The Afon Senni is a river in Powys, Wales which rises in the Fforest Fawr section of the Brecon Beacons National Park and flows northwards for 11 km to a confluence with the River Usk at Sennybridge. The river rises as Blaen Senni to the north of the peak of Fan Nedd and east of Fan Gyhirych (51.8621°N, -3.5931°W), occupying a deep glacial trough until it reaches the hamlet of Heol Senni. Before reaching Sennybridge (51.9455°N, -3.5723°W), it flows past the village of Defynnog, where it is joined on its left bank by its sole significant tributary, the Nant Treweryn (or Treweren). The river's name probably derives from a personal name 'Senni'. The river is designated as a special area of conservation in respect of its population of three species of lamprey, twaite shad, European bullhead, Atlantic salmon and otter.