medieval troubadour, count of Poitiers and duke of Aquitaine and of Gascony (1086–1127), son of William VIII and grandfather of the famous Eleanor of Aquitaine.

William IX spent most of his life in warfare, including leading an unsuccessful Crusade to the Holy Land (1101–02) and battling the Moors near Cordova (1120–23). His fame rests chiefly, however, on his being the first poet in the Provençal (Provençal literature) language whose works have come down to us. His chansons, or songs, are boisterous, amorous, humorous, usually delicate but sometimes coarsely obscene and tend, in the fashion of courtly love, to idolize one's lady love.