On January 30, 1607, a massive wave from the ocean surged up Bristol Channel in the United Kingdom, flooding more than 500 km2 of lowland along 570 km of coast. It killed 2000 people and is considered Britain’s worst natural disaster on land. The wave occurred on a fine day and surprised inhabitants. Contemporary descriptions of the event have many of the characteristics of accounts of recent catastrophic tsunamis. Geomorphic evidence for tsunamis in the channel can be found in the form of transported and imbricated boulders, bedrock sculpturing on coastal platforms and ramps, and, at isolated locations, wholesale erosion of the coastal landscape. Hydrodynamic calculation of the height of the tsunami and flow velocities can be derived from boulder dimensions. Tsunami wave height increased from 4 m in the outer Bristol Channel to more than 6 m within the inner Severn Estuary. Theorized flow velocities range between 11.8 and 18.1 m s1, increasing up the estuary. Under topographic enhancement, these depths and velocities may be sufficient to generate bedrock sculpturing, which is indeed observed at a few locations on rocky headlands in the channel. Interpolation of the amount of cliff retreat at Dunraven Bay indicates that an imbricated boulder train was deposited by tsunami sometime between 1590 AD and 1672 AD, a time span that encompasses the January 30, 1607, event.