The claim was made by researchers who carried out the most extensive study ever of the famous lake and its mythical beast.
More than 250 samples of water were taken from the very depths of Loch Ness - the British Isles' largest and second deepest body of fresh water.
They have now been studied and the research project has now said Nessie is in fact probably a massive EEL.
Geneticist Professor Neil Gemmell, from the University of Otago in New Zealand, said: "There is a very significant amount of eel DNA.
"Eels are very plentiful in Loch Ness, with eel DNA found at pretty much every location sampled - there are a lot of them.
"So - are they giant eels?
"Well, our data doesn't reveal their size, but the sheer quantity of the material says that we can't discount the possibility that there may be giant eels in Loch Ness.
"Therefore we can't discount the possibility that what people see and believe is the Loch Ness Monster might be a giant eel.
"Divers have claimed that they've seen eels that are as thick as their legs in the loch, whether they're exaggerating or not - I don't know - but there is a possibility that there are very large eels present in the loch.
"Whether they are as big as around 4m as some of these sightings suggest - well, as a geneticist I think about mutations and natural variation a lot, and while an eel that big would be well outside the normal range, it seems not impossible that something could grow to such unusual size."
He added: "Further investigation is needed to confirm or refute the theory, so based on our data, giant eels remain a plausible idea."
The myth dates back to the 6th Century - but theories were discounted, including that Nessie could be a shark, a sturgeon, or a catfish, or even a plesiosaur.
Professor Gemmell said: "We can't find any evidence of a creature that's remotely related to that in our environmental-DNA sequence data."So, sorry, I don't think the plesiosaur idea holds up based on the data that we have obtained."
DNA from each sample was captured, extracted and sequenced then compared against global DNA databases to reveal a comprehensive picture of life present in the Loch - examining the bacteria, the fish, and everything else in between.
Professor Gemmell added: "What I'm most satisfied with is that we came here to study environmental DNA, and our analysis has captured everything we thought is in the loch.
"We now have an excellent database which if compared to any future testing could enable us to identify trends and changes in the Loch environment."