Bergmann’s Rule is the propensity for species-mean body size to decrease with increasing temperature. Temperature-dependent oxygen limitation has been hypothesized to help drive temperature–size relationships among ectotherms, including Bergmann’s Rule, where organisms reduce body size under warm oxygen-limited conditions, thereby maintaining aerobic scope. Temperature-dependent oxygen limitation should be most pronounced among aquatic ectotherms that cannot breathe aerially, as oxygen solubility in water decreases with increasing temperature. We use phylogenetically-explicit analyses to show that species-mean adult size of aquatic salamanders with branchial or cutaneous oxygen uptake becomes small in warm environments and large in cool environments, whereas body size of aquatic species with lungs (i.e., that respire aerially), as well as size of semi aquatic and terrestrial species do not decrease with temperature. We argue that oxygen limitation drives the evolution of small size in warm aquatic environments for species with aquatic respiration. More broadly, the stronger decline in size with temperature observed in aquatic vs terrestrial salamander species mirrors the relatively strong plastic declines in size observed previously among aquatic vs terrestrial invertebrates, suggesting that temperature-dependent oxygen availability can help drive patterns of plasticity, micro- and macroevolution.