Alien primarily refers to:
- Alien (law), a person in a country who is not a national of that country
- Enemy alien, the above in times of war
- Extraterrestrial life, life which does not originate from Earth
- Specifically, a lifeform with extraterrestrial intelligence
- For fictional extraterrestrial life, see Extraterrestrials in fiction
- Introduced species, a species not native to its environment
Alien(s), or The Alien(s) may also refer to: