The Scottish SPCA believe the animals may have been the victims of the puppy farming industry.
An undercover officer with the animal charity said they believed the pups were aged between six and 12 weeks and they were possibly a Spaniel breed.
They were discovered by in a field next to a layby on the A77 between Cairnryan and Ballantrae on 18 November.
Warning: there is a photo of the dead puppies further down this story. Some people may find it disturbing.
The Scottish SPCA officer said: "The A77 is the main link between Scotland and Northern Ireland, which is where the majority of puppy farm suppliers are based.
"The central belt of Scotland is also where the majority of potential customers are."
Earlier this year a BBC Scotland investigation exposed the illegal trade.
The officer added: "We suspect they were part of a shipment from Northern Ireland and tragically died during the ferry journey before being callously dumped.
"We are currently awaiting the results of a post-mortem examination to establish the cause of death.
"Anyone with information is being urged to contact our Animal Helpline on 03000 999 999."
The Scottish SPCA has urged people to be vigilant if they buy a puppy this Christmas.
The officer said: "Trading puppies from puppy farms is an abhorrent practice and those involved have no regard for the welfare of the animals involved.
"We are very concerned that many people will be unwittingly supporting the puppy farming industry this Christmas by buying from unscrupulous breeders and sellers.
"The pups are often bred and kept in poor conditions, resulting in disease and infection.
"While the puppy may appear fine when purchased, problems usually begin to show shortly afterwards, by which point money has already exchanged hands and the seller is long gone."
The charity issued the following advice on buying a new dog:
Four dead puppies have been found dumped in a field in Dumfries and Galloway.