The rat, which has no fear of humans, measures 82cm long, placing it among the largest species of rat known anywhere in the world.
The creature, which has not yet been formally described, was discovered by an expedition team filming the BBC programme Lost Land of the Volcano.
It is one of a number of exotic animals found by the expedition team.
Like the other exotic species, the rat is believed to live within the Mount Bosavi crater, and nowhere else.
"This is one of the world's largest rats. It is a true rat, the same kind you find in the city sewers," says Dr Kristofer Helgen, a mammalogist based at the Smithsonian National Museum of Natural History who accompanied the BBC expedition team.
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