Mr. Standfast is the third Richard Hannay book by John Buchan, first published in 1919. It is the successor to The Thirty-Nine Steps and Greenmantle. Set in the later years of World War I, Brigadier Hannay is recalled from active service on the Western Front to undertake a secret mission hunting for a dangerous German agent in Britain.

Mr. Standfast is the third of five Richard Hannay novels by John Buchan, first published in 1919 by Hodder & Stoughton, London. It is one of two Hannay novels set during the First World War, the other being Greenmantle (1916); Hannay's first and best-known adventure, The Thirty-Nine Steps (1915), is set in the period immediately before the war started.The title refers to a character in John Bunyan's Pilgrim's Progress, to which there are many other references in the novel; Hannay uses a copy of Pilgrim's Progress to decipher coded messages from his contacts, and letters from his friend Peter Pienaar.

John Buchan, 1st Baron Tweedsmuir PC GCMG GCVO CH (26 August 1875—11 February 1940) was a Scottish novelist, historian and Unionist politician who served as Governor General of Canada, the 16th since Canadian Confederation.