Tsunami damage to archaeological sites in Japan has been recognized since the 1980s, but the Great Tōhoku-oki Earthquake and tsunami of 11 March 2011 stimulated geologists and archaeologists to find evidence of previous tsunami in Japan, investigate the responses of earlier inhabitants to tsunami, and assess the probability of future occurrences. Excavated sites on the Sendai Plain, partially inundated in this recent tsunami, have been crucial in this endeavor, with recovered data at times contradicting historical sources. Great progress has been made in the science of identifying tsunami deposits and understanding their nature and distribution, aiding in their recognition at archaeological sites. This article provides an introduction to the nature of tsunami waves and their causes, resources available for studying past tsunami worldwide, and difficulties in identifying tsunami sediments. Seventeen case studies of sites where tsunami deposits have been investigated throughout the Japanese and Ryukyu archipelagos are presented. Tsunami can be included within my conception of ‘tectonic archaeology,’ archaeology that must methodologically deal with the influence of plate tectonics on the islands. Earthquakes, volcanoes, and most tsunami relate to the subduction zone setting of Japan; thus, to fully understand the site remains of previous tectonically derived disasters demands knowledge of plate tectonics, seismology, volcanology, sedimentology, and wave physics among others. Integrating these spheres of knowledge into archaeological research opens new avenues of interpretation, including understanding why many Middle Yayoi settlements on the Sendai Plain were abandoned, not to be reoccupied for 400 years.