Police raced to the vehicle parked in King Street car park, in Glasgow city centre, yesterday morning.
A concerned member of the public contacted officers after spotting the dog.
Police said that advice was given to the owners, who returned to the vehicle shortly after officers arrived.
Animal welfare experts warned that 20°C outside, can see the temperature inside a car reach 45°C within an hour.
PC Stephen McAllister said: "Cars can reach high temperatures quickly and dogs can overheat in minutes.
"Leaving a window open or providing water is simply not good enough."
Scotland's animal rescue charity and vets across Glasgow have also received reports of distressed dogs left inside warm vehicles.
Chief Superintendent Mike Flynn, of the Scottish SPCA, said: "Most people know what it's like to get into a car after it's been sitting in the sun.
"It's unbearable and we immediately roll down the windows or turn on the air conditioning.
"Imagine being trapped inside a hot vehicle wearing a fur coat for minutes, possibly hours, and you are some way to understanding how a dog left inside a hot vehicle feels.
"Dogs can't sweat the way humans do and need to pant to cool down which can lead to severe dehydration and hyperventilation."
Last summer, a Glasgow man was convicted after leaving his two dogs inside a hot vehicle.
The 54-year-old was banned from keeping animals for 10 years.