In Irish mythology Medb (Medhbh, Maeve) is queen of Connacht during the events of the Ulster Cycle. Her father was Eochaid Feidlech, the High King of Ireland. Her husband is Ailill, athough she had several husbands before him, all of whom were kings of Connacht while they were married to her. She was probably originally a sovereignty goddess, whom a king would ritually marry as part of his inauguration. A separate character, Medb Lethderg, performs a similar function in Tara. Her name is said to mean 'she who intoxicates', and is cognate with the English word 'mead'; it is likely that the sacred marriage ceremony between the king and the goddess would involve a shared drink.

Medb's first husband was Conchobar of Ulster, but that marriage didn't last. She then murdered Conchobar's next wife, her own sister Eithne, while she was pregnant. Eithne's son, Furbaide, was born by posthumous caesarian section. Conchobar later raped Medb after an assembly at Tara.

She demanded her husband satisfy her three criteria - that he be without fear, meanness or jealousy. The latter was particularly important as she had many lovers. She also insisted that she be equal in wealth with her husband, and started the Táin Bó Cúailnge (Cattle Raid of Cooley) when she discovered that Ailill was one powerful stud bull richer than her. Fighting on her side in that campaign was Fergus mac Róich, exiled former king of Ulster and one of Medb's lovers. It is reported that it took seven men to satisfy her, or Fergus once. She had Conall Cernach kill Ailill after he had contrived Fergus's death.

In her later years she often went to bathe in a pool on an island. Furbaide took a rope and measured the distance between the pool and the shore, and practiced with his sling until he could hit an apple on top of a stake Medb's height from that distance. The next time he saw Medb bathing he put his practice to good use and killed her with a piece of cheese in revenge for his mother.

Medb was the mother of Findabair by Ailill.