The male Staffordshire bull terrier cross was found by a member of the public on Wednesday (November 21)Â in a car park at the ramparts in Portsmouth at around 9am.
The old, tan-coloured dog, who was on a grass verge in a black Adidas sports bag, wrapped in a duvet in the bag, was rushed to The Stubbington Ark RSPCA Animal Shelter by the member of public who found him but died before arrival.
RSPCA inspector Charlotte Coggins said: '˜This poor dog has been discarded rubbish and simply left to suffer when he was in desperate need of help.
'˜It's heartbreaking to think of him alone and frightened having been left to die in a bag in this freezing weather.
'˜We're very grateful to the person who found him for showing kindness and compassion and caring for him in his final moments.'
Fleas, overgrown nails and a small growth were some of the signs of neglect found by vets at the centre.
Ins Coggins added: '˜He was alive when he was found but sadly died before arrival at our centre.
'˜He was riddled with fleas and had overgrown nails and muscle wastage on his head and a small growth on his right hock.
'˜We have no way of knowing where he came from, and he has no microchip, so we are appealing to the public to come forward with information.
'˜Anyone who has any information should contact our appeals line on 0300 123 8018, and leave a message for myself. All calls will be treated with confidence.'