There are occasional reports of belugas off the British coast, though very rarely this far south.
Ecologist and ornithologist Dave Andrews could not hide his surprise as he tweeted videos of the mammal in the Thames off Coalhouse Fort in Tilbury, Essex.
Lucy Babey, head of science and conservation at ORCA, a United Kingdom charity that works with whales and dolphins, also urged caution.
Whales are rarely seen in the Thames, and the sighting has sparked conerns for the beluga's health.
"Hopefully instinct will soon kick in and the beluga will leave the estuary and go out into the North Sea and then head north where it should be", Mr Lott said.More news: Jurgen Klopp urges Liverpool to take the Carabao Cup seriously
"He or she is obviously very lost and quite possibly in trouble".
The British Divers Marine Life Rescue, which helps with rescues of stranded cetaceans and other marine animals, said they were sending their area coordinator down to the river to monitor the situation. They are known for their bulbous forehead, known as a "melon", which allows them to make various facial expressions due to its flexibility.
"We're hoping if we give it enough space and keep an eye on it, it will find its own way out of the Thames to an environment that's more appropriate for it".
Ranging from 13ft to 20ft in length, they are common in the Arctic Ocean's coastal waters - but migrate southwards in large herds when the sea freezes over.
In 2006, a northern bottle-nosed whale died after stranding itself in the Thames.