The ad will appear on the back of buses and creates an optical illusion, which shows an obese man seemingly making a single deck bus tilt due to his weight.
The copy asks, "Time to go vegan?" The charity says it is trying to make the point that meat, dairy products and eggs are prime culprits in Britain's obesity crisis and going vegan is the way to reduce weight.
The ad was made in-house with photography by Pal Hansen.
Yvonne Taylor, senior programmes manager, Peta UK, said: "People who insist on stuffing themselves with Sunday roasts, bacon, cheese and eggs are throwing their health under the bus.
"Going vegan is a delicious, easy way to help dump unwanted weight, save animals' lives and keep your body running at peak efficiency on high-octane veggie power."