William Johnstone may refer to:
- William Johnstone, 1st Marquess of Annandale (1664–1721), Scottish nobleman
- Sir William Pulteney, 5th Baronet (1729–1805), born William Johnstone, Scottish Member of Parliament
- William Johnstone (VC) (1823–1857), Victoria Cross recipient
- Sir William Johnstone, 2nd Baronet (?–1727), Member of Parliament for Dumfries Burghs, and Dumfriesshire
- Will B. Johnstone (1883–?), writer
- William W. Johnstone (1938–2004), American author
- Billy Johnstone, Australian rugby league player
- William Johnstone (artist) Scottish artist
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