Llywarch ap Hyfaidd was a king of Dyfed, an independent kingdom in southwest Wales. He was the son of Hyfaidd ap Bleddri and is thought to have inherited the kingdom of Dyfed after his father's death in c. 892. Sometime soon after Llywarch's death at the beginning of the tenth century, Dyfed became part of the new kingdom of Deheubarth, ruled by Hywel Dda who was married to Llywarch's daughter Elen.