William I Longsword (c. 900 – 942) is the second ruler of Normandy, from 927 until his assassination.

He is sometimes anachronistically dubbed "Duke of Normandy", even though the title duke (dux) will not come into common usage until the 11th century.

William is known at the time by the title count (Latin comes) of Rouen.

Flodoard—always detailed about titles—consistently refers to both Rollo and his son William as principes (chieftains) of the Norse.