Llanwrthwl is a village in Powys, mid Wales. Llanwrthwl lies off the A470 road, north by road from Builth Wells and Newbridge-on-Wye and south of Rhayader. It lies on the River Wye and River Elan and the village is accessed by a bridge over the Wye. In 1833, its population was 517; its population in 1841 was 568; in 1845, its population was 563.