In J.R.R. Tolkien's The Silmarillion, the Forest of Brethil was a cluster of woods bordering Dorthonion, which was probably originally part of Doriath.

It was to Brethil that the House of Haleth removed after dwelling east of the river Gelion. Some Drúedain also dwelt there.

The main feature of the forest was the hill of Amon Obel, upon which stood Ephel Brandir, the main settlement of the Haladin.

The river Taeglin crossed the forest.