Gigantothermy is a phenomenon with significance in biology and paleontology, whereby large, bulky ectothermic (cold-blooded) animals are more easily able to maintain a constant, relatively high body temperature than smaller animals by virtue of their greater volume to surface area ratio. A bigger animal has proportionately less of its body close to the outside environment than a smaller animal of otherwise similar shape, and so it gains heat from, or loses heat to, the environment much more slowly.

The phenomenon is important in the biology of ectothermic megafauna, such as large turtles (particularly the Leatherback Sea Turtle), dinosaurs, and aquatic reptiles like ichthyosaurs and mosasaurs. It is also present in certain large fish, most notably the great white shark. Gigantotherms, though almost always ectothermic, generally have a body temperature and metabolic rate similar to that of endotherms.