Rhydderch ap Dyfnwal (fl. 971) was an eminent tenth-century Cumbrian who slew Cuilén mac Illuilb, King of Alba in 971. Rhydderch was possibly a son of Dyfnwal ab Owain, King of Strathclyde, and could have ruled as 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.