Minimal direct evidence exists in the rock record of dinosaurs and mammals behaving as predators and prey, respectively. A newly discovered Late Cretaceous trace fossil association of digging traces of maniraptoran theropod dinosaurs and mammalian den complexes indicates a predator-prey relationship. Three distinct associated trace fossils occur within a floodplain siltstone-mudstone bed of the Upper Cretaceous Wahweap Formation in southern Utah, United States. One trace fossil morphology and its extramorphological variants record digging by a maniraptoran theropod dinosaur, possibly a dromeosaurid or troodontid. The other two are interpreted as mammalian den complexes. The proximal association of these trace fossils suggests that dinosaurs used excavation techniques to prey on mammals.