Briccriu (Bricriu, Briccirne, Bricne), is a warrior, poet and troublemaker in the Ulster Cycle of Irish Mythology.

He once held a lavish feast for Conchobar mac Nessa and the heroes of Ulster in his house at Dún Rudraige (modern Dundrum, County Down), but knowing his reputation the Ulstermen had to be threatened to attend. First Briccriu threatened to set the Ulster warriors at odds with each other, then to set father against son and mother against daughter, but the Ulstermen finally agreed to come when he threatened to set the two breasts of each Ulster woman beating against each other.

At the feast he promised the "champion's portion" to Cúchulainn, then to Conall Cernach, then to Lóegaire Búadach, and the three heroes proceeded to compete for the honour. Challenges were set, some judged by Ailill and Medb of Connacht, some by Cú Roí of Munster. At every test set Cúchulainn came out top, but neither Conall nor Lóegaire would accept the decision.

Finally Cú Roí, disguised as a giant churl, challenged each of the three to behead him, then allow him to return and behead them in return. Only Cúchulainn passed this test, and was judged worthy of the champion's portion.

(Two motifs in this story, the champion's portion and the beheading challenge, are mentioned by the Greek writer Posidonius as practices of the ancient Celts. The beheading challenge is also central to the Middle English Arthurian poem Sir Gawain and the Green Knight.)

Briccriu followed Fergus mac Róich into exile in Connacht following the Deirdre affair. He died there at the end of the Táin Bó Cuailnge (Cattle Raid of Cooley), trampled to death by the two bulls as they fought.

The townland of Loughbrickland , near Banbridge, County Down, is thought to derive from Irish Loch Briccrend or Briccriu's Lake.