Newcastle Emlyn (Welsh: Castellnewydd Emlyn) is a town and community on the River Teifi, in West Wales. The community is wholly in Carmarthenshire, but the built up area also includes Adpar north of the Teifi, which is in Ceredigion. Adpar was formerly called Trefhedyn and was historically a borough in its own right. Newcastle Emlyn grew around its castle, which dates from the 13th century. At the 2021 census the community had a population of 1,144. The built up area (including Adpar) had a population of 1,883 at the 2011 census.

The town takes its name from the cantref of Emlyn, an administrative district in medieval Dyfed.

Newcastle Emlyn Castle, was first mentioned in Brut y Tywysogion in 1215, when it was seized by Llewelyn the Great (Welsh: Llywelyn Fawr). It was captured by the Welsh during the revolt of 1287–1288 and also by Owain Glyndŵr in 1403.

Cawdor Hall was completed in 1892.

The Teifi Valley Railway is nearby, although the town has not had a passenger train service since 1952.

Newcastle Emlyn hosted the Urdd National Eisteddfod in 1981.

There are two tiers of local government covering Newcastle Emlyn, at community (town) and county level: Newcastle Emlyn Town Council (Cynfor Tref Castell Newydd Emlyn) and Carmarthenshire County Council (Cyngor Sir Gâr). The town council meets at Cawdor Hall on Market Square. North of the Teifi, Adpar forms part of the community of Llandyfriog in Ceredigion. As well as Llandyfriog, the Newcastle Emlyn community is also bordered by those of Llangeler and Cenarth, both of which are in Carmarthenshire.

Newcastle Emlyn was historically part of the ancient parish of Cenarth. Newcastle Emlyn and Adpar have always been administratively separate for local government purposes, although between 1832 and 1885 Newcastle Emlyn was in the Adpar parliamentary borough, which formed part of the Cardigan Boroughs constituency.