Newcastle Emlyn (Welsh: Castellnewydd Emlyn) is a town straddling the border of the counties of Ceredigion and Carmarthenshire in west Wales and lying on the River Teifi; it is also a community entirely within Carmarthenshire. The community is bordered by the communities of Llangeler and Cenarth, both being in Carmarthenshire; and by Llandyfriog in Ceredigion. Adpar is the part of the town that lies on the Ceredigion side of the River Teifi. It was formerly called Trefhedyn and was an ancient Welsh borough in its own right.