Doctors said the 9-year-old Havanese, called Zsazsa, would have died within a couple of days if she had not been found.
Severely dehydrated, she was so weak she could not stand up.
Police were called to a flat in Budapest after neighbours noticed that Zsazsa's elderly owner had not been seen for some time.
"The dog was lying next to the dead body, and was so weak that she could not stand up. We had to drag her away," said head of animal rescue group Allatmento Liga, Gabor Pataki.
It is thought many animals would have died after such a trauma.
Since being rescued, Zsazsa has been wagging her tail again after getting back on her feet.
Police said her owner, a 66-year old woman, appeared to have died of natural causes.