Ystradgynlais (English: /ˌʌstrədˈɡʌnlaɪs/;[2] Welsh: [ˌəstradˈɡənlais] ⓘ) is a town in southwest Powys, Wales.

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.

The first documented written evidence of iron working in the area was at Ynyscedwyn and is of a deed of release dated 1729.

Ystradgynlais hosted the 1954 National Eisteddfod, an annual Welsh festival of literature, dance, and music.

Dan yr Ogof caves are a short journey from the town centre, passing Craig-y-Nos Castle and country park.

In 2020 a Purple Plaque was installed at the building to mark Eunice Stallard's life and activity as a peace campaigner.

"[17] In 2016 The Stephen Lewis Tristars Aquathlon in Ystradgynlais won the Welsh triathlon event of the Year 2016.

[citation needed] National Cycle Route 43 passes by the southern edge of the town on the line of the former Swansea Vale Railway which linked Swansea via the Neath and Brecon Railway at Coelbren with Brecon.