This study shows diatom fossil assemblage of the 2011 Tohoku-oki earthquake tsunami deposits and underlying soil in Arahama area, Sendai Plain, Miyagi Prefecture, northeastern Japan. All the major component species with >2% in relative abundance, 27 species belonging to 22 genera, are freshwater species. Surirella angustata and Nitzschia nana are common species in paddy field soil of the inter-ridge marshes, and their abundance in tsunami deposits increases toward inland. This suggests that diatom assemblage in tsunami current became similar as that of soil landward during the tsunami run up the plain. On the other hand, Hantzschia amphioxys and Luticola mutica are abundant in soil on beach ridges. In the tsunami deposits, they showed higher abundance at the sites just behind the beach ridges, and gradually decreased landward. This suggests that they probably transported from beach ridges toward inland due to erosion by the tsunami current. Transport distance of terrestrial diatom fossils is estimated to be a few hundred meters.