The eight-year-old reptile, Atlas, appeared to have drowned in his water bowl at Ben Waterfall's home in Swimbridge, Devon.
The horrified family doctor saw the tortoise half submerged in the bowl and picked him up to find he was not breathing.
But the determined 34-year-old acted fast, and spent six minutes giving his pet mouth to mouth until the tortoise started breathing again, the Daily Mail reported.
Dr Waterfall had been tending his garden when he had walked past Atlas's run and noticed that his pet was in serious trouble.
"I saw his back feet sticking up out of the water. I pulled him out but he was not breathing and was completely floppy," Waterfall said.
"I did six minutes of mouth-to-mouth before he slowly started breathing and blinking," he added.
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"I was quite emotional at the time. I first started doing it and I had my mouth over his whole head, mouth, nose and nostrils. Then I changed it to sort of breathing into his nostrils," the doctor said.
"He has a run outdoors which had some water in a concrete container about ten inches long, the same width as his shell. He had just fallen in. He was essentially dead," he added.
Tortoises are terrestrial, which means that they live on land. Unlike aquatic turtles, tortoises lack webbed feet.
If a tortoise by accident falls into a pond or swimming pool it could sink to the bottom like a rock and drown.
Tortoises generally have lifespans comparable with those of human beings, and some have lived for more than 150 years.
The pet owner took Atlas to the vet to be checked out and he was given antibiotics in case he had any water in his lungs.