Rhaeadr Mawddach is a waterfall on the Afon Mawddach within Coed y Brenin Forest Park — "Forest of the Kings" — in Snowdonia, north Wales. After rainfall it becomes a powerful cascade, dropping over 35 metres in three distinct stages.

Although not as tall as the nearby Pistyll Cain on the Afon Gain, Rhaeadr Mawddach carries a considerably greater volume of water.

The waterfall sits close to the remains of the Gwynfynydd Gold Mine, and the two can be visited together on a moderate woodland trail that follows the steep-sided Mawddach valley north from the Tyddyn Gwladys car park.

Viewpoints exist both at river level, accessed via a path through the old mining site, and from above the falls along the forest track.