Quarter Bach (Welsh: Cwarter Bach) is a community located in the east of Carmarthenshire, Wales.

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 Cefnbrynbrain, Rhosaman 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.

According to the 2001 Census, 75.2% of people in Quarter Bach can speak Welsh, the highest percentage of any ward in Carmarthenshire and indeed in the southern half of Wales.