The dangerously thin beluga whale had no digestive activity for unknown reasons, conservation group Sea Shepherd France confirmed saying vet exams were done after it was hauled out of the water following hours of preparation.
It had been removed from the river in preparation for a transfer to a saltwater basin in Normandy in a bid to save its life.
Conservationists had tried unsuccessfully since Friday to feed it fish.
Photos posted by Sea Shepherd France showed the white mammal lying on a big net used to get it out of a river lock.
A vet team was planning to transport the 13ft whale to a coastal spot in the north-eastern French port town of Ouistreham for “a period of care”, according to Lamya Essemlali, president of Sea Shepherd France.
Authorities planned to keep the whale in its temporary saltwater home for two to three days of surveillance and treatment before towing it out to sea.
The lost beluga was first seen in the French river, far from its Arctic habitat, last week.
It weighed about 1,700lb.
Authorities said while the move carried its own mortality risk because of the stress on the animal, the whale could not have survived much longer in the Seine’s freshwater habitat.
They were hopeful it would survive after it responded to a cocktail of antibiotics and vitamins administered in the last few days and rubbed itself on the lock’s wall to remove patches that had appeared on its back.