Bergmann’s rule suggests that animals in colder climates tend to be larger, but does this pattern hold across all vertebrates? While traditionally thought to apply more strongly to endotherms, evidence remains inconsistent across species and ecosystems. We analysed body size trends across latitude in major terrestrial vertebrate—amphibians, reptiles, birds, and mammals—to assess the rule’s validity. Birds and mammals followed Bergmann’s rule, while amphibians and reptiles showed no consistent pattern. In addition, we observed a stronger support for Bergmann’s rule in endotherms compared to ectotherms at the order/family level, but a slightly higher support at the species level. Contrary to expectation, however, Bergmann’s rule is not stronger at the population level than at the species level. These results highlight that Bergmann’s rule is context-dependent, shaped by factors like body temperature regulation and evolutionary history, and underscore the need to consider these differences when predicting species’ vulnerability to climate change.