Molly, a seven-year-old Labrador, is no longer allowed to interact with patients amid hygiene concerns at Western Isles Hospital, Stornoway.
The ban was revealed in a report by the Healthcare Environment Inspectorate (HEI).
The dog's owner Valerie Macleod, 66, from Stornoway, said: "They said it was because she wasn't a therapy dog. But I never claimed she was. They also said it was for health and safety and lots of other things."
Mrs Macleod said patients on the ward appreciated Molly's presence: "They didn't talk because of their illnesses, but you could see it in their eyes."
She said Molly was a "very placid" dog.