<p>The Atlantic Niño is the dominant mode of interannual climate variability in the equatorial Atlantic, exerting major relevance in the climate system, with local and remote socioeconomic impacts. It is projected to weaken under enhanced greenhouse warming. A variety of factors were suggested to modulate the Atlantic Niño, such as the slowdown of the Atlantic Meridional Overturning Circulation (AMOC), whose effect, however, has not yet been fully understood. Here, we perform fully coupled model simulations by maintaining the AMOC strength under greenhouse warming to isolate and quantify the AMOC effect on the Atlantic Niño during the twenty-first century. We find that the Atlantic Niño’s response to the AMOC slowdown varies seasonally, throughout its development and peak stages, and is mostly tenuous, statistically non-significant, as a result of two opposing mechanisms. A major southward shift of the Intertropical Convergence Zone strengthens ocean-atmosphere coupling, which prevails in July and enhances Atlantic Niño variability. On the other hand, the relative warming of the equatorial Atlantic significantly enhances heat flux damping, which dominates in May, June, and August, suppressing the variability of the Atlantic Niño.</p>