In Irish mythology, Mac Gréine of the Tuatha Dé Danann was a son of Cermait, son of the Dagda.

He and his brothers Mac Cuill and Mac Cecht killed Lug in revenge for their father. The three brothers became joint High Kings of Ireland, rotating the sovereigty between them a year at a time, covering twenty-nie or thirty years depending on the source consulted. They were the last kings of the Tuatha Dé Danann before the coming of the Milesians.

Mac Gréine's given name was Céthur. He was named Mac Gréine after his god, Grian, the sun. His wife was Ériu.