Mr. Standfast is a 1919 novel by John Buchan, the third book in his Richard Hannay series of espionage thrillers. It follows The Thirty-Nine Steps (1915) and Greenmantle (1916). Set during World War I, the novel blends espionage, adventure, and a touch of romance.

Richard Hannay, now a brigadier-general, is pulled from the front lines of World War I and assigned to a secret intelligence mission in Britain. His task is to uncover German spies operating in England and Scotland, particularly those who have infiltrated pacifist groups to undermine the war effort.

Mr. Standfast is considered one of Buchan's finest thrillers, balancing action with deep character development. The Richard Hannay series influenced later spy fiction, including Ian Fleming's James Bond and John le Carré's works.