William II (December 1153 – 11 November 1189), called the Good, was king of Sicily from 1166 to 1189. From surviving sources William's character is indistinct. Lacking in military enterprise, secluded and pleasure-loving, he seldom emerged from his palace life at Palermo. Yet his reign is marked by an ambitious foreign policy and a vigorous diplomacy. Champion of the papacy and in secret league with the Lombard cities, he was able to defy the common enemy, Frederick Barbarossa. In the Divine Comedy, Dante places William II in Paradise. He is also referred to in Boccaccio's Decameron (tale IV.4, where he reportedly has two children, and tale V.7).

William was nicknamed "the Good" only in the decades following his death. It is due less to his character than to the cessation of the internal troubles that plagued his father's reign and the wars that erupted under his successor. Under the Staufer dynasty his reign was characterised as a golden age of peace and justice. His numeral is contemporary and he himself used it.

William was born in Palermo to William I and Margaret of Navarre. At the age of twelve his father died, and he was placed under the regency of his mother. In 1171 he was declared adult and until then the government was controlled first by the chancellor Stephen du Perche (1166–1168), cousin of Margaret, and then by Walter Ophamil, archbishop of Palermo, and Matthew of Ajello, the vice-chancellor.

In 1168, du Perche was overthrown by a coup, while the revolts claimed that William was murdered and du Perche planned to have his brother marry Constance, aunt of William who was confined to Santissimo Salvatore, Palermo as a nun from childhood due to a prediction that "her marriage would destroy Sicily", to claim the throne, despite the existence of Henry, Prince of Capua brother of William.

An effort by Bertrand II, archbishop of Trani, to negotiate the hand of Byzantine Princess Maria for William yielded no fruit and led to his breaking up with Byzantine Emperor Manuel I Comnenus in 1172. In 1173, the Emperor Frederick Barbarossa proposed William's marriage with his daughter, Beatrice, but William refused to offend the pope.

In the same year the death of Henry, Prince of Capua marked a potential succession crisis: it was said that William II had Constance, the last legitimate heir to the throne, appointed heir and sworn fealty in 1174, but she remained confined in her monastery.

In 1174 and 1175 William made treaties with Genoa and Venice and his marriage in February 1177 with Joan, daughter of King Henry II of England and Duchess Eleanor of Aquitaine, marks his high position in European politics. Although Joan produced no surviving heir, William showed no intention to annul the marriage.

In July 1177, William sent a delegation of Archbishop Romuald II of Salerno and Count Roger of Andria to sign the Treaty of Venice with the Emperor. In 1184, he released 30-year-old Constance from convent, engaged her to the Emperor's son, the future Emperor Henry VI to secure the peace, and married her off on January 1186, causing a general oath to be taken to her as his heir presumptive. This step, of great consequence to the Norman realm, was possibly taken that William might devote himself to foreign conquests, or aiming to prevent Tancred, Count of Lecce, an illegitimate cousin of William, to claim the throne.