Ystradgynlais is a town located in southwest Powys, Wales, situated on the River Tawe. Historically, it was positioned within the boundaries of the former county of Brecknockshire. It serves as the second-largest town in the county of Powys.
The community includes the areas of Cwmtwrch, Abercraf, and Cwmgiedd. According to the 2011 census, the town had a population of 8,092, while the broader Ystradgynlais subdivision, which forms part of the Swansea Urban Area, recorded a population of 10,248. The town is characterized by a high proportion of Welsh language-speakers.