Synchronous records of secular change in temperature and atmospheric CO2 concentration (pCO2) spanning 107 yr time scales from warm, ice-free periods are rare in the geologic record. Here we compile records of pCO2 and sea-surface temperature (SST) change that closely correspond for a 12 m.y. interval of the Late Triassic hothouse, through a review of the chronostratigraphy between the terrestrial Newark Basin (northeastern USA) and Tethyan marine sections. We use this interval to put empirical constraints on Earth system sensitivity (ESS), which is the equilibrium change in Earth’s surface temperature for a doubling of atmospheric pCO2 that accounts for both fast and slow feedbacks in the climate system. We find that ESS is 3.82 °C for a doubling and 3.88 °C for a halving of pCO2, in close agreement with empirical and model calibrations from younger ice-free periods. These findings provide further evidence that ESS may be enhanced during a hothouse state, and that atmospheric pCO2 is a dominant control on Earth’s climate even in an ice-free world.