Although a growing body of evidence has indicated that tuna can thermoregulate and have body temperatures that are decoupled from immediate changes in ambient temperature, demonstrating the extent and time-course of body temperature changes in tuna moving through their natural environments has proved to be elusive. Here we use body temperature data telemetered from free-ranging fish to demonstrate short-latency physiological thermoregulation in bigeye tuna. We used a recently developed modeling system to determine the magnitude and time-course of the whole-body thermal conductivity changes that would result in the body temperature changes observed in fish in the wild. The results indicate rapid, 100 to 1000-fold changes in whole-body thermal conductivity that occur in response to quickly changing ambient temperatures. Coupling this physiological response with behavioral thermoregulation expands the foraging space of these animals by permitting activity in wide ranges of water temperatures and depths.