Clive Oxley, 49, decided to act after he noticed the cocker spaniel locked inside a tourist’s Kia in a seaside car park in St Ives, Cornwall.
The owner returned soon after and screamed when she saw police by her car, before admitting she had left the dog for an hour and a half while she went to Pizza Express.
Mr Oxley said: “There is no reason at all to leave a dog in that state. But for something as insignificant as going for a pizza is unbelievable.
“The woman came back and started screaming at me. She said she’d just been to Pizza Express. By the time she got there the police were there too.
“When she came back she said ‘oh God, my dog. It is lucky it is not dead. I told her ‘too right you are lucky it is not dead.’
“She was a holidaymaker and was really embarrassed. She claimed she’d only gone for an hour and a half. Inside it was estimated to be 125F (52C).”
The panting dog was struggling to breathe inside the vehicle, that had been parked up last Friday afternoon, when temperatures reached about 15C (59F) in the south west.
Mr Oxley added: “That poor dog was absolutely boiling in a tiny cage with no water and no air.
“You couldn’t even touch the roof of the car, it was flipping boiling.
“I had to smash the window and get the dog out. I would do what I did again any time if I needed to.
“I would tell anyone in my situation – just smash it and get it out. It was virtually breathless when we finally got it out. It wouldn’t have had long left.
“She got the dog and walked off down the road. Ten minutes later we saw her husband driving his car to the police station.”
Devon and Cornwall Police said people had the right to break into a car if they believe a dog is in distress.
A spokesman said: “With warm weather, we always see a rise in the number of reports of dogs being left in cars.
“The temperature can rise very quickly inside a car during the summer months, dogs do not have the ability to cool themselves down and therefore can suffer from heatstroke which can be fatal.
“Police have the right to break the window of a car to free the animal if they believe that a dog is in distress.
“If a member of public sees a dog in a car in a warm day please call the police immediately and we will advise from there.”
Top pic: SWNS