subduction in the Izu-Bonin trench, where effective normal force on the frictional plate interface is tectonically reduced.