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.

Rhys ap Gruffydd or ap Gruffudd was the ruler of the kingdom of Deheubarth in south Wales from 1155 to 1197. Today, he is commonly known as The Lord Rhys, in Welsh Yr Arglwydd Rhys, although this title may have not been used in his lifetime. He usually used the title "Proprietary Prince of Deheubarth" or "Prince of South Wales", but two documents have been discovered in which he uses the title "Prince of Wales" or "Prince of the Welsh". Rhys was one of the most successful and powerful Welsh princes, and, after the death of Owain Gwynedd of Gwynedd in 1170, the dominant power in Wales.