Scientists dug up a gigantic jawbone, teeth and scales belonging to the shark which lived 89 million years ago.
The bottom-dwelling predator had huge tooth plates, which it likely used to crush large shelled animals such as giant clams.
Palaeontologists already knew about the shark, but the new specimen suggests it was far bigger than previously thought.
The scientists who made the discovery, published in the journal Cretaceous Research, last week also released details of other newly discovered giant plankton-eating fish that swam in prehistoric seas for more than 100 million years.
But this new fish, called Ptychodus mortoni, is both bigger and more fierce, having a taste for flesh rather than plankton.
It may even have been the largest shellfish-eating animal ever to have roamed the Earth.