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

Mister X may refer to:

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
The Amazing Mr X (comics), Britain's first superhero, who first appeared in The Dandy comic in 1944
Mr. X (Elite Beat Agents), a character from the Elite Beat Agents series
Mr. X (Streets of Rage), the villain in the video game series Streets of Rage
The villain in the video game The Ship
The final villain in the video game Kung-Fu Master
The pseudonym of Dr. Wily in Mega Man 6, which was later applied to the Archie Mega Man character Xander Payne.
The pseudonym of Major Grigori Agabekov in KGB
The pseudonym of Olga in Metal Gear Solid 2
T-103, AKA Mr. X, a Tyrant beast in Resident Evil 2
The Multi-billionaire owner of stunt island in Lego Stunt Rally
Film and television
Mister X (1958 film), a 1958 Soviet musical film directed by Yuli Khmelnitsky
Mister X (1967 film), a 1967 Italian superhero film directed by Piero Vivarelli also known as Avenger X
Mr. X (1987 film), a 1987 Bollywood Sci-Fi movie starring Amol Palekar directed by Khwaja Ahmad Abbas
Mr. X (2015 film), a 2015 Bollywood movie starring Emraan Hashmi
Mr. X, a recurring puppet on The Canadian Howdy Doody Show
Mr. X, a recurring arch-criminal of comic Arabic films played by Fouad el-Mohandes
Mr. X, in the cartoon series The X's
Mr. X, a villain in the anime series Lupin III.
Mr. X (Simpsons), Homer's pseudonym in the Simpsons episode "The Computer Wore Menace Shoes"
X (The X-Files), a character in the television series The X-Files
The Amazing Mr. X, a.k.a. The Spiritualist, a 1948 thriller film
Mr. X, an assassin in the 2008 film Wanted, played by David O'Hara
Mr. X, another name for the Observer 'September' in the television series Fringe
Refers to another character who appears in the Season 3 episode "Lysergic Acid Diethylamide" from Fringe
Mr. X, the pseudonym for Goofy in the animated short Aquamania
Two characters in various Teenage Mutant Ninja Turtles animated series associated with Mondo Gecko
A crime boss/terrorist in the 1987 cartoon
An alias for the character Xever Montes/Fishface in the 2012 cartoon
The placeholder name the farmer is given when he gets amnesia in the Shaun the Sheep Movie
Mister X (band), a street punk group from Belarus
"Mr X", a song by Ultravox from their album Vienna
"Mr X", a song and single by Pauline Murray And The Invisible Girls
"Mister X", a character in the operetta Die Zirkusprinzessin (The Circus Princess) by Emmerich Kálmán
"Mr.X", American DJ also known as Afrika Islam
Donald E. Graves (1929–2008), American Kremlinologist, from the 1982 Washington Post article "The Secret Files of Mr. X"
George F. Kennan (1904–2005), American historian, in his essay "The Sources of Soviet Conduct"
Carl Sagan (1934–1996), American astronomer, in an essay on his experiences with marijuana
Norman Wexler (1926–1999), American screenwriter, in Bob Zmuda's Andy Kaufman Revealed
Gaston Defferre (1910–1986), French Politician, using this pseudonym during the 1965 Presidential Elections
Ben Zygier, a prisoner who was held in Israel
The former longtime boyfriend of American radio talk show host, Robin Quivers
Aeros Mister X, a Ukrainian paraglider
"Mr. X", nickname of American golfer Miller Barber (1931–2013)
Mr. X, a masked professional wrestler in the 1930s and 1940s, one of the identities of Cyclone Mackey
Mr. X, a masked professional wrestler in the WWF in the 1970s, one of the identities of Jerry Balisok
Mr. X, a masked professional wrestler in the WWF in the 1980s, one of the identities of Dan Marsh
Mr X, a novel by Peter Straub
Mr. X, a player character in the board game Scotland Yard and its spin-off, Mister X
Mister X, an alias used by Mayuka Kondō in the manga and anime series, Oniichan no Koto Nanka Zenzen Suki Janain Dakara ne!!