Two fossils found in China show what the animal looked like.
Scientists say its hammer-shaped skull helped it to feed on underwater plants.
Only a handful of marine reptiles, living or extinct, are known to be vegetarian.
Scientists used modelling clay, like Play-Doh, to re-make the jaw to work out how the animal fed. They used toothpicks to represent the teeth.
Olivier Rieppel of the Field Museum in Chicago said:
"We looked at how the upper and lower jaw locked together, and that's how we proceeded and described it."
"It's definitely a reptile that no one would have thought to exist - look at it, it's crazy!"
The first fossils of the creature were discovered a few years ago.
It was named Atopodentatus unicus, which is Latin for "unique strangely toothed".
A crocodile-sized creature that lived 242 million years ago was the first known vegetarian reptile to live in the sea, according to new evidence.