Subduction zones feature a large elevation difference between the deep oceanic trench and the high mountains on land, which causes stress in the upper plate. We investigate how the topographic stress affects the aftershock seismicity of large subduction earthquakes using analytical and numerical stress models. We show that topographic stress has a large influence on aftershock seismicity and promoted most of the seismicity after the Tohoku-Oki (Japan) and Maule (Chile) earthquakes.