The Thirty-Nine Steps is an adventure novel by the Scottish author John Buchan. It first appeared as a serial in Blackwood's Magazine in August and September 1915 before being published in book form in October that year by William Blackwood and Sons, Edinburgh.

It is the first of five novels featuring Richard Hannay, an all-action hero with a stiff upper lip and a miraculous knack for getting himself out of sticky situations.

The novel is set during May and June 1914; Europe is close to war and spies are everywhere. Richard Hannay has just returned to London from Rhodesia in order to begin a new life, when a freelance spy called Franklin P. Scudder calls on him to ask for help. Scudder reveals to Hannay that he has uncovered a German plot to murder the Greek Premier and steal British plans for the outbreak of war.

Scudder claims to be following a ring of German spies called the Black Stone. John Buchan, 1st Baron Tweedsmuir (1875–1940) was a Scottish novelist and historian.