Ecologist Dave Andrews made the sighting, saying the mammal has been feeding near barges around Gravesend in Kent.
He shared videos online, saying it hasn’t moved more than 200m in any direction.
The British Divers Marine Life Rescue warned the public to watch it from the shore, and not to get too close to the whale.
Spokeswoman Julia Cable added that the whale was “swimming strongly”, calling its appearance “extremely rare”, according to the BBC.
Rescuers are hoping it will swim out of the Thames and back into the sea, rather than heading inland.
The Thames whale sighting is the first in years for London’s river, with the last whale being seen three years ago.
A northern bottlenose whale swam up the Thames in 2006, which led to the river being closed over the animal’s two-day plight. Its journey was ill-fated, with the animal eventually dying.