Chris Morrison noticed an unusual sound from the stomach of his black labrador, Oscar, who he regularly walks on a golf course near his home in Dunfermline, Fife.
When he took the five-year-old to a vet, he was stunned to find 13 balls, each weighing around 45 grams (1.6oz) and around four centimetres (1.57 inches) in diameter, were lodged in the dog's stomach.
Mr Morrison, a planning administrator, said one of the balls had been in her stomach so long that it had turned black and was decomposing.
He said: "He finds golf balls like truffles. We're not sure how long exactly this happened over - but it must have been a fair period, several months at least.
"I felt his stomach and heard them rattling around. He normally brings a few home, but I had no idea he had eaten so many.
"It is normally around the ninth and twelfth fairways that we go around - and he just goes and searches for them wherever the golfers lose them.
"The vet hadn't seen anything like it - it was bizarre. I thought he might have had a couple in there, but not 13. He is a black lab so he is a fair size - but to swallow 13 is quite amazing.'
The balls were removed two weeks ago in an hour-long operation conducted by Bob Hesketh, 40, a vet from Rosyth.
He said: "It was like a magic trick. I opened him up and felt what I thought was two or three golf balls. But they just kept coming until we had a bag full.
"I think they must have been in there for several months, one was all black and the shell was swollen. It is incredible really. We occasionally find things like bottle corks but this is a bit different."
Oscar is now on the road to making a full recovery after a special post-operation diet of watered down food, but he now has to wear a muzzle during his walks.
Mr Morrison added: "He does get a bit frustrated now and again. He couldn't go running around straight away but he is now off the lead again."