Abstract The generation mechanism of the 1741 Oshima-Oshima tsunami, which is considered to be the most destructive tsunami that has ever originated in the Japan Sea, has been the subject of much debate. The tsunami caused about 2,000 casualties along the Hokkaido and northern Honsu coasts and inflicted damage as far as the Korean Peninsula. The tsunami source is located between recent tsunamigenic earthquakes, but there is no historical record of an earthquake in 1741. In contrast, the records indicate volcanic activity of Oshima-Oshima, including a large-scale sector collapse, although the volume change associated with the subaerial landslide is too small to explain the observed tsunami heights. Recent marine surveys indicate that the landslide extended to the ocean bottom with a volume change of about 2.5 km3, nearly an order of magnitude larger than the subaerial slide. On the basis of mapped bathymetry, the generation of the tsunami is calculated using a simple kinematic landslide model. The tsunami propagation is computed in two different grids—a 6″ grid around the source and a 1′ grid for the entire Japan Sea. A parameter search of the model shows that the observed tsunami heights are best explained by a horizontal slide velocity of 40 m/s and a rise time of 2 min. The large vertical drop of the landslide and the coupling of the submarine landslide and tsunami are the main factors responsible for the large tsunami.