The Cretaceous thermal maximum was a major turning point in the history of Earth's climate. This interval of peak warmth in the Turonian has been attributed to very high atmospheric pCO2 resulting from rapid outgassing rates, although crustal cycling rates peaked in the Aptian–Albian interval. On the basis of coupled ocean-atmosphere model simulations of the middle Cretaceous, we hypothesize that the formation of an Atlantic gateway could have contributed to the Cretaceous thermal maximum. Differences between prerifting and postrifting climate experiments demonstrate substantial regional oceanographic changes in the North and South Atlantic Basins that are consistent with oxygen isotopic evidence used to infer a Cretaceous thermal maximum. The model results help reconcile the paleoclimate record inferred from foraminiferal δ18O with our understanding of climate dynamics and Cretaceous tectonism.