Old Castle Lachlan, is a ruined 15th-century castle on the shore of Loch Fyne, within the Cowal Peninsula, in Argyll and Bute, Scotland. It was the stronghold of Clan MacLachlan until 1746 when it was attacked by British Government forces. New Castle Lachlan was built as a replacement in 1790, around 1 kilometre (0.62 mi) to the north-east.