Mr X (novel), by Peter Straub

Carl Sagan, American astrnomer, in an essay on his experiences with marijuana

George F. Kennan, American historian, in his essay "The Sources of Soviet Conduct"

Mister X or Mr. X is commonly used as a pseudonym for someone whose name is secret or unknown. Mister X may refer to:

Mr. X (Simpsons), Homer's pseudonym in the Simpsons episode "The Computer Wore Menace Shoes"

Mr X is the title of a song by Ultravox from their Vienna album

X (The X-Files), a character in the television series The X-Files

The longtime boyfriend of American radio talk show host, Robin Quivers

Mister X (Marvel Comics), a supervillain in the Marvel Universe

Mister X (Vortex), the protagonist in Dean Motter's comic book series of that title

Norman Wexler, American screenwriter, in Bob Zmuda's Andy Kaufman Revealed

The Amazing Mr. X, a.k.a. The Spiritualist, a 1948 thriller film

Miss X

Mister X (Amalgam Comics), a psychic member of the JLX