the Xenomorph, is a endoparasitoid extraterrestrial species that is the primary antagonist of the Alien film series. The species made its debut in the 1979 film Alien, and reappeared in its sequels Aliens (1986), Alien 3 (1992), and Alien Resurrection (1997). It has also appeared in the series' two spinoffs Alien vs. Predator (2004) and Aliens vs. Predator: Requiem (2007), as well as the series' subsidiary literature and video games.

Unlike many other recurring enemy extraterrestrial races in science fiction, the aliens are not an intelligent civilization, but predatory creatures with no higher goals than the propagation of their species and the destruction of life that could pose a threat. Like wasps or termites, aliens are eusocial, with a single fertile queen breeding a caste of sterile warriors.

The aliens' life cycle, in which their offspring are violently implanted inside living hosts before erupting from their chests, is in many ways their signature aspect. Their design deliberately evokes many sexual images, both male and female, to illustrate its blurring of human sexual dichotomy.

The alien design is credited to Swiss surrealist and artist H. R. Giger, originating in a lithograph called Necronom IV and refined for the series' first film, Alien.