Body shape plays a fundamental role in organismal function and it is expected that animals will evolve body proportions that best exploit their ecological niche. Terrestrial tetrapods have evolved a disparate array of body plans over the past 350 million years, but to-date this diversity in body shape and its relationship to ecology and behaviour have not been systematically quantified. Here we analyse body proportions in 411 extinct and extant terrestrial tetrapods spanning most major taxonomic, locomotor and dietary groups. We show that most body segments scale with negative allometry across terrestrial tetrapods as a whole but find statistical support for quadratic relationships that suggest differential scaling in small-medium versus large animals. Statistical analyses of shape differences and allometric trends in different locomotor and dietary groups highlight key adaptations in body proportions that mechanistically underlie the exploitation of key ecological niches, such as flight and hyper carnivory, as well revealing patterns of changing body proportions during major macroevolution events, such as bipedal-quadrupedal transitions. Overall, our results emphasise that changing body proportions played an important role in the broad-scale ecological diversification of terrestrial tetrapods.