Atmospheric CO2 possibly doubled during Oceanic Anoxic Event (OAE) 1a, likely in response to submarine volcanic outgassing. Despite being important for our understanding of the consequences of carbon cycle perturbations, the response of the climate system to this increase in greenhouse forcing is poorly constrained. Here we provide a new sea-surface temperature (SST) record from the mid-latitude proto–North Atlantic based on the organic geochemical TEX86 paleothermometer. Using different calibrations, including the newly developed Bayesian Spatially-varying Regression (BAYSPAR) deep-time analogue approach, we demonstrate that SSTs increased by ∼2–4 °C during OAE 1a and decreased by ∼4–6 °C at its end, both simultaneous with changes in δ13Corg, which we argue reflects changes in pCO2. We demonstrate that a clear latitudinal SST gradient prevailed during OAE 1a, contrary to the generally accepted view that a nearly flat SST gradient existed during OAE 1a and the Early Cretaceous. These results are more consistent with climate model simulations of the Cretaceous that have failed to produce flat SST gradients.