Temperature seasonality, the difference between summer and winter temperature, is an important component of the climate. Knowledge about seasonality during periods of extreme warmth in Earth history may help us to better understand climate dynamics during global warming. We have established intrashell stable isotope profiles of shallow-water gastropods from the warm early middle Eocene Epoch in order to determine the seasonality of coastal sea surface temperature (SST). Oxygen isotope profiles of shells from Texas and Mississippi suggest a seasonality of 8–9 °C along the early middle Eocene U.S. Gulf Coast, with a winter temperature of 19 °C and a summer temperature of 27–28 °C. Relative to the present temperatures in the area, the Eocene summer temperature was similar, whereas the winter temperature was 7–8 °C higher. A probable reason is a smaller impact than today by cold continental air from the north because of higher continental winter temperatures. Isotope profiles of shells from southern England indicate early middle Eocene seasonality similar to present day, about 10–12 °C, whereas the mean temperature was 8–10 °C higher in Eocene time. These data confirm previous temperature estimates of the early middle Eocene Epoch in France. Using the gastropod δ18O temperatures in this and previous studies, we have established preliminary maps of winter and summer SSTs of the Eocene North Atlantic. The latitudinal temperature gradient in the summer seems to have been insignificant, whereas in the winter the gradient was prominent but significantly less steep than today. Increased oceanic heat transport has been suggested as a primary mechanism for the early Cenozoic warm climate. Strong oceanic heat transport significantly reduces the seasonality of coastal SST and surface air temperature. As the seasonality in western Europe seems to have been similar to that today, oceanic heat transport was probably not essentially higher than at present. Atmospheric properties and feedback effects were likely of more importance for maintaining the reduced latitudinal temperature gradient.