The Mysterious Stranger is a novel attempted by the American author Mark Twain. He worked on it intermittently from 1897 through 1908. Twain wrote multiple versions of the story; each involves a supernatural character called "Satan" or "No. 44". All the versions remained unfinished (with the debatable exception of the last one, No. 44, the Mysterious Stranger).

In 1590, three boys, Theodor, Seppi, and Nikolaus, live relatively happy simple lives in a remote Austrian village called Eseldorf (German for "Assville" or "Donkeytown"). The story is narrated by Theodor, the village organist's son. Other local characters include Father Peter, his niece Marget, and the astrologer.