Pumpkin - partial to leftover ice cream and crisps - is the only Scottish representative in PDSA Pet Fit Club.
The seven-year-old ginger and white tabby will compete against six other pets from across the UK, including dogs, other cats, and a rabbit.
Weighing 1st 8lbs (10.2kg), Pumpkin's target is to lose 7lb (3.2kg).
The pet cat will now go on a strict six-month diet and exercise programme.
Owner Chelsea Mullen, 21, from the Bridge of Dee area of Aberdeen, has owned Pumpkin and his sister Munchkin - who is not overweight - since they were kittens.
She said: "They both seem to eat the same amount but the difference is that Pumpkin simply doesn't do any exercise. He just lies there.
"Several years ago Pumpkin got stuck in the cat flap. It was so stressful because he was completely wedged in, couldn't go forwards or backwards.  He's avoided it ever since.
"We would like Pumpkin to lose weight so he can start having fun and learn how to play with both his toys and his sister Munchkin again, but most of all we want him to lose the weight in order to keep him healthy."
She added: "He's a very happy, friendly and loving cat but has turned into a couch potato.
"His daily exercise now consists of moving from the bed or sofa to the food bowl and back again.
"He's very reluctant to go outside because that involves getting off the sofa and he doesn't even move if he sees a fly.
"Pumpkin tends to eat dry food and he does like cat treats. He also loves indulging in leftover ice cream and crisps.  He always tries anything to get food and claws at you until you give in."
PDSA senior vet Fiona Gregge, who will be helping to overseeing Pumpkin's diet, said: "Pet obesity is a growing issue that affects millions of UK pets.
"Carrying excess weight can have serious health risks and increase the chances of pets suffering from life-limiting and life-threatening diseases including arthritis, diabetes and heart disease.
"With the help of PDSA Pet Fit Club, Chelsea is making the necessary lifestyle changes to help get Pumpkin down to a healthier weight.
"We will support him every step of the way over the next six months to ensure they succeed."
An overweight cat from Aberdeen who got stuck in a cat flap has been chosen as a finalist in a major pet slimming competition.