Studying the origin of avian thermoregulation is complicated by a lack of reliable methods to measure body temperatures in extinct dinosaurs. Evidence from bone histology and stable isotope analysis often relies on uncertain assumptions about the relationship between growth rate and body temperature, or the isotopic composition (δ18O) of body water. By contrast, clumped isotope (Δ47) paleothermometry, which relies on the binding of 13C to 18O, provides a robust tool for determining the body temperatures of dinosaurs, but has yet to be applied across a broad phylogenetic range while accounting for the influence of paleoenvironmental conditions. Applying this method to well-preserved fossil eggshells, we find that the three major clades of dinosaurs, Ornithischia, Sauropodomorpha, and Theropoda, were characterized by warm body temperatures. Dwarf titanosaurs may have exhibited similar body temperatures to larger sauropods, although these conclusions are provisional in light of uncertainties in the taxonomic assignment of dwarf titanosaur eggshell. Regardless, our results reveal that metabolically controlled thermoregulation was the ancestral condition for Dinosauria.