The addanc (also called adanc, addane, afanc, avanc, abhac, abac) is a lake monster from Welsh folklore that also appears in Celtic and British folklore. Its exact description varies; it is described alternately as resmbling a crocodile, beaver or dwarf, 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 in Bettws-y-Coed that was named for the creature.

The addanc 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. Its thrashings caused massive flooding which ultimately drowned all inhabitants of Britain save for two people, Dwyfan and Dwyfach, from whom the later inhabitants of the British Isles descended.

According to one version of the myth, one which locates it in Llyn Llion, Hugh Gadarn's oxen dragged the addanc out of the lake; once it was out of the water, it was powerless and could be killed. An alternate myth 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 laid. It was finally dragged away to the lake Cwm Ffynnon, or killed by Peredur.

Some later British legends ascribe the creature's death to King Arthur or to Percival (Peredur's name in the Arthurian legend).