Arenig Fach is a mountain in southern Snowdonia, north Wales. It is a peak in the Arenig range and lies to the north of its parent peak Arenig Fawr.

It lies close to Llyn Celyn alongside the A4212 between Trawsfynydd and Bala. Carnedd y Filiast and Foel Goch (Arenigs) is to the east, while Moel Llyfnant, Gallt y Daren and Arenig Fawr are to the south.

The summit itself is known as Carnedd Bachgen, named after the ancient cairn, Carnedd y Bachgen (which lies just north-east of the summit), and is the highest point at the north end of a wide plateau. There is a trig point and a substantial shelter cairn. The mountain rises from the Migneint moorland and looks like a bulky heathery "lump" from most directions. However, its north-eastern face has steep cliffs with a small corrie lake, Llyn Arenig Fach, at their bottom. The last glacier occupation of this corrie is thought to have happened during the Loch Lomond Stadial.