Ystradgynlais (Welsh: [ˌəsdradˈɡənlais], English /ˌʌstrədˈɡʌnlaɪs/) is a town on the River Tawe in southwest Powys. The town, which is the largest in the historic county of Brecknockshire and the second largest in the administrative area of Powys, grew around the ironmaking, coal mining and watchmaking industries.