For other personages and places called Fergus, see Fergus.

In Irish mythology, Fergus (or Fearghus) mac Róich (or mac Róeg) is the former king of Ulster during the events of the Ulster Cycle. He was tricked out of the kingship by Ness, who made her seven-year-old son Conchobar mac Nessa king in his place.

After Conchobar's intended bride, Deirdre, had eloped with Naoise, Fergus was sent to offer the fugitive couple safe passage home. However Fergus was waylaid and Conchobar had Naoise and his brothers murdered before forcibly marrying Deirdre. Fergus, outraged by this violation of his word, took his followers into exile in Connacht, where they were taken in by Ailill and Medb. Fergus became Medb's lover, and he and his Ulster exiles fought on her side in the Táin Bó Cúailnge (Cattle Raid of Cooley), against his foster-son Cú Chulainn.

Fergus married Flidais, a deer-goddess, and was renowned for his sexual prowess and his enormous phallus (Fergus means "virility"; mac Róich means "son of a great horse"). He wielded the legendary sword, Caladbolg, which he once used to slice off the tops of three hills. He was killed at the instigation of Ailill out of jealousy for his affair with Medb.