The Alien (also known as a Xenomorph XX121 or Internecivus raptus, or simply a xenomorph) is a fictional endoparasitoid extraterrestrial species that serves as the title antagonist of the Alien film series. The species made its debut in the film Alien (1979) and reappeared in the sequels Aliens (1986), Alien 3 (1992), and Alien Resurrection (1997). The species returns in the prequel series, first with a predecessor in Prometheus (2012) and a further evolved form in Alien: Covenant (2017). It also featured in the crossover films Alien vs. Predator (2004) and Aliens vs. Predator: Requiem (2007), with the skull and tail of one of the creatures respectively appearing briefly in Predator 2 (1990) and The Predator (2018), and will return in the FX television series Alien (2023). In addition, the Alien appears in various literature and video game spin-offs from the franchises.

The Aliens' design is credited to Swiss surrealist and artist H. R. Giger, originating in a lithograph titled Necronom IV and refined for the series's first film, Alien. The practical effects for the Alien's head were designed and constructed by Italian special effects designer Carlo Rambaldi. Species design and life cycle have been extensively augmented, sometimes inconsistently, throughout each film.

Unlike many other extraterrestrial races in science fiction (such as the Daleks and Cybermen in Doctor Who, or the Klingons and Borg in Star Trek), the Aliens are not sapient toolmakers — they lack a technological civilization of any kind, and are instead primal, predatory creatures with no higher goal than the preservation and propagation of their own species by any means necessary, up to and including the elimination of other lifeforms that may pose a threat to their existence. Like wasps or termites, Aliens are eusocial, with a single fertile queen breeding a caste of warriors, workers, or other specialist strains. The Aliens' biological life cycle involves traumatic implantation of endoparasitoid larvae inside living hosts; these "chestburster" larvae erupt from the host's body after a short incubation period, mature into adulthood within hours, and seek out more hosts for implantation.