Research found that 80% of vets have reported growing numbers of fat pets, even though 90% of owners don't think they've got a podgy animal.
Experts reckon it's because people mistake the signs of an animal being sad for them being hungry.
They also discovered that a quarter of pets have suffered from behavioural problems like stress and depression.
Overweight pets, like people, are in danger from diseases like diabetes, high blood pressure and arthritis.
Sophie de Pelet, a veterinary adviser for the survey, said owners should check their pets' weight regularly, monitor what they eat and make sure they get regular exercise.