According to the UK-based Whale and Dolphin Conservation group, there have been just 20 sightings of beluga whales off the United Kingdom coast before today's incident.
Dave Andrews, a consultant ecologist and ornithologist according to his Twitter profile, shared footage of the whale near Coalhouse Fort in Essex on Twitter.
Posting the shocking clip on Twitter, he said: 'Can't believe I'm writing this, no joke - beluga in the Thames off Coalhouse Fort.
Beluga whale, also known as a white whale, are usually found in the Arctic Ocean and the seas and coasts around North America, Russia and Greenland - making today's sighting a rare occurrence.
Lucy Babey, head of science and conservation at ORCA, a United Kingdom charity that works with whales and dolphins, also urged caution.
The Beluga poked its head out of the water on multiple occasions.
Another said: "Almost unbelievably, it would seem that there is now a beluga whale in the Thames".
"We do have quite a lot of plastic bags, which could be quite an issue", she said.
The RSPCA said: "We are working with other agencies to monitor the situation and ready to provide appropriate assistance if requested", it said.
After British Divers Marine Life Rescue were alerted, they confirmed the sighting, noting that the whale seemed to be "swimming strongly".
In 2006, an 18ft (5m) northern bottle-nosed whale died after becoming stranded in the Thames.
They range from 13ft (3.9m) to 20ft (6.1m) in length and have distinctive rounded foreheads.
This sea creature made a whale of a trip.