The young common seal, nicknamed Freddie Mercury, had entertained locals in Barnes for more than a week after he began sunbathing at low tide near Hammersmith Bridge.
The seal suffered a fractured flipper and dislocated joint, and vets made the decision to euthanise him after concluding his prognosis was “extremely poor”.
Scotland Yard has confirmed officers are investigating the seal’s death and want to speak to the dog’s owners, as well as any witnesses to the attack. The RSPCA has ben informed.
The dog was recalled by its owner who reportedly then left the scene.
British Divers Marine Life Rescue (BDMLR) medics treated his wounds before taking him to the South Essex Wildlife Hospital (SEWH) in Tilbury by boat with help from the Port of London authority and the brigade.
A statement issued by SEWH on Monday explained that following X-rays, the prognosis was “extremely poor” and Freddie would have to be put down.
On Monday, a hospital spokesman said on Facebook: “At this stage, we believe the only ethical and fair option we have is to end his suffering.”
The spokesman added: “We know this is a well-loved seal from the amazing response we have had to his story, but he is not the only seal we have had to care for that this has happened to.
“Please folks do not go near seals and always, always, keep dogs on leads and under control.”
Freddie had originally been rescued as a newborn last year in the Netherlands but he was rescued again in France and now twice in the UK, the animal hospital added.