Dinefwr Castle is a ruined castle overlooking the River Towy near the town of Llandeilo, Carmarthenshire, Wales. It lies on a ridge on the northern bank of the Towy, with a steep drop of one hundred feet to the river. Dinefwr was the chief seat of the Dinefwr dynasty of the Kingdom of Deheubarth. The castle is a Grade I listed building.

Llandeilo is a town and community in Carmarthenshire, Wales, situated at the crossing of the River Towy by the A483 on a 19th-century stone bridge. Its population was 1,795 at the 2011 Census. It is adjacent to the westernmost point of the Brecon Beacons National Park. The town is served by Llandeilo railway station on the Heart of Wales Line.