For tsunami seawalls installed along coastline, measures for tsunami disaster assuming overflow are required from the viewpoint of failsafe. Therefore, the tsunami force verified under overflow condition is necessary for the optimal design. Some estimation models of the tsunami forces acting on land structures have already been proposed but they were of a non-overflow condition. In this study, physical model tests for the tsunami force acting on vertical seawalls were conducted to examine the tsunami force characteristics under overflow conditions, and an estimation method of the overflowing tsunami force was proposed and verified.