Its source is the Tal-y-llyn Lake (Welsh: Llyn Mwyngil) just south of the Cadair Idris massif and its mouth is in the Cardigan Bay area of the Irish Sea to the north of Tywyn.

The River Dysynni flows from the western end of Tal-y-llyn Lake in a southwesterly direction to its confluence with the Nant Gwernol at Abergynolwyn.

The upper course of the river follows a fault line but at Abergynolwyn must have been blocked by a major landslide, the scar from which can still be seen on the side of the valley.

The River Dysynni was thus forced to adopt a new course, and pushed its way through the hills northwestwards to find a new course in the parallel valley to the north.

It is a scenic valley, with the Cadair Idris massif to the north and the huge rock face of Craig yr Aderyn to the south.