The snail, which can grow up to 15mm, was believed to have slowly died out due to a deterioration of water quality and a lowering in the level of Wales's largest natural lake.
Searches at the lake by conservationists in 1960, 1964 and 1989 failed to find any trace of the elusive snail.
Mike Howe, of the Countryside Council for Wales, said: "It shows that the species is a bit more robust than we thought. Until the snail was found last month there have been no reported live specimens anywhere in the UK since 1991. As far as we know, this is the only currently known population - which is good kudos for us but bad news for the snail."
He added: "It used to be fairly widespread in southern England but most of the population has disappeared because of pollution. It is now one of the rarest freshwater snails in Europe."
Dr Rod Gritten, of Snowdonia National Park Authority, said: "As owners of the lake we always consider Llyn Tegid to be one of the jewels in the crown of the National Park. A find of this nature makes it even more important that it is conserved in an appropriate manner."
Conservationists will now examine ways of preserving the species' habitat for the future. "Until we know more about the snail it is too early to say exactly how we will manage the population," said Mr Howe.
A spokeswoman for English Nature said: "The species is a major rarity and it is on our species recovery list. We are conducting a survey with the Environment Agency at the moment to see if there are any in places where we have records of them."Reuse content