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The assessment of seismic hazards along subduction zone coastlines provides important information regarding the frequency and magnitude of earthquakes and tsunamis that can be expected in the future. Unlike many subduction zone coastlines that involve one tectonic plate subducting under another, seismic hazard assessments for the Kanto region of Japan are complicated by the presence of a nearby triple junction; where one continental (CON) and two oceanic plates (PHS, PAC) collide. The CON/PHS (Sagami Trough) and CON/PAC (Japan Trench) boundaries are recognized earthquake sources. However, historical and geological evidence of a large PHS/PAC (Izu-Bonin Trench) earthquake has been lacking and decades worth of instrumental data point to low seismicity along this boundary. Here we show that two unusually large tsunamis are evidenced by sandy deposits preserved along 50 km of coastline in the Kanto region. The oldest of them, deposited about 1,000 years ago, contains evidence consistent with tsunami deposits reported elsewhere (e.g., marine foraminifera, rip-up clasts, pebbles, erosional base) and represents a previously unknown prehistoric earthquake. In computer simulations, this earthquake deposited sand that extended too far inland to represent any known historical earthquake originating from the CON/PHS and CON/PAC boundaries alone. Rather, the greater inland inundation points to significantly greater displacement on the CON/PHS and CON/PAC boundaries, which may be unrealistic, or much smaller displacement along the previously unconsidered PHS/PAC megathrust. This plate-boundary fault adds another source for earthquakes in Tokyo and tsunamis in the Pacific Ocean.