Castle Sween, also known as Caisteal Suibhne, and Caistéal Suibhne, is located on the eastern shore of Loch Sween, in Knapdale, south of the forestry village of Achnamara on the west coast of Argyll, Scotland. Castle Sween is thought to be one of the earliest stone castles built in Scotland, having been built in the late 11th century. The castle's towers were later additions to wooden structures which have since vanished.