To assess long-term tsunami inundation history, we studied a wetland on the Pacific coast of Aomori, Japan, at the northern end of the Japan Trench. We found five sand sheets interbedded in freshwater mud and peat, three of which contained brackish diatoms indicating deposition by marine inundation. We identified the youngest sand sheet, deposited AD 1480–1770, as a tsunami deposit based on its lateral extent (>1 km) from the shoreline at the time of its deposition. Although this area has been struck by many tsunamis generated by earthquakes along the Japan Trench as well as along the Kuril and Peru–Chile trenches, no tsunami recorded at least in the past 120 years has inundated the coast as far inland as this tsunami deposit is distributed. We therefore infer that it was deposited by an unusually large tsunami. Historical and geological evidence for tsunamis in north-east Japan suggests that possible sources are the AD 1611 Keicho earthquake along the Japan Trench, a 17th-century earthquake along the Kuril Trench, or an unknown large earthquake.