In the remarkable footage, said to have been filmed at a lake in China, a black shape can be seen surfacing through the waves.
The shocked onlookers claimed it was about 16 to 19 feet (five to six metres) in length, as it did not completely come out of the water and all of it could not be seen.
In the video, swimmers in the lake are heard to gasp as it seems to rear its head.
Commentators have said it resembles the mythical Loch Ness Monster, but this was filmed thousands of miles away from Scotland at the Mount Lu Xihai Resort in Jiujiang City.
The video has been seen by more than five million people on social media.
But, one wildlife expert said it was likely to be a Chinese sturgeon – a critically endangered fish that can grow up to 16.4 feet.
He suggested it had escaped the Luoping Sturgeon Farm which he said had operated for eight years – more than enough time for the specimen to grow to the length of the mystery creature.