The Afon Ysgethin is a short river in Gwynedd, Wales. Flowing entirely within Snowdonia National Park it rises beneath the peaks of Y Llethr and Diffwys within the Rhinogs mountain range and runs in a generally ESE direction towards Cardigan Bay.

Headwater streams flow into a small upper lake, Llyn Dulyn, and Llyn Bodlyn reservoir. Originally a natural lake, Llyn Bodlyn was enlarged in 1894 to provide Barmouth with water. The river emerging from the reservoir is crossed by a historic track at Pont Scethin. This old stone arch bridge ("probably 18th century") was once used by packhorses and drovers moving between Harlech and London.

A further 3 km downstream the river enters a narrower wooded valley and is crossed by Pont Fadog. An inscription on the bridge states that it was improved in 1762 by a mason called H. Edward, who was commissioned by William Vaughan, a member of the powerful owner family of nearby Cors-y-gedol hall (see Dyffryn Ardudwy).

There are two further crossings at the village of Tal-y-bont: by the A496 road and by the Cambrian Coast railway. After another 1 km, the Afon Ysgethin enters the sea.

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  Llyn Bodlyn
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  Pont Scethin and Moelfre
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  Pont Fadog
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  Inscription on Pont Fadog: W.V. 1762. H. Ed. saer (saer: mason)
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  Coed Cors-y-gedol, the small "rain forest" below Pont Fadog

52°46′36″N 4°06′53″W / 52.77677°N 4.11480°W / 52.77677; -4.11480