Llanddeusant is a very sparsely populated community in the Black Mountain (range) of the Brecon Beacons National Park in Carmarthenshire, Wales. It is about 5 miles southeast of Llangadog. Llanddeusant lies within the Llansadwrn & Llangadog / Myddfai & Llanddeusant ward, which had a population of 2,412 at census 2001. The boundaries were changed and most of the population was shown under the Llangadog community. The remaining population at the 2011 census was 220 only. The name, meaning "church of two saints", is supposed to originate from the fact that Teilo and Saint David are believed to have met there. The 'Old Red Lion Inn' is now a Youth hostel.