Cathbad is the chief druid in the court of Conchobar mac Nessa in the Ulster Cycle of Irish Mythology.

In his younger days he was a warrior, leading a landless band of twenty-seven men. Once he led a raid on the house where the Ulster princess Ness was brought up, killing all twelve of her foster-fathers. Ness sought revenge, becoming a warrior and leading her own band of twenty-seven. However, one day when she went off on her own to bathe in a pool, Cathbad came between her and her weapons and demanded her as his wife. She had no choice but to agree.

Ness had a son, Conchobar mac Nessa, although whether his father was Cathbad or Fachtna Fáthach, the High King of Ireland and Ness's lover, the stories do not agree.

Cathbad had the ability to foretell the future. He was present at the birth of Deirdre and prophesied her tragic fate, but Conchobar ignored him. On another occasion, the young Cúchulainn overheard Cathbad prophesy that anyone who took arms on that day would have everlasting fame but a short life; he immediately ran to Conchobar and asked to be armed.