Arenig Fach is a Hewitt of 688.8 m in Gwynedd. It sits below Arenig Fawr, only 4.7 km to the south and 165.2 m higher.
Its nearest higher summit, Moel Llechwedd (817 m, 4.4 km south), is a listed top not profiled here. The summit is marked by a trig point. It falls on two OS 1:50,000 sheets, 124 and 125.