William I (d. May 7, 1166) was king of Sicily from 1154 to 1166.

He was the fourth son of Roger II and Elvira Alfonso of Castile , and grew up with little expectation of ruling. The deaths of his 3 older brothers between 1138 and 1148 changed matters, though when his father died William was still not well-prepared to take his place.

William's title "the Bad" seems little merited and expresses the bias of the historian Falcandus and the baronial class against the king and the official class by whom he was guided. It is obvious, however, that William was far inferior in character and energy to his father, and was attached to the semi-Muslim life of his gorgeous palaces of Palermo.

The real power in the kingdom was at first exercised by Maio of Bari , a man of low birth, whose title ammiratus ammiratorum was the highest in the realm. Maio continued Roger's policy of excluding the nobles from the administration, and sought also to curtail the liberties of the towns. The barons, always chafing against the royal power, were encouraged to revolt by Pope Adrian IV, whose recognition William had not yet sought, by the Byzantine Emperor Manuel I Comnenus and the German emperor Frederick I.

At the end of 1155 Greek troops recovered Bari and began to besiege Brindisi. William, however, was not devoid of military energy; landing in Italy he destroyed the Greek fleet and army at Brindisi (May 28, 1156) and recovered Bari. Adrian came to terms at Benevento (June 18), abandoned the rebels and confirmed William as king, and in 1158 peace was made with the Greeks.

These diplomatic successes were probably due to Maio; on the other hand, the African dominions were lost to the Almohads (1156-1160), and it is possible that he advised their abandonment in face of the dangers threatening the kingdom down from the north. The policy of the minister led to a general conspiracy, and in November 1160 he was murdered in Palermo by Matthew Bonello , leader of the Sicilian nobles.

For a while the king was in the hands of the conspirators, who purposed murdering or deposing him, but the people and the army rallied round him; he recovered power, crushed the Sicilian rebels, had Bonello blinded, and in a short campaign reduced the rest of the Regno. Thus freed from feudal revolts, William confided the government to men trained in Maio's school, such as the grand notary, Matthew d'Agello . His latter years were peaceful; he was now the champion of the true pope against the emperor, and Alexander III was installed in the Lateran Palace in November 1165 by a guard of Normans. William died on May 7, 1166.

William married Margaret of Navarre, daughter of Garcia IV Ramirez of Navarre . They had 4 sons:

• Roger (d. 1161), duke of Apulia
• Robert
• William II of Sicily
• Henry (d. 1172), prince of Capua