Ystradgynlais is a town in southwest Powys, Wales. It is located on the River Tawe, and was within the boundaries of the former county of Brecknockshire. The town has a high proportion of Welsh language-speakers. The community includes Cwmtwrch, Abercraf and Cwmgiedd, with a population of 8,092 in the 2011 census; it is the second-largest town in Powys. It forms part of the Swansea Urban Area where the Ystradgynlais subdivision has a population of 10,248.