In Arthurian legend, Morgause (also known as Anna-Morgause) is the half-sister of King Arthur who slept with him and produced Mordred, the incestuous heir that would lead to Camelot's downfall. She is the daughter of the Duke of Cornwall and Lady Igraine. She is married to King Lot, an enemy of Arthur in the rebellions following Arthur's coronation.

She has four siblings:

• Gormant , her brother
• Elaine, her sister
• Morgan le Fay, her sister
• Arthur Pendragon, her half-brother from her mother's affair with Uther Pendragon

She has five children, all sons. From eldest to youngest they are:

• Gawaine, who becomes one of King Arthur's greatest knights
• Agravaine
• Gaheris
• Gareth
• Mordred, her son with her half-brother

Her children play key roles in the story of Arthur and his kingdom. She is active in Le Morte d'Arthur.

In modern variations, the defining role of Morgause (that of sleeping with her brother) is often assigned to Morgan le Fay and the character of Morgause sometimes ignored entirely.