Greenmantle is the second novel in John Buchan's Richard Hannay series. It is a sequel to The Thirty-Nine Steps and follows Hannay as he embarks on a dangerous espionage mission across Europe and the Middle East.

Richard Hannay, now a Major in British Intelligence, is assigned a top-secret mission: investigate rumors of a mysterious figure, "Greenmantle," who is expected to lead a religious uprising against the British in the Middle East. The Germans, hoping to use Islamic forces to their advantage, are backing this unknown leader.

Greenmantle remains an influential espionage novel and is often praised for its fast-paced narrative, exotic settings, and intricate political intrigue. It also predicted the use of religious extremism in global conflicts, making it surprisingly relevant today.