The PCs, who became concerned about the "sleeping" animal after being alerted by a member of the public, attempted to wake it from the back seat of parked car.
When it would not be roused, they smashed the window of the Mercedes coupe fearing it would be harmed in the 18C temperatures.
It was only when they reached inside to rescue the King Charles Spaniel that they realised it was merely a stuffed cuddly toy worth £4.50.
Owner Gordon Williams, 80, has now told how he returned to his car to find a seat covered in broken glass and an official note.
It reads: "Smashed your window re concern for animal on rear seat."
The police force has agreed to pay to the damage to allow the retired mining engineer to replace his window.
Mr Williams, from Mansfield, Nottinghamshire, told the Sun newspaper: "I guess they felt like idiots."
The widower said he had bought the toy, named Little Willy, on holiday before leaving him in his car on Monday.
He said: "He'd been living quite happily on the car's back seat. But I've decided he will be safer sleeping on a chair in my kitchen."
Nottinghamshire Police said: "The call was made in good faith by a passer-by. In this case officers believed a dog could be suffering due to the heat of the day and its life was potentially at risk, so felt there was no other option but to enter the vehicle.
"We will be speaking to the owner of the vehicle about paying for the damage.
"This is a perfect example of why drivers need to think about what they leave on view in their car, both in terms of other people's perception of the objects and in a bid to deter opportunist thieves."
