Ystradgynlais (Welsh: [ˌəstradˈɡənlais], ) is a town on the River Tawe in southwest Powys, Wales. It is the second-largest town in the principal area and county of Powys. Ystradgynlais is in the historic 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; the built-up area population was 10,248.