Sexual dimorphism is challenging to detect among fossils due to a lack of statistical representativeness. The Angeac-Charente Lagerstätte (France) represents a remarkable 'snapshot' from a Berriasian (Early Cretaceous) ecosystem and offers a unique opportunity to study intraspecific variation among a herd of at least 61 coeval ornithomimosaurs. Herein, we investigated the hindlimb variation across the best-preserved specimens from the herd through 3D Geometric Morphometrics and Gaussian Mixture Modeling. Our results based on complete and fragmented femora evidenced a dimorphism characterized by variations in the shaft curvature and the distal epiphysis width. Since the same features vary between sexes among modern avian dinosaurs, crocodilians, and more distant amniotes, we attributed this bimodal variation to sexual dimorphism based on the extant phylogenetic bracketing approach. Documenting sexual dimorphism in fossil dinosaurs allows a better characterization and accounting of intraspecific variations, which is particularly relevant to address ongoing taxonomical and ecological questions relative to dinosaur evolution. Editor's evaluation This important contribution offers a convincing analysis of the challenging topic of sexual dimorphism in dinosaurs. Unlike previously published contributions, which are ambiguous, this paper, based on 61 ornithomimosaur fossils, makes a compelling case for measurable differences between male and female individuals. Of particular note, the use of rigorous statistical approaches, a major strength of this manuscript, sets this study apart from previous attempts to tackle this question. Morphological changes are carefully analysed and put into a broader comparative context: the conclusions of this paper allow for interesting comparisons between non-avian dinosaurs and extant groups (e.g., crocodilians, birds, mammals). As such, this manuscript will be of interest to a diverse audience, including palaeontologists, zoologists, and evolutionary biologists.