AbstractCeratopsidae represents a group of quadrupedal herbivorous dinosaurs that inhabited western North America and eastern Asia during the Late Cretaceous. Although horns and frills of the cranium are highly variable across species, the lower jaw historically has been considered to be relatively conservative in morphology. Here, the lower jaws from 58 specimens representing 21 ceratopsoid taxa were sampled, using geometric morphometrics and 2D finite element analysis (FEA) to explore differences in morphology and mechanical performance across Ceratopsoidea (the clade including Ceratopsidae,TuranoceratopsandZuniceratops). Principal component analyses and non‐parametric permutedmanovas highlight Triceratopsini as a morphologically distinct clade within the sample. A relatively robust and elongate dentary, a larger and more elongated coronoid process, and a small and dorso‐ventrally compressed angular characterize this clade, as well as the absolutely larger size. By contrast, non‐triceratopsin chasmosaurines, Centrosaurini and Pachyrhinosaurini have similar morphologies to each other.ZuniceratopsandAvaceratopsare distinct from other taxa. No differences in size between Pachyrhinosaurini and Centrosaurini are recovered using non‐parametric permutedanovas. Structural performance, as evaluated using a 2D FEA, is similar across all groups as measured by overall stress, with the exception of Triceratopsini. Shape, size and stress are phylogenetically constrained. A longer dentary as well as a long coronoid process result in a lower jaw that is reconstructed as relatively much more stressed in triceratopsins.