Walter (Sydney) Masterman (19 December 1876 – 16 May 1946) was an English author of mystery, fantasy, horror and science fiction.
Initially educated at Tonbridge School and Weymouth College, he entered Christ's College, Cambridge in 1897, where he was a football blue.
Served as a lieutenant in the 3rd Battalion Welsh Regiment in the Second Boer War, 1900–02; was made a captain in 1901.
He was the joint headmaster of Horsmonden School, Kent, 1903-05.
Also served in the Great War as a Major with the Welsh Regiment (1914–19).