Llywarch ap Hyfaidd (died) was a king of Dyfed, an independent kingdom in southwest Wales.[1] He was the son of Hyfaidd ap Bleddri.Sometime after his death, Dyfed became part of the new kingdom of Deheubarth, ruled by Hywel Dda who was married to Llywarch's daughter Elen.[2]
Upon Llywarch's death, the kingdom passed briefly to his brother Rhodri, but Hywel soon consolidated his rule, eventually merging Dyfed with his paternal inheritance as the new kingdom of Deheubarth. Later Welsh tradition held that Hywel inherited Dyfed peacefully through his supposed marriage to Llywarch's daughter Elen in a manner similar to the stories told about his great-grandfather Merfyn's acquisition of Gwynedd, his grandfather Rhodri's acquisition of Powys, and his father's acquisition of Ceredigion, all of this despite female inheritance of land having no place in the Welsh law of the period. However, the repeated attacks of Cadell and Hywel were recorded in Asser's Life of King Alfred, where it states Hyfaidd was replaced by his brother Rhodri, although the cause of his death is unknown.