Conservationists are anxious that the animal, which is normally found thousands of miles away in the Arctic, heads out to sea.
The rare marine mammal was first seen near Gravesend, Kent, and appeared to be “swimming strongly” and feeding in the estuary.
However rescue teams are on standby in case it gets into danger.
Rob Lott, a marine mammal scientist at Whale and Dolphin Conservation (WDC), said the cetacean was being monitored in case it “live strands” on a sandbank.
“It’s a monitoring operation at the moment, obviously the longer it stays in the Thames estuary then it will become more of a concern, so we’ll see what happens in the morning,” he told BBC Radio 5Live.
Beluga whales can grow up to 20ft in length and are usually at home in the icy waters around Greenland, Svalbard or the Barents Sea.
They are also a social species, so to see one by itself in the Thames is “concerning”, Lott said.
“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,” the scientist explained.
The British Divers Marine Life Rescue said it was sending its area coordinator down to the river to monitor the situation.
The network of volunteer “marine mammal medics” has whale rescue pontoons at the ready that can be used to re-float stranded animals in an effort to move them to safety.
A spokeswoman for the organisation said the beluga’s visit to the Thames was a “very rare occurrence”, and urged people not to go out in boats to get a close look at the whale, but to watch it from the shore.
The whale was first spotted by ecologist Dave Andrews, who tweeted footage of the white animal.
“Can’t believe I’m writing this, no joke – BELUGA in the Thames off Coalhouse Fort,” he said.
The last reported sighting of beluga in UK waters was in 2015, when two were spotted off the Northumberland coast and one in Northern Ireland.
Among the theories of how the beluga whale ended up in the Thames is that it followed a shoal of fish into the waterway.
Schooling fish such as herring, capelin and cod are among the usual sources of food for the species.
Danny Groves, from WDC said: “This is a High Arctic species thousands of miles from where it should be in Greenland, Svalbard or the Barents Sea, they are usually associated close to the ice.
“He or she is obviously very lost and quite possibly in trouble.”
A spokesman for the RSPCA said: “The RSPCA is aware of reports of a whale, possibly a beluga, in the Thames.
“We are working with other agencies to monitor the situation and ready to provide appropriate assistance if requested.”
In 2006, a whale died after it swam up the river into central London despite efforts to rescue the animal.