Lochan na h-Achlaise is an irregular shaped, somewhat triangular or heart shaped, freshwater loch on Rannoch Moor, in the Argyll and Bute area of the Scottish Highlands.

The loch (lake) is situated between The Black Mount and Rannoch Moor. The loch is separated from Loch Ba by the A82 road to Fort William. The scenery is considered one of the most atmospheric ones in Scotland. Most of Rannoch Moor is marshland and is hard to access.