The rise of dinosaurs during the Late Triassic was a pivotal event in the history of life on Earth, which led to them becoming dominant members of terrestrial ecosystems throughout the remainder of the Mesozoic. However, the drivers of this geographic expansion and explosion in biodiversity have been poorly understood. Early studies have hypothesized that the extinction of co-occurring vertebrate groups such as aetosaurs, rauisuchians, and therapsids at the end-Triassic mass extinction, provided early dinosaurs with the opportunity to diversify into new niches. However, this pattern could instead be a response to climatic changes during this interval, especially given the increasing evidence that climate played a key role in constraining Triassic dinosaur distributions. Our work is the first to quantitatively explore the opportunistic expansion model by examining changes in dinosaur and tetrapod ‘‘climatic niche space’’ across the Triassic-Jurassic boundary. We found that Late Triassic sauropodomorph dinosaurs occupied more restricted climatic niche spaces than other dinosaurs, and were excluded from the hottest climate zones at low latitudes. The geographic expansion of sauropodomorphs after the mass extinction is linked to the expansion of their preferred climatic conditions. Evolutionary model-fitting analyses provide evidence for an important evolutionary shift from cooler to warmer climatic niches during the origin of Sauropoda, the clade that later in the Mesozoic became the iconic long-necked forms. Together, our results provide support for the key role of climate in the ascendancy of dinosaurs.