The functional anatomy of the hindlimb of bipedal dinosaurs has been intensively studied. Yet, surprisingly little work has been done concerning functional adaptations in the digits for terrestrial locomotion. While complete and articulated pes skeletons are scarce, pes shape is abundantly recorded by fossil footprints. Here we aim to elucidate the significance of footprint shape and size for locomotion using a large sample (n = 303) of tridactyl dinosaur footprints from a broad range of geographic localities and time slots. Size and shape variation is characterised separately for theropods and ornithischians, the two principal trackmaker taxa. At smaller sizes, theropod footprints are best discriminated from ornithischian footprints based on their smaller interdigital angle and larger projection of digit III, while at larger sizes digital widths are effective discriminants. Ornithischian footprints are shown to increase in size from the Early Jurassic to the Late Cretaceous, a trend not observed in theropod footprints. Size and function are argued to be important determinants of footprint shape, and an attempt made to infer function from shape. Digit III projection and length-to-width ratio of the footprints are negatively correlated with size in both trackmaker groups, while digit impression widths are positively correlated with size only in ornithischians. Digit III projection is proposed to be positively correlated with cursorial ability. Increased interdigital angles are associated with a decrease in digital widths, possibly representing an adaptation for stability. Footprints are found to yield great unrecognised potential for the understanding of the functional morphology of dinosaur feet.