Merfyn ap Rhodri (died c. 900) was a late 9th-century Aberffraw prince of Gwynedd. He is sometimes credited with ruling Powys after the death of his father Rhodri the Great in AD 878. In the accounts where he is credited as a king, he is reported to have lost his realm to an invasion by his brother Cadell, King of Ceredigion. Merfyn's death may be connected to the incursion into Anglesey by the Viking Ingimundr in the first decade of the 10th century.
The drowning of his son Haearnddur, or "Haardur", was reported by both the Chronicle of the Princes and the Annals of Wales. The first places it in the year 953; Phillimore's reconstruction of the latter's dating would place it in 956.
- Middle Welsh: Oed Crist 953, y boddes Haearnddur fab Merfyn. Cambrian Archaeological Association. Archaeologia Cambrensis: Chronicle of the Princes, p. 24. W. Pickering, 1864. Accessed 19 Feb 2013.
- Medieval Latin: haardur .f(ilius). meuruc mersus est. Annals of Wales (B text), p. 10. (in Latin)
- Harleian MS. 3859. Op. cit. Phillimore, Egerton. Y Cymmrodor 9 (1888), pp. 141–83. (in Latin)