Video footage showed the white mammal surfacing in the Thames near Gravesend, Kent, though no official identification has been made.
The British Divers Marine Life Rescue said the presence of the beluga is "concerning as it's not a common species, however it's swimming strongly and feeding", according to Sky News.
Rescue teams are on standby in case the whale, which usually lives in the Arctic, gets into danger.
Lucy Babey, head of science at Orca, a marine conservation charity, said: "This is the most southerly recording of a beluga in the UK".
Whale and Dolphin Conservation Society spokesman Danny Groves said: "This one is thousands of miles from where it should be".
Officials say they're monitoring the situation from afar too.
"Beluga whales inhabit cold, Arctic waters off Greenland, Svalbard and in the Barents Sea", the group said.
"He or she is obviously very lost and quite possibly in trouble".
"We certainly don't want people trying to rescue it". "There have only been around 20 sightings of beluga whales off the United Kingdom coast previously, but these have occurred off Northumberland, Northern Ireland and Scotland".
In 2006, a northern bottle-nosed whale died after stranding itself in the Thames.
They range from 13ft (3.9m) to 20ft (6.1m) in length and have distinctive rounded foreheads.
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".
Belugas are able to produce sounds such as chirps, clicks, whistles and squeals, giving them the nickname "the canary of the sea".