Rhydderch ap Dyfnwal (fl. 971) was an eminent tenth-century Cumbrian, and possibly a son of Dyfnwal ab Owain, King of Strathclyde. Rhydderch appears on record in about 971, when he is said to have killed Cuilén mac Illuilb, King of Alba, a man said to have abducted and raped Rhydderch's daughter. Following Cuilén's death, the Cumbrian Kingdom of Strathclyde endured an invasion by Cuilén's successor, Cináed mac Maíl Choluim, King of Alba. This Scottish attack could have been a retaliatory raid for Rhydderch's actions, and may have been undertaken in the context of restoring Scottish authority over the Cumbrian realm. If Rhydderch ever ruled as king it must have been before 973, when Dyfnwal's son, Máel Coluim, is accorded the title king.
Rhydderch flourished during the reign of Cuilén mac Illuilb, King of Alba (died 971). The latter's undisputed reign as King of Alba seems to have spanned from 966 to 971, and appears to have been relatively uneventful. Cuilén's death in 971 is noted by several sources. According to the ninth–twelfth-century Chronicle of the Kings of Alba, he and his brother, Eochaid (died 971), were killed by Britons. The fifteenth–sixteenth-century Annals of Ulster also reports that Cuilén fell in battle against Britons, whilst the twelfth-century Chronicon Scotorum specifies that Britons killed him within a burning house. The Chronicle of the Kings of Alba locates Cuilén's fall to "Ybandonia". Although this might refer to Abington in South Lanarkshire, a more likely location may be preserved by the twelfth–thirteenth-century Chronicle of Melrose. This source states that Cuilén was killed at "Loinas", a placename which seems to refer to either Lothian or the Lennox, both plausible locations for an outbreak of hostilities between Scots and Britons. In any event, the account of Cuilén's demise preserved by the twelfth-century Prophecy of Berchán is somewhat different. According to this source, Cuilén met his end whilst "seeking a foreign land", which could indicate that he was attempting to lift taxes from the Cumbrians. The Chronicle of the Kings of Alba identifies Cuilén's killer as Rhydderch, describing him as the son of a man named Dyfnwal, and further reports that Rhydderch slew Cuilén for the sake of his own daughter. The thirteenth-century Verse Chronicle, the Chronicle of Melrose, and the fourteenth-century Chronica gentis Scotorum likewise identify Cuilén's killer as Rhydderch, the father of an abducted daughter raped by the king.