Clan Maclachlan, also known as Clan Lachlan, is a Highland Scottish clan that historically centred on the lands of Strathlachlan on Loch Fyne, Argyll on the west coast of Scotland. The clan claims descent from Lachlan Mor, who lived on Loch Fyne in the 13th century, and who has left his name upon the countryside he once controlled: places such as Strathlachlan, Lachlan Castle and Lachlan Bay. Tradition gives Lachlan Mor a descent from an Irish prince, Anrothan O'Neill. Clan Maclachlan has been associated with other clans, such as Clan Lamont, Clan MacEwen of Otter, Clan MacNeil of Barra, and the MacSweens: as all claim descent from Anrothan O'Neill who left Ireland for Kintyre in the 11th century. From this descent the clan claims a further descent from the legendary Niall Noigíallach, High King of Ireland, who lived from the mid 4th century to early 5th century.

The clan took part in the Jacobite Risings as loyal supporters of the Stuart kings of Scotland. The seventeenth chief of the clan was killed in the Battle of Culloden in 1746. Following the Jacobite defeat, a Government warship is said to have damaged the clan seat of old Castle Lachlan.