| Walter S. Masterman | |
| Birth Name: | Walter Sydney Masterman |
| Birth Date: | 19 December 1876 |
| Birth Place: | Wimbledon, London, United Kingdom |
| Death Place: | Brighton, United Kingdom |
| Alma Mater: | Christ's College, Cambridge |
| Occupation: | Author |
Walter (Sydney) Masterman (19 December 1876 – 16 May 1946) was an English author of mystery, fantasy, horror and science fiction.
Masterman was born in Wimbledon, London on 19 December 1876, the son of Thomas William Masterman. Initially educated at Tonbridge School and Weymouth College, he entered Christ's College, Cambridge in 1897, where he was a football blue. Played football with his brother H W Masterman for amongst others Tunbridge Wells and is referred to in the "History of the Football Association" published in 1953 for an article he wrote in 1911 on the dispute between the Amateur Football Association and the Football Association. 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).
Walter S. Masterman died in Brighton on 16 May 1946.