Afon Tanat (River Tanat) is a river in northern Powys, Wales. Its source is close to the Cyrniau Nod mountain, to the north of Lake Vyrnwy. The river flows in a generally east-south-east direction until it joins the River Vyrnwy near Llansantffraid-ym-Mechain. For a short distance prior to its confluence it flows within western Shropshire, England.
Its tributaries include the Afon Eirth, Afon Rhaeadr, Afon Iwrch and Afon Goch.
Coordinates: 52°47′N 3°07′W / 52.783°N 3.117°W