The eight-week-old animal, who has been named Murkle, was discovered by a member of the public in Caithness last week.
They contacted Scottish SPCA with concerns for the cub after spotting her at Shilvrag Top, Occumster, Lybster.
She was taken to vets in Thurso and then cared for overnight by senior inspector Audrey Gunn before being taken to the charity's National Wildlife Rescue Centre in Clackmannanshire.
Ms Gunn said the youngster, who was in a bad way when she was discovered on October 3, appears to be perking up on a diet of fresh trout.
She said: "Murkle was very sick, weak and lethargic when she was found.
"Sadly, it's possible her mother has been killed or frightened off. She wouldn't have been able to survive on her own as otters usually remain with their parents until they're around a year old.
"After some time in a heated room at the vets Murkle became a lot brighter.
"I was advised to feed her trout and I gave the fishmonger a bit of a laugh when I told him who I was shopping for. He joked that Murkle was better fed than him.
"Murkle happily munched her way through the trout and keenly searched for more, which was a really positive sign."
Over the longer term, staff at the centre will work to return Murkle to the wild.
Ms Gunn said: "She has settled in well at our National Wildlife Rescue Centre with the staff member who is fostering her but soon they'll adopt a hands-off approach to allow her to develop a healthy and natural fear of humans. This is vital if she's to survive in the wild.
"We're very hopeful that in around 12 months we'll be able to release Murkle back into her natural habitat fit and healthy, and able to fend for herself."