The River Rhythallt (Welsh: Afon Rhythallt) is a river in Gwynedd, North Wales whose source is Llyn Padarn. It flows in a northwesterly direction past the village of Brynrefail, Gwynedd and changes its name to Afon Seiont downstream of Pont Rhythallt in Llanrug. The Afon Seiont eventually reaches the sea at Caernarfon.