The Lieutenant of the Duchy of Aquitaine was an officer charged with governing the Duchy of Aquitaine on behalf of the King of England. Unlike the seneschalcy of Gascony, the lieutenancy was not a permanent office. Lieutenants were appointed in times of emergency, due either to an external threat or internal unrest. The lieutenant had quasi-viceregal authority and so was usually a man of high rank, usually English and often of the royal family.
Aquitaine, a grand fief in southwestern France, was a possession of the English crown from 1154, when the Duke of Aquitaine and Gascony inherited the English throne, until it was finally conquered by the French at the end of the Hundred Years' War (1453).