The predatory frog, Beelzebufo ampinga, lived between 70 million and 65 million years ago in what is now Africa.
"We knew it was big; we knew it was almost certainly predatory," said study co-author Susan Evans, a paleontologist at the University College London.
"What the new material has shown us is that it was even more heavily armoured than we imagined," Evans said.
It's not clear what the frogs used the body armour for, but one possibility is that the sculptured bones may have been an adaptation to a dry environment that allowed the frogs to burrow underground, where they were less likely to bake in the hot sun, Evans said.
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The armour may also have been protection, helping the frog fend off dinosaurs and crocodiles that prowled during that time, researchers said.
A few bone fragments were first discovered from a mystery frog in Madagascar in 1998, but it was only in 2008 that they had enough pieces to identify the species, which they dubbed the devil frog, or Beelzebufo ampinga.
The study also downgraded the amphibian's size - instead of being the biggest frog that ever lived, it may be closer to the size of an African bullfrog, which grows to about 25.4 centimetres across.
The study was published in the journal PLOS ONE.