The Afon Tarell (Welsh for 'River Tarell') is a 20-kilometre (12.4-mile) river in Powys, Wales, rising to the west of the Brecon Beacons in Brecon Beacons National Park at an elevation of approximately 562 metres near Pen y Fan, and flowing north then north-east before joining the River Usk at Llanfaes on the southern edge of Brecon, at an elevation of around 135 metres.

The river's upper reaches pass through open moorland over red sandstone geology. As it descends into the Tarell valley, the underlying rock changes to limestone, and the river becomes more enclosed, running through dense deciduous woodland of beech, ash, oak, and willow. Throughout this wooded section the river races over rocky, shallow stretches of riverbed. The entire length of the valley is followed by the A470 road, passing through the hamlets of Libanus and Tai'r Bull. The most significant tributary is Nant Cwm Llwch, a crystal-clear mountain stream filtered through black peat that emerges from the glacial lake Llyn Cwm Llwch beneath Pen y Fan, joining the Tarell on its right bank between Tai'r Bull and Llanfaes.

The Afon Tarell is designated as a Special Area of Conservation for its populations of three species of lamprey, twaite shad, European bullhead, Atlantic salmon, and otter. Dippers are also noted along the river. These conservation designations are worth bearing in mind when planning any paddling activity on the river.

The river is characterised as a small, technically demanding watercourse. Angling descriptions note it as "hard fishing" and "over-shaded," with a rocky, shallow riverbed — conditions that suggest a challenging, low-volume stream rather than a straightforward paddle. The wooded middle and lower sections become more inaccessible as the river progresses through the valley. Access to at least one point near Libanus is via a public right of way beside a chapel, leading to a bridge over the river.