The River Geirionydd (Afon Geirionydd in Welsh) is a river in Snowdonia, North Wales. It is a tributary of the River Crafnant (Afon Conwy), which flows into the River Conwy (Afon Conwy), which is the main river of the Conwy valley.
It flows from Llyn Geirionydd down a steep gorge before joining the River Crafnant. It is less than a mile in length.
The river passes the former Klondyke Mill, which used its waters. Klondyke was a mining and milling complex connected with some of the metal mines of the Gwydir Forest by means of a tramway which ran alongside Llyn Geirionydd. From above the Mill wagons used to enter the building via an aerial ropeway.