Marine heatwaves are of growing concern around the world. We use a state-of-the-art ensemble of downscaled climate models to project how often heatwaves will occur in the future across northwestern Europe under a high-emission scenario. The projections show that, without emission reductions, heatwaves will occur more than half of the time in the future. We show that the seafloor is expected to experience much more frequent heatwaves than the sea surface in the future.