Dave Andrews, a consultant ecologist and ornithologist according to his Twitter profile, shared footage of the whale near Coalhouse Fort in Essex on Twitter.
He added the beluga had been feeding around barges on the river for around an hour and had not moved more than 200m either direction on the river.
British officials are checking out reports that a beluga whale is swimming in the River Thames.
Images posted on Twitter showed a white whale in the water.
Some'fin' unusual has been spotted in the River Thames prompting much excitement on social media.
Whale and Dolphin Conservation Society spokesman Danny Groves said as the whales were high Arctic species, "this one is thousands of miles from where it should be".
The whale that was spotted Tuesday was "obviously very lost and quite possibly in trouble", the Whale and Dolphin Conservation said.
Weather Authority: Rain showers likely on Tuesday
Here's the weather where you live: A few areas of patchy fog this morning with temperatures in the lower 70s. After a wet and stormy Tuesday , rain chances will taper off into the day on Wednesday .
"We are working with other agencies to monitor the situation and ready to provide appropriate assistance if requested", it said.
"We would urge that the whale is given space and disturbance is kept to a minimum", he said.
They are considered "extremely sociable mammals" and live in pods. People have been warned not to approach it, and to stay on the shore.
In 2006, a whale died after it swum up the Thames into central London despite efforts to rescue the animal.
Despite the latest whale sighting prompting excitement on social media, conservationists expressed their worry for the animal. The last sighting of a Beluga whale was some three years ago, off the coast of Northumberland, in northern England.
"Beluga whales are not usually seen in British waters, so it's very rare", spokeswoman for the organization Julia Cable said.