1. [Welsh rhi, ruler; dyrch, exalted (?)]. Sixth-century (d. 570) Welsh king of Strathclyde in what is now Scotland; sometimes bearing the epithets Hen [old] and Hael [generous]. Cited in the Triads as one of the three most generous men in the Isle of Britain, along with his cousins Nudd Llaw Ereint and Mordaf, he is much associated with early stories of Myrddin. He defeated Gwenddolou fab Ceido at the battle of Arfdderydd (573/5). St Kentigern found the ring in the salmon for Rhydderch. He is now identified with the latinized figures of Rodarchus and Rederech.

2. See WHITE BOOK OF RHYDDERCH.