The Afanc (Welsh pronunciation: [ˈavank], sometimes also called Addanc, [ˈaðank]) is a lake monster from Welsh mythology. Its exact description varies; it is described variously as resembling a crocodile, beaver or dwarf-like creature, and is sometimes said to be a demon. The lake in which it dwells also varies; it is variously said to live in Llyn Llion, Llyn Barfog, near Brynberian Bridge or in Llyn yr Afanc, a lake near Betws-y-Coed that was named after the creature.

The afanc was a monstrous creature that, like most lake monsters, was said to prey upon any foolish enough to fall into or swim in its lake.

One of the earliest descriptions of it is given by the 15th century poet Lewys Glyn Cothi, who described it as living in Llyn Syfaddon or Llangorse Lake, now in Powys.

One tale relates that it was rendered helpless by a maiden who let it sleep upon her lap; while it slept, the maiden's fellow villagers bound the creature in chains. The creature was awakened and made furious; its enraged thrashings crushed the maiden, in whose lap it still lay. It was finally dragged away to the lake Cwm Ffynnon, or killed by Peredur.