Five porpoises were seen swimming between Tower Bridge and the Thames Barrier in London on Friday morning.
The Metropolitan Police's Marine Policing Unit said the pod was seen moving to the Lambeth area, before three of them disappeared from view, leaving two heading west past Battersea Power Station.
Speaking to the Daily Telegraph, The Marine Conservation Society said it was rare to see the creatures, which are close relatives of dolphins, in the Thames.
A spokesman said: "It is unusual for five animals to be in the river as porpoises tend to be seen alone, and in smaller groups.
"It isn't possible to say whether the tidal surge is the cause of the porpoises being in the river, but it could well be related, either to the tidal movement, or as a response to the particularly rough seas beforehand.
"The outer Thames Estuary would have been relatively sheltered from northerly offshore winds."
Stephen Mowat, a marine conservationist for ZSL, told The Guardian: "With the condition of the Thames improving we are getting more and more sightings of marine mammals such as porpoise and seals.
"They follow prey fish up the river so with the high tides to get more fish further up the Thames. Because central London is a more heavily populated area more sightings will be reported."