The Afon Clarach () is a small river discharging into Cardigan Bay north of Aberystwyth. It flows through a largely agricultural and forested landscape except in its lowest reaches where its banks are crowded with holiday developments. The Clarach is formed at the confluence of the Afon Stewi and Nant Seilo at Penrhyncoch, with both their headwaters originating in low hills at the edge of the Plynlimon range. The course of the river takes it past the Institute of Biological, Environmental and Rural Sciences (a part of Aberystwyth University), and the village of Bow Street. Between Bow Street and Llangorwen, the river is joined by the Bow Street Brook.