Officials in the Eure department of Normandy said late Wednesday that images suggested it was a beluga separated from its pod, though they did not specify its size nor where exactly it was seen.
An adult beluga can reach up to four metres (13 feet) in length, and while they migrate away from the Arctic in the autumn to feed as ice forms, they rarely venture so far sout.
“Studies of its health are underway to determine the best measures to take to ensure its chances of survival,” the Eure regional authorities said.
In May, a killer whale — technically part of the dolphin family — was found dead in the Seine between the port city of Le Havre and Rouen.
The animal had been stranded in the river, which flows through Paris to the Channel, and was unable to find its way back to the ocean despite attempts by officials to guide it.
The Eure authorities said lone belugas do sometimes swim farther south than usual, and are able to temporarily survive in fresh water.