Newcastle Emlyn is a town on the River Teifi, straddling the counties of Ceredigion and Carmarthenshire in West Wales. It is also a community entirely within Carmarthenshire, bordered by those of Llangeler and Cenarth, also in Carmarthenshire, and by Llandyfriog in Ceredigion. Adpar is the part of town on the Ceredigion side of the River Teifi. It was formerly called Trefhedyn and was an ancient Welsh borough in its own right. The area including Adpar had a population of 1,883 according to the 2011 census.

The River Teifi in Wales forms the boundary for most of its length between the counties of Ceredigion and Carmarthenshire, and for the final 3 miles (4.8 km) of its total length of 76 miles (122 km), the boundary between Ceredigion and Pembrokeshire. Its estuary is northwest of Cardigan, known in Welsh as Aberteifi, meaning 'mouth of the Teifi'. Teifi has formerly been anglicised as "Tivy".