Dinefwr Castle (sometimes anglicized as Dynevor) is a Welsh castle overlooking the River Tywi near the town of Llandeilo, Carmarthenshire, Wales. It lies on a ridge on the northern bank of the Tywi, with a steep drop of one hundred feet to the river. Dinefwr was the chief seat of the Principality of Deheubarth. The castle is a Grade I listed building.