Llywarch ap Hyfaidd (died c. 904) was the king of Dyfed, Wales until its conquest in 904 or 905 by Cadell ap Rhodri and his son Hywel Dda, rulers of Seisyllwg. Kingship passed briefly to his brother, Rhodri, until rule was consolidated by Hywel. Hywel married Llywarch's daughter, Elen, to legitimise his claim to the throne. He subsequently merged Seisyllwg and Dyfed into the new kingdom of Deheubarth.