Geoffrey Plantagenet (September 23 1158 – August 19 1186) was Duke of Brittany between 1181 and 1186, through his marriage with the heiress Constance. Geoffrey was the fourth son of King Henry II of England and Duchess Eleanor of Aquitaine. After a relative uneventful period as Duke, Geoffrey died suddenly in Paris, stamped to death by his horse, after a fall during a tournament. Constance succeeded him and ruled Brittany until 1196, when she abdicated to their son Arthur.

During his life, Geoffrey was also the presumptive heir of his older brother Richard, the Lionheart, who remained single at the time. His premature death opened the way of John Lackland to the throne of England, which he occupied in 1199. Geoffrey's son Arthur of Brittany, fought his uncle John for the throne and was subsequently murdered. Geoffrey's daughter Eleanor was imprisoned at Corfe and died in 1241 still a prisoner of the crown.

See also: Dukes of Brittany family tree – British monarchs family tree – Other politically important horse accidents