The interpretation of diatom biostratigraphy is a little used technique in the study of onshore tsunami sedimentation. Evidence is presented from examples from three tsunamis in which sediments were either observed to have been deposited; documentary evidence attests to the presence of sediment sheets, or finally, in the case of palaeo tsunamis, morphological and stratigraphical evidence permits the interpretation of tsunami inundation and the subsequent deposition of sediment. Examples from Scotland and Canada, and preliminary results from Indonesia, illustrate distinctive diatom assemblages that are associated with tsunami sedimentation. These assemblages contrast with the sediments immediately underlying and overlying the tsunami sediment. Data is presented showing an often chaotic assemblage, attributable to the tsunami waves crossing many distinctive diatom habitats, from fully marine planktonic and benthic species, through the varied intertidal zone and finally over the onshore terrestrial environments. The deposits attributable to tsunami inundation, particularly in Scotland, disclose a high proportion (often in excess of 65%) of broken diatom valves, and an over-representation of centric species due to their greater resistance to erosion. Together the information derived from known historical tsunamis and from palaeo tsunamis with good stratigraphical and dating control provide a good basis for the study of the diatom characteristics of modern tsunamis.