It's known for inhabiting the murky waters of Loch Ness.
But mysterious footage shows that the famous creature - whose existence has been debated for years - may have taken a bit of a holiday to London.
The video was shot while Youtube user Penn Plate was travelling on the Emirates Air Line cable cars near Greenwich.
The footage shows a humped figure making its way through the huge river near the O2 Arena - before sinking back under the water.
Penn said: 'This was on the cable car in Greenwich yesterday. Something huge was moving under the water and then briefly surfaced.
'Are there whales in the Thames? Or is it some weird submarine?'
But it is possible the whole stunt was set up as an elaborate April Fool's joke.
The Emirates Air Line offers views across the river to its thousands of monthly passengers.
And while it's unlikely that Penn really did see the Loch Ness Monster, it wouldn't be the first time an unusual creature has been spotted in the capital's river.
In January 2006, a northern bottle-nosed whale spent several days in the Thames after becoming lost while heading for the deep waters of the North Atlantic.
The whale sadly died on its way back to sea after capturing the hearts of thousands who came out to watch it swim through central London.