Llansteffan Castle (Welsh: Castell Llansteffan) is a privately owned castle in Llansteffan, Carmarthenshire, Wales, overlooking the River Towy estuary in Carmarthen Bay.

The castle sits on a much older Iron Age promontory fort, proving Llansteffan has been inhabited for several millennia. The hill where the castle stands commands the River Towy estuary. The hill would have been stripped of trees so that foot soldiers were vulnerable to attack by archers.

The original earthworks can still be seen and were used as part of the modern castle's defence system—the castle proper rests within the earthwork rings.

The castle was built by the Normans after 1100 as part of their invasion of Wales and granted to the Marmion family before passing to the Camvilles through marriage.

It was captured by Maredudd ap Gruffydd in 1146 against the forces of Maurice FitzGerald, Lord of Lanstephan and his brother William FitzGerald, Lord of Emlyn who were the leading Norman settlers of the region, members of the Fitzgerald dynasty.

The castle was retaken by the Normans in 1158. Llywelyn the Great recaptured the castle for the Welsh in 1215 and taken back by the Camville family sometime after 1223. The castle fell to Llywelyn ap Gruffudd in 1257 but returned to the Camvilles by the 1260s.

By 1367, it was described as in a poor state.

The castle was captured twice by the forces of Owain Glyndŵr in 1403 and c.1405. It was recaptured by Sir John Pennes in 1408. The castle was later granted to the Crown and the two-tower Gatehouse was converted into a residence.