It was feeding around moored barges and did not move more than 200 metres during the day.
Known as the "canary of the sea" due to their chirps, clicks and whistles, beluga whales can range from 13 ft to 20 ft in length and have distinctive round foreheads, known as "melons".
Rescue teams are on standby in case the whale, which usually lives in the Arctic, gets into danger.
The animal was spotted this afternoon near Coalhouse Fort in East Tilbury, Essex.
The Whale and Dolphin Conservation society said the whale could be ill, or young and separated from his or her pod.
"The River Thames beluga whale has reportedly headed back out to sea after discovering how much it costs to rent a one bed in zone 1", tweeted one user, who couldn't resist using the day's news to take a swipe at London's housing market.
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"We are surprised and a little saddened that it has resurfaced again - we had hoped it had gone out to deeper waters and gone out to where it should be", said Lucy Babey, head of science and conservation at the ORCA charity.
It said it was ready to provide help to the whale if asked to do so by other agencies.
The last reported sighting of beluga in United Kingdom waters was in 2015, when two were spotted off the Northumberland coast and one in Northern Ireland.
He urged people to give the whale "space and minimise disturbance".
The rare marine mammal, nicknamed Benny, was seen near Gravesend yesterday and appeared to be swimming strongly and feeding in the estuary.
A bottle-nosed whale became stranded in the Thames for two days in 2006, but died from convulsions as it was being rescued. They can be found in the waters around Russia, Alaska, Canada, West Greenland, and Svalbard.