Using a spherical model of postseismic deformation, for the first time we have computed the global contribution of large earthquakes to the relative sealevel variations in the twentieth century. We have found that great earthquakes have the overall tendency to produce a sealevel rise, and that they affect the measurements taken at those tide-gauge sites that are commonly employed to obtain global estimates of sealevel rise. Though on a global scale most of the signal is associated with thrust events, on a regional scale the effects of great transcurrent earthquakes cannot be neglected. Depending on the viscosity of the asthenosphere, the contribution of earthquakes to the long-term sealevel changes amounts to at least 0.1 mm/yr. Thus, the climate-driven long-term sealevel changes deduced by tide-gauge observations may be slightly, but not negligibly, overestimated.