Castle MacEwen (Scottish Gaelic: Caisteal Mhic Eoghainn) is a ruined fort on the east shore of Loch Fyne in Argyll, Scotland. It is near Kilfinan, around 5 kilometres (3.1 mi) south of Otter Ferry. The site was a medieval dun, which was superseded by a promontory fort, and later by a medieval homestead. The site is linked to the Clan MacEwen who are associated with the Loch Fyne area.