A "giant herring" measuring 3.5 metres has been discovered off Sweden's western coast - the first such fish found in the Scandinavian country in more than 130 years, a maritime museum spokesman said.

The Regalecus glesne, known as the King of Herrings or giant oarfish, was found dead in the small fishing village of Bovallstrand on Sweden's west coast, about 90 kilometres from the Norwegian border.

"Down at the water, there was something big floating. At first we thought it was a big piece of plastic.

"But then we saw an eye. I went down to check and saw that it was this extremely strange fish," Kurt Ove Eriksson, the passer-by who found the specimen, told daily Svenska Dagbladet.

The rarely seen regalecus, the world's longest bony fish, can reach up to 12 metres.

"The last time we saw a King of Herrings in Sweden was in 1879," the House of the Sea museum in Lysekil, where the fish was taken to, said in a statement.

"We don't know much about the species," it said, "but believe it lives in deep waters, at least 1,000 metres deep, and many believe it's at the origin of the sea serpent myth," or stories of mythological sea creatures like the Loch Ness Monster.

The dead fish, which was frozen at the museum, had a deep cut through its body and was missing its beautiful, typical back fin, the museum said, adding the fish might be added to an exhibit on sea monsters planned later this year.