Abstract Microplastics have become widely dispersed throughout the marine environment in and around Japan since the 1960s, which corresponds to the onset of mass plastic production and use in this country. Our study documents a possible abrupt microplastic depositional event in continental shelf and deep-sea environments triggered by a tsunami. The sediment layers contaminated by microplastics correspond with sedimentary horizons where 137 Cs signals were measured, indicating deposition after 1960s nuclear tests. The microplastics were observed in the 2011 Tohoku-Oki tsunami deposits. Tsunamis can thus contribute to the wide dispersal of microplastics from coastal to deep-sea areas, and these anthropogenic particles can be used to date very recent turbidite deposits.