The Morrígan or Mórrígan, also known as Morrígu, is a figure from Irish mythology. The name is Mór-ríoghan in modern Irish before the spelling reform, and it has been translated as "great queen" or "phantom queen".

The Morrígan is mainly associated with war and fate, especially with foretelling doom, death, or victory in battle. In this role she often appears as a crow, the badb. She incites warriors to battle and can help bring about victory over their enemies. The Morrígan encourages warriors to do brave deeds, strikes fear into their enemies, and is portrayed washing the bloodstained clothes of those fated to die. She is most frequently seen as a goddess of battle and war and has also been seen as a manifestation of the earth- and sovereignty-goddess, chiefly representing the goddess's role as guardian of the territory and its people.

The Morrígan is often described as a trio of individuals, all sisters, called "the three Morrígna". In mythology membership of the triad is given as Badb, Macha, and the Morrigan, who may be named Anu. It is believed that these were all names for the same goddess. In modern sources Nemain may also be named as one of the three Morrigan along with Badb and Macha, although her inclusion is unclear. The three Morrígna are also named as sisters of the three land goddesses Ériu, Banba, and Fódla. The Morrígan is described as the envious wife of The Dagda and a shape-shifting goddess, while Badb and Nemain are said to be the wives of Neit. She is associated with the banshee of later folklore.

There is some disagreement over the meaning of the Morrígan's name. Mor may derive from an Indo-European root connoting terror, monstrousness, cognate with the Old English maere (which survives in the modern English word "nightmare") and the Scandinavian mara and the Old East Slavic "mara" ("nightmare"); while rígan translates as "queen". This etymological sequence can be reconstructed in the Proto-Celtic language as *Moro-rīganī-s. Accordingly, Morrígan is often translated as "Phantom Queen". This is the derivation generally favoured in current scholarship.

In the Middle Irish period, the name is often spelled Mórrígan with a lengthening diacritic over the o, seemingly intended to mean "Great Queen" (Old Irish mór, "great"; this would derive from a hypothetical Proto-Celtic *Māra Rīganī-s). Whitley Stokes believed this latter spelling was due to a false etymology popular at the time. There have also been attempts by modern writers to link the Morrígan with the Welsh literary figure Morgan le Fay from the Matter of Britain, in whose name mor may derive from the Welsh word for "sea", but the names are derived from different cultures and branches of the Celtic linguistic tree.

The earliest sources for the Morrígan are glosses in Latin manuscripts and glossaries (collections of glosses). The 8th century O'Mulconry's Glossary says that Macha is one of the three morrígna. In a 9th-century manuscript containing the Vulgate version of the Book of Isaiah, the word Lamia is used to translate the Hebrew Lilith. A gloss explains this night hag as "a monster in female form, that is, a morrígan." Cormac's Glossary (also 9th century), and a gloss in the later manuscript H.3.18, both explain the plural word gudemain ("spectres") with the plural form morrígna.

The Morrígan's earliest narrative appearances, in which she is depicted as an individual, are in stories of the Ulster Cycle, where she has an ambiguous relationship with the hero Cú Chulainn. In the Táin Bó Regamna ("The Cattle Raid of Regamain"), Cú Chulainn encounters the Morrígan, but does not recognise her, as she drives a heifer from his territory. In response to this perceived challenge, and his ignorance of her role as a sovereignty figure, he insults her. But before he can attack her she becomes a black bird on a nearby branch. Cú Chulainn now knows who she is, and tells her that had he known before, they would not have parted in enmity. She notes that whatever he had done would have brought him ill luck. To his response that she cannot harm him, she delivers a series of warnings, foretelling a coming battle in which he will be killed. She tells him, "It is at the guarding of thy death that I am; and I shall be."

In the Táin Bó Cúailnge ("The Cattle Raid of Cooley"), Queen Medb of Connacht launches an invasion of Ulster to steal the bull Donn Cuailnge; the Morrígan, like Alecto of the Greek Furies, appears to the bull in the form of a crow and warns him to flee. Cú Chulainn defends Ulster by fighting a series of single combats at fords against Medb's champions. In between combats, the Morrígan appears to him as a young woman and offers him her love and her aid in the battle, but he rejects her offer. In response, she intervenes in his next combat, first in the form of an eel who trips him, then as a wolf who stampedes cattle across the ford, and finally as a white, red-eared heifer leading the stampede, just as she had warned in their previous encounter. However, Cú Chulainn wounds her in each form and defeats his opponent despite her interference. Later, she appears to him as an old woman bearing the same three wounds that her animal forms had sustained, milking a cow. She gives Cú Chulainn three drinks of milk. He blesses her with each drink, and her wounds are healed. He regrets blessing her for the three drinks of milk, which is apparent in the exchange between the Morrígan and Cú Chulainn: "She gave him milk from the third teat, and her leg was healed. 'You told me once,' she said,'that you would never heal me.' 'Had I known it was you,' said Cú Chulainn, 'I never would have.'" As the armies gather for the final battle, she prophesies the bloodshed to come.