Caramel, aged four months, was taken in a burglary in Harlow, Essex, on 3 June.
The sick puppy belongs to Lexie Taylor, 8, who has juvenile arthritis, a condition which improved since she began walking her pet.
Mother Tracy Taylor said: "The search reached 13m people on Facebook. A woman saw reports and realised she had bought Caramel. She was returned yesterday."
The chocolate-coloured Chihuahua-cross has a lump on her stomach caused by a hernia and also a suspected tumour. She was scheduled to visit the vet shortly after she was stolen.
Three days after the theft the family posted Caramel's story on Facebook in the hope she would be recognised and returned.
In a little over a week 14,000 people had "liked" the page.
A lost pets website has also posted the story, and almost 60,000 people saw that post, Mrs Taylor said. Messages of support came in from as far away as India and Canada.
The family had been particularly keen to have Caramel back because of the puppy's positive effect on their daughter's health.
"Ever since Caramel was born she and Lexie have been inseparable. It has really helped with Lexie's mobility because she has been walking Caramel so much," Mrs Taylor said.
Late on Monday afternoon Mrs Taylor's husband Richard received a call from a woman who said she had bought a Chihuahua puppy, and having seen the publicity, thought it might be the family's pet. The dog is now safely back home.
"We are just the happiest family in the world right now," Mrs Taylor said.
A stolen Chihuahua puppy has been returned to its owners after the search for her went viral on Facebook.