Dùn an Achaidh, sometimes Anglicised as Dun Acha, is a dun located near the village of Acha on the Inner Hebridean island of Coll. It is considered the best preserved dun on the island. The site of Dùn an Achaidh is located at grid reference NM18335456. According to local tradition, the dun was the stronghold of, and named after, the son of a Norse king. The early 20th century antiquary Erskine Beveridge considered it as one of the four most interesting fortifications on Coll.

Coll is an island located west of the Isle of Mull in the Inner Hebrides of Scotland. Coll is known for its sandy beaches, which rise to form large sand dunes, for its corncrakes, and for Breacachadh Castle. It is in the council area of Argyll and Bute.