The charity conducted an assessment of almost 30,000 dogs across the UK over the past four years and found 35% of pets were carrying too many pounds.
Four years ago the figure was 21%, and PDSA warns that if the trend continues, almost half of all dogs could be overweight by 2013.
The charity said obesity posed a threat to the animals' health and lifespan.
Senior veterinary surgeon Sean Wensley said: "Overweight pets are less mobile, less willing to play and more likely to develop a number of serious health conditions.
"Ultimately, owners control their pet's diet and exercise. The good news is it's never too late to achieve positive change with their veterinary practice."
As part of efforts to highlight the problem PDSA is relaunching its annual pet slimming competition, Pet Fit Club, which aims to encourage owners to get their pets to lose weight.
In 2009, it was won by Labrador Keano, from Plymouth, who after shed 1st 6lbs (10kg).
He was so overweight at 7st 2lbs (46kg), he could not roll over on his back before joining the PDSA scheme, but regular walks and proper dog food helped him find a new lease of life.
More than a third of dogs in the UK are overweight, veterinary charity PDSA has warned.