William II, Count of Hainaut (1307 – 26 September 1345) was William IV of Avesnes, William IV of Holland and William III of Zeeland from 1337 to his death, succeeding his father, William I. He married Joanna, Duchess of Brabant and Limburg in 1334, but had no issue.
William fought in France as ally of the English (he was the brother in law of Edward III). He went on crusade against the Saracens and the Lithuanians. He besieged Utrecht, because his one time favourite bishop John van Arkel of Utrecht had turned against him. He died during one of the battles of Warns against the Frisians in 1345.
William was succeeded by his sister, who was married to emperor Louis IV. Hainaut, Holland and Zeeland became a part of the imperial crown domains.
| Preceded by William I |
Count of Hainaut | Succeeded by Louis the Bavarian and Margaret II |
| Count of Holland and Zeeland |