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.