Patch needed surgery after wolfing down the mask, which his owner believes happened without her noticing during a walk in Thrapston, Northamptonshire.
It was only because the cockapoo had also eaten a sock from a tumble dryer that he was taken in for treatment.
Vets noticed the metal nose clip – which they said could have proved fatal – on an X-ray, and safely removed it.
Both the face mask and the sock were retrieved from the three-year-old’s stomach by vet Riccardo Minnelli, from Abington Park Vets.
Initially he was only trying to find the sock, but “when we took a radiograph to check the position, we could see there was a piece of metal”, Mr Minnelli said.
“That didn’t make sense for a sock but as it was still accessible in the stomach, we were able to go in with an endoscope, which has a camera.
“We pulled it out along with the sock and when we untangled it, we found it was a disposable face mask.”
Owner Sue Fox said: “We didn’t even realise Patch had got the face mask at first.
“He has a habit of eating things around the house and he’d got into our tumble dryer and took a small sock.
“When he didn’t bring it back up, we knew we had to get help and took him to the vets.”
Mr Minnelli said had the metal clasp in the mask moved into the intestine and caused an obstruction, it could have become “life-threatening in the long run”.
“So, get your pet to your vet straight away if you suspect it has eaten one,” he added.
The surgery happened at the end of November. Speaking after Patch was given the all-clear after two months of check-ups and treatment, Mrs Fox pleaded with people to put their masks and gloves in a bin.
“It’s so thoughtless and dangerous” to discard them, she said.