In Welsh mythology and folklore, Cŵn Annwn (Welsh pronunciation: [kuːn ˈanʊn], "hounds of Annwn"; singular Ci Annwn (Welsh pronunciation: [kiː ˈanʊn]) were the spectral hounds of Annwn, the otherworld of Welsh myth. They were associated with a form of the Wild Hunt, presided over by either Arawn, king of Annwn in Pwyll Pendefig Dyfed (Pwyll, Prince of Dyfed), the First Branch of the Mabinogi and alluded to in Math fab Mathonwy (Math, the son of Mathonwy) the Fourth Branch of the Mabinogi, or by Gwyn ap Nudd as the underworld king and king of the fair(y) folk is named in later medieval lore.

In Wales, they were associated with migrating geese, supposedly because their honking in the night is reminiscent of barking dogs.

Hunting grounds for the Cŵn Annwn are said to include the mountain of Cadair Idris, where it is believed "the howling of these huge dogs foretold death to anyone who heard them".

According to Welsh folklore, their growling is loudest when they are at a distance, and as they draw nearer, it grows softer and softer. Their coming is generally seen as a death portent.