William I (born c. 1085 and died October 1129) was the Count of Forcalquier and Marquis of Provence from 1094. He was the second son of Count Ermengol IV of Urgell and the only son of Adelaide of Forcalquier, heiress of Count William Bertrand of Provence.
William married Gersende, daughter of Count Guigues III of Albon. They had two sons, Guigues of Forcalquier and Bertrand I.