The female Staffordshire bull terrier cross was found in a shocking state in Bolton on Sunday afternoon after apparently being dumped by her owners.
Along with bleeding from the badly grazed tumour, the dog had chronic eye issues and her claws were piercing through her paw pads because they were so overgrown.
She had to be put to sleep because she was in so much pain.
The brindle-coloured dog was found by a member of the public at about 3pm on Sunday near a field in West Meade.
RSPCA inspector Kathryn Kellegher urged anyone with any information to come forward.
“This poor dog was in such a shocking state when she was picked up as a stray in West Meade in Bolton,” she said.
“Sadly, she had a lump on her stomach which was so large it had started to drag on the floor and she was bleeding all over the pavement.
“It was distressing to see a dog in this poor state. I rushed her to the vets immediately but sadly the vets had to put her to sleep to end her suffering.
“If anyone recognises this dog or has any information about where she may have come from, please contact the inspectorate appeal line in strictest confidence on 0300 123 8018.”