Fossilised bones bear tooth-marks that could only have come from another member of the same species.
The ferocious offender is a giant meat-eater the length of a double-decker bus that lived 65 to 70 million years ago.
Scientists believe the two-legged creature, named Majungatholus atopus, may have dined on its own kind to survive.
There were dramatic food and water shortages at the time, and many animals became extinct.
"It appears as though Majungatholus atopus exploited all available resources during stressful episodes, even if it meant dining on members of its own species," says Raymond Rogers of Macalester College in St Paul, Minnesota, US, who led the research team.
The discovery is no great surprise as many large predators, such as lions, have been known to use cannibalism as a feeding strategy.
The skull of the cannibal (Image: Greg Helgeson, Macalester College)