The Atlantic Niño is one of the most important tropical patterns of interannual climate variability, with major regional and global impacts. How global warming will influence the Atlantic Niño has been hardly explored, because of large climate model errors. We show for the first time that the state-of-the-art climate models robustly predict that equatorial Atlantic Niño variability will weaken in response to global warming. This is primarily because subsurface and surface temperature variations decouple as the upper equatorial Atlantic Ocean warms. The weakening is predicted by most (>80%) models following the highest emission scenarios in the Coupled Model Intercomparison Project Phases 5 and 6 considered here. These indicate a reduction in variability by the end of the century of 12-17%, and as much as 25% when accounting for model errors. Weaker Atlantic Niño variability will have major consequences for global climate and the skill of seasonal predictions.