The prehistoric creature has scaly skin similar to a crocodile and an impressive set of teeth.
It was found, already dead, by schoolboy fisherman Shawn Brown in the Grand Union Canal at Wigston.
The 14-year-old took a picture of his 10 ins-long discovery and showed it to a number of aquarists who managed to identify it.
The armoured suckermouth catfish normally lives in Panama, Costa Rica and South America.
Experts say it is the first time one has been found in our waterways.
It is thought to have been released into the canal after growing too big for somebody's aquarium but could not survive in the colder water.
John Hall, who runs All Seasons Angling tackle shop in Wigston, said: "We are only 300 yards from where Shawn found it and he came in here and showed me the photo. He is a good little fisherman and as soon as he saw it knew it didn't belong here.
"He went home and looked it up on the internet to try and see what it was but he had to send it off to experts to identify it.
"It had teeth as well and I'm sure it was scare a lot of anglers who saw it swimming up river. But it looks worse than it actually is."
The armoured suckermouth catfish – Hypostomus plecostomus in Latin – are herbivores and use their distinctive mouths to hoover up algae off rocks.
Their tough armoured plating acts as a defence mechanism to ward off predators in tropical waters.
They pose no threat to humans although it is not known what effect they would have on native fish if they were to ever breed here.
Ian Wellby, a fisheries scientist at Brooksby College, Leics, said: "It is not something you want in your freshwaters but it is quite harmless.
"It is the first one I have ever heard of in Britain before. It is a warm water fish and could not survive our winters."
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