Fred, a 10-year-old labrador has become "dad" to the baby birds at Mountfitchet Castle, a tourist attraction near Stansted Airport in Essex.
The ducklings' mother was nowhere to be found when staff noticed them waddling around alone, but "Fred... immediately took to them and has been babysitting ever since."
They have even followed him into the castle moat for a swim, staff said.
The reconstructed Norman castle is home to a number of rescued animals, but Fred is "the resident dog".
A spokeswoman said staff were "very worried" when the ducklings' mother disappeared on Thursday, leaving them "looking a little bit lost".
But Fred, who belongs to one of the attraction's owners, Jeremy Goldsmith, stepped in to help.
"We brought the ducklings into the house as they are too young to fend for themselves, and Fred just took them under his paw - rather than his wing," said Mr Goldsmith.
"He's got a lovely nature and has grown up around rescued animals."
The ducklings sleep in the dog basket with Fred and follow him around as though he was their mother.
"They absolutely adore him and he has now resigned himself to being a stay-at-home dad looking after the nine baby ducklings," Mr Goldsmith said.
Mr Goldsmith said he only expected Fred's babysitting services to last another few weeks, after which the ducklings will be free to stay at the castle or leave.