Quarter Bach is a community located in Carmarthenshire, Wales. Prior to 23 December 1881, the area was a part of the parish of Llangadog. It is at the foot of the Black Mountain, in the far east of the county. The main settlement here is Upper Brynamman, though it also includes , and Ystradowen, as well as a substantial amount of open moorland. The community is bordered by the communities of: Cwmamman; Llangadog; and Llanddeusant, all being in Carmarthenshire; by Ystradgynlais in the unitary authority of Powys; and by Cwmllynfell and Gwaun-Cae-Gurwen in the unitary authority of Neath Port Talbot.