More than 2,100 UK households were surveyed on behalf of the Pet Food Manufacturers' Association (PFMA).
Some 62% of dog owners and 72% of cat owners believed they could do little or nothing about their pets' weight, according to the survey.
Only a quarter of respondents said they had ever sought advice on their pets' weight.
"The figures are alarming, and the problem won't go away without a fundamental shift in owners' attitude to feeding their pets," said PFMA chief executive Michael Bellingham.
By Lindsay Beaton
As work continues on creating a new nutrition label that focuses on simplifying information for consumers, challenges remain.
By Tang Yu