The armoured giant was 8ft long, weighed 400lb and terrorised the seas 400 million years ago.
The huge scorpion prowled the ocean floor but may also have crawled on to the land, scientists said.
After a foot-long fossilised claw was found in a German quarry, British expert Dr Simon Braddy said: "This is an amazing discovery.
"We have known for some time that the fossil record yields monster millipedes, super-sized scorpions, colossal cockroaches, and jumbo dragonflies.
"But we never realised until now just how big some of these ancient creepy-crawlies were."
The scorpion, called Jaekelopterus rhenaniae, was found by scientists including Markus Poschmann.
It had eight legs, two paddles for swimming plus two huge serrated claws. The creatures, called eurypterids, may be the ancestors of all arachnids - the group which includes scorpions and spiders. (AP)