The collie dog had been left in a car parked at North Devon District Hospital in Barnstaple on Friday afternoon.
The dog's owner was an outpatient visiting the hospital for a procedure.
Diana Lewis, founding member of North Devon Animal Ambulance, was called to the scene by a worried member of the public who saw the dog in distress.
She said: "I received the call about a very distressed dog in a hot car.
"I responded within minutes to find the dog had already died.
"I called 999 and the police arrived immediately. It is a terribly traumatic, heartbreaking end to a dog's life."
The dog, which was in a cage, was lying down in the back of the car when it was seen by a couple who pulled up in the parking bay behind.
The pair ran into the hospital to make the phone call to Mrs Lewis.
She said: "I cannot believe people are still keeping dogs in hot cars, despite all the publicity there is about it.
"The hospital even has RSPCA posters in the hospital telling people not to leave dogs in the car."
A police spokesman said the dog had already died when officers arrived at the hospital.
The police have passed the investigation to the RSPCA.
A spokesman for Northern Devon Healthcare NHS Trust, which runs the hospital, said: "We can confirm the dog had been left in a car by a patient on Friday afternoon.
"Our on-site policeman is now conducting a joint investigation with the RSPCA.
"We would urge any patient coming to hospital to leave their dog at home.
"Even during a short stay for an appointment, the temperature inside a car can rise very sharply and put the dog's life at risk."
Mrs Lewis said in a similar case a dog was left for an hour and 40 minutes while its owners went to an event in Braunton on Sunday. Luckily that dog survived unharmed.
She said: "Everyone knows about dogs and hot cars and the repercussions.
"A very beautiful dog has now died unnecessarily. How do you explain that kind of loss?
"The owner will be devastated and ridden with grief for the rest of her life."