A recently discovered tyrannosaurid metatarsal IV (SWAU HRS13997) from the uppermost Cretaceous (Maastrichtian) Lance Formation is heavily marked with several long grooves on its cortical surface, concentrated on the bone's distal end. At least 10 separate grooves of varying width are present, which we interpret to be scores made by theropod teeth. In addition, the tooth ichnospecies Knethichnus parallelum is present at the end of the distal-most groove. Knethichnus parallelum is caused by denticles of a serrated tooth dragging along the surface of the bone. Through comparing the groove widths in the Knethichnus parallelum to denticle widths on Lance Formation theropod teeth, we conclude that the bite was from a Tyrannosaurus rex. The shape, location, and orientation of the scores suggest that they are feeding traces. The osteohistology of SWAU HRS13997 suggests that it came from a young animal, based on evidence that it was still rapidly growing at time of death. The tooth traces on SWAU HRS13997 are strong evidence for tyrannosaurid cannibalism—a large Tyrannosaurus feeding on a young Tyrannosaurus.