Yr Arddu is a 389m high mountain peak in Snowdonia, Wales. It lies approximately 2.5km south east of the village of Beddgelert and just over 1km from the rather more famous mountain Cnicht. It is a rugged hill with widespread rock outcrops and cliffs. A small lake, Llyn yr Arddu, is located on the flanks of the mountain.
The craggy nature of the summit is due to the presence of lavas and tuffs (volcanic ash deposits) dating from the Ordovician period when Snowdonia was characterised by numerous volcanic eruptions; Yr Arddu is a particularly good location to see the record of Ordovician volcanism. The Yr Arddu Tuffs may be the earliest eruptive phase of the Lower Rhyolitic Tuff Formation.