In the Ulster Cycle of Irish mythology, Cú Roí (Cú Ruí, Cú Raoi) mac Dáire is a king of Munster and a sorcerer who can change his form at will. His name probably means "hound of the battlefield". He is thought by some to be a euhemerised god.

When Cúchulainn, Conall Cernach and Lóegaire Búadach were incited to compete for the champion's portion by Briccriu, Cú Roí was one of those who judged between them. Like all the other judges, he chose Cúchulainn, but Conall and Lóegaire refused to accept his verdict. When the three heroes returned to Ulster, Cú Roí appeared to each in the guise of a hideous churl and challenged them to behead him, then allow him to return and behead them. Only Cúchulainn was brave and honourable enough to submit himself to the churl's axe, so he was declared champion.

In exchange for his choice of the spoils, Cú Roí joined Cúchulainn on a raid of Inis Fer Falga (possibly the Isle of Man), again in disguise. They stole treasure and abducted Blathnat , daughter of the king of the island, who loved Cúchulainn. But when Cú Roí was asked to choose his share, he chose Blathnat. Cúchulainn tried to stop him taking her, but Cú Roí cut his hair and drove him into the ground up to his armpits, before escaping, taking Blathnat with him.

Later, Blathnat betrayed Cú Roí to Cúchulainn, who beseiged his fort and killed him. In one version of the story, Cú Roí's soul was hidden in an apple in the belly of a salmon which lived in a stream in the mountains of Slieve Mish , and only surfaced once every seven years; Blathnat discovered the secret and told Cúchulainn, who killed the fish, enabling him to kill Cú Roí. However Ferchertne, Cú Roí's poet, enraged at the betrayal of his lord, grabbed Blathnat and leaped off a cliff, killing her and himself.

Cú Roí's nephew, Conganchnes, tried to avenge him, but was killed by Celtchar. His son, Lugaid mac Con Roí, later succeeded in avenging him by killing Cúchulainn, but was himself killed by Conall Cernach.

The Iron Age ruin of Caherconree (Irish Cathair Con Raoi, Cú Roí's castle) in the mountains of Slieve Mish, on the Dingle Peninsula, County Kerry, preserves Cú Roí's name.