Lugaid Riab nDerg (Riabhdhearg, Réoderg, Sriab nDearg, "Red Stripes") was a legendary High King of Ireland. He was the foster-son of Cúchulainn.

He was a son of the three Findemna (fair triplets), sons of Eochaid Feidlech. The triplets went to war with their father to try and take the High Kingship. The night before the battle their sister Clothra tried to persuade them to call it off, but to no avail. They had no heirs, so she took all three to bed for fear they would die without issue, and Lugaid was conceived. Some texts say his epithet comes from the red stripes that divided his body into three, indicating that he had three fathers. Others, more prosaically, explain his stripes as battle scars.

Incest features further in Lugaid's story. He slept with Clothra himself, conceiving Crimthann Nia Náir.

His foster-father, Cúchulainn, split the Lia Fáil, the coronation stone at Tara which roared when the rightful king stood or sat on it, with his sword when it failed to roar under Lugaid. It never roared again except under Conn of the Hundred Battles.

Lugaid married Derbforgaill , a princess from Scandinavia. She had come to Ireland in the form of a swan to seek out Cúchulainn, whom she loved, but Cúchulainn shot her down with a stone from his sling which penetrated her womb, and in sucking it out he violated a taboo which meant he could not marry her himself. Instead, he gave her to Lugaid.

One winter the women of Ulster held a competition in which they tried to send their urine furthest into a pillar of snow, saying that the winner would be the most sexually attractive. Derbforgaill won, and out of jealousy the other women beat and mutilated her. When Lugaid arrived home he noticed that the snow on the roof of her house had not melted, and realised she was close to death. He arrived in time to see her die, and died of grief himself.

See Lugaid for other figures of the same name, and Lug for the god the name derives from.