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Modern open-ocean sea-surface temperatures rarely exceed ∼28–29 °C, and the same has been thought to represent a rough maximum for past tropical climates. However, new isotopic estimates from the uppermost Cenomanian in the tropical western North Atlantic suggest that mixed-layer temperatures reached ∼33–34 °C (± 2 °C) during the middle Cretaceous hothouse. Uppermost Cenomanian tropical sea-surface temperatures may have been as much as 4–7 °C warmer than the highest modern mean annual temperatures. Such extreme conditions suggest that warm tropical oceans could have driven substantially intensified atmospheric heat transport near the Cenomanian-Turonian boundary. The tropical “thermostat” was set higher than today, challenging the hypothesis of tropical climate stability.