Cuilén mac Illuilb (died 971) was an early mediaeval King of Alba. He was a son of Illulb mac Custantín, King of Alba, and member of Clann Áeda meic Cináeda, a branch of the Alpínid dynasty. During the tenth century, the Alpínids rotated the kingship of Alba between two main dynastic branches. Dub mac Maíl Choluim, a member of a rival branch of the kindred, seems to have succeeded after Illulb's death in 962. Cuilén soon after challenged him but was defeated in 965. Dub was eventually expelled and slain in 966/967. Whether Cuilén was responsible for his death is uncertain.
Following Dub's fall, Cuilén appears to have ruled as undisputed king from 966–971. Little is known of Cuilén's short reign other than his own death in 971. According to various sources, he and his brother, Eochaid, were slain by Britons. Some sources identify Cuilén's killer as Rhydderch ap Dyfnwal, a man whose daughter had been abducted and raped by the king. Rhydderch was evidently a man of eminent standing, and seems to have been a son of Dyfnwal ab Owain, King of the Cumbrians, and could have possibly ruled the Kingdom of the Cumbrians at the time of Cuilén's death.
After Cuilén's assassination, the kingship of Alba may have been assumed by another member of Clann Áeda meic Cináeda, Cináed mac Maíl Choluim, a man who appears to have launched a retaliatory raid against the Cumbrians. There is evidence indicating that Cináed faced considerable opposition from Cuilén's brother, Amlaíb, a man who was accorded the title King of Alba in Irish sources recording his death at Cináed's hands in 977. Cuilén's son, Custantín, eventually succeeded Cináed as king. There is evidence to suggest that Cuilén had another son, Máel Coluim.