Marine heatwaves (MHWs)-periods of extremely high ocean temperatures in specific regions-have occurred in all of Earth's ocean basins over the past two decades, with severe negative impacts on marine organisms and ecosystems. However, for most individual MHWs, it is unclear to what extent they have been altered by human-induced climate change. We show that the occurrence probabilities of the duration, intensity, and cumulative intensity of most documented, large, and impactful MHWs have increased more than 20-fold as a result of anthropogenic climate change. MHWs that occurred only once every hundreds to thousands of years in the preindustrial climate are projected to become decadal to centennial events under 1.5 degrees C warming conditions and annual to decadal events under 3 degrees C warming conditions. Thus, ambitious climate targets are indispensable to reduce the risks of substantial MHW impacts.