The B4518 road is a road in Powys, central Wales, with a total length of . It begins at in Rhayader near the junction of the A470 road and the A44 road and leads eventually to the A470 again at Llanbrynmair at . En route going north from Rhayader it passes through the following settlements: St Harmon, Pant-y-Dwr, Tylwch, Llanidloes (where it crosses the A470 again), Staylittle, Pont Crugnant, Pennant, Bont Dolgadfan, Llan and Plas Esgair. It passes through the Clywedog Valley and Clywedog Reservoir, built in 1964.