The Afon Tanat (River Tanat) is a river in northern Powys, Wales, rising in the glaciated upland valleys of the Berwyn Mountains. It originates at the confluence of Cwm Rhiwarth and Cwm Pennant streams near Llangynog, then flows eastward through a narrow valley before joining the River Vyrnwy, itself a tributary of the River Severn. For a short distance prior to its confluence with the Vyrnwy, near Llansantffraid-ym-Mechain, the river passes through western Shropshire, England. The valley is characterised by steep-sided walls less than 1 km wide, with valley floors around 150 m above sea level rising to surrounding hills of 300 m. The river's gravel-cobble bed and pool-riffle structure support water crowfoot and wildlife including otters.

The river is graded II and is considered a beautiful touring river. A popular paddling route runs approximately 16 km from Llangynog to Llanyblodwel, suitable for intermediate paddlers and typically taking 3 to 5 hours depending on water levels. Water levels must reach at least 2 feet over the road at the put-in for the river to be navigable. The access point is off the B4391 Bala to Pant road. No portages are required along the route. The total navigable length is noted as 24 km on some accounts. Nearby rivers include the Vyrnwy and Severn. The relevant OS maps are sheets 125 (Bala & Lake Vyrnwy) and 126 (Shrewsbury). The Access Officer for the river is Dave Proctor, Capel Bethania, Cwm Cownwy, Llanwddyn, SY10 0NJ, reachable at 01691 870615.

The Tanat Valley features enclosed fields, small farms, and medieval settlements shaped by its glacial history. The river has been noted as a possible inspiration for Tolkien's work on The Lord of the Rings. Angling is also popular on the river, with the Upper Tanat Fishing Club managing approximately 7 miles of water for brown trout and grayling, which are among the top fish species present alongside European grayling. Paddlers should be aware that angling activity may be present along the river corridor.