Large-scale variation in mammalian body size has often been found to be related to environmental conditions. A general finding among large herbivores is that body size increases with decreasing temperature (Bergmann's rule), because animals with larger body size have better heat conservation or fasting tolerance, or because higher quality forage occurs in colder environments. Using a data set on the skeletal morphology of Norwegian red deer (Artiodactyla, Cervidae: Cervus elaphus) spanning the last approximately 7,100 years, we document an inverse relationship between climatic conditions and body size. The size of Norwegian red deer, as estimated from both teeth and weight-bearing bones, was significantly larger during the warmer and wetter middle Holocene than it is today. However, the reduction in body size does not seem to be related to changing climatic conditions. Rather, this decrease happened during a period of large-scale human-mediated habitat fragmentation, increased populations of domestic herbivores, and heavy hunting pressure that reduced population density. The size of teeth was reduced as much as, or even more than, the size of weight-bearing bones, which indicates an evolutionary response rather than phenotypic plasticity to changing forage and environmental conditions. Decreased body size may be a general response in wild ungulates to a more human-dominated landscape, resulting from reduced access to optimal habitats and high adult hunting mortality.