Sessile organisms with thermally sensitive developmental trajectories are at high risk from climate change. For example, oviparous reptiles with temperature-dependent sex determination (TSD) may experience strong (potentially disastrous) shifts in offspring sex ratio if reproducing females are unable to predict incubation conditions at the time of oviposition. How then have TSD reptile taxa persisted over previous periods of extreme climatic conditions? An ability of embryos to move within the egg to select optimal thermal regimes could buffer ambient extremes, but the feasibility of behavioral thermoregulation by embryos has come under strong challenge. To test this idea, we measured thermal gradients within eggs in semi-natural nests of a freshwater turtle species with TSD, manipulated embryonic thermoregulatory ability, and modeled the effects of embryonic thermoregulation on offspring sex ratios. Behavioral thermoregulation by embryos accelerated development and influenced offspring sex ratio, expanding the range of ambient conditions under which nests produce equal numbers of male and female offspring. Model projections suggest that sex ratio shifts induced by global warming will be buffered by the ability of embryos to influence their sexual destiny via behavioral thermoregulation.