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.