William I (1120 or 1121 7 May 1166), called the Bad or the Wicked , was the second king of Sicily, ruling from his father's death in 1154 to his own in 1166. He was the fourth son of Roger II and Elvira of Castile.

William's title "the Bad" seems little merited and expresses the bias of the historian Hugo Falcandus and the baronial