This article is about the ruler of Normandy. For William Longsword, Earl of Salisbury, see: William de Longespee, 3rd Earl of Salisbury . There was also a William of Montferrat nicknamed William Longsword.

William Longsword (died December 17, 942) was jarl (ruler) of Normandy.

Little is known about his early years. He may have been born before his father Rollo settled in northern France, and succeeded Rollo sometime around 928. It appears that he faced a rebellion early in his reign, from Normans who felt he had become too Frankified. The following years are obscure. In 939 William became involved in a war with Arnulf I of Flanders, which soon became intertwined with the other conflicts of the reign of Louis IV of France. He was killed by followers of Arnulf while at a meeting to settle their conflict. His son Richard I of Normandy succeeded him.