In the fictional universe of Warhammer 40,000, a Genestealer is an alien creature and the name of one of the add-on packs for the game Space Hulk.

Genestealers are often said to be a rip-off of the creature from the film Alien. They have a similar appearance - in particular the heads within heads, or ovipositor - and they act in a similar predatory fashion, with the same goal of implanting their 'eggs' into a human host.

Unlike the creature in Alien however this impregnation is not a violent act, as the genestealer uses hypnosis. Additionally, an egg is not implanted to hatch explosively from the human later, but rather the host gives birth to a genestealer hybrid as it would give birth to a normal human. These hybrids are a cross species between the genestealer's and their host. Unlike their genestealer parent they are capable of speech and tool usage, though they share the same wish to impregnate others. In this fashion the genestealer genes become more and more recessive in the appearance of each subsequent generation however in their mind they are still fully devoted to expanding the hive mind of the genestealers.

Genestealers are part of the Tyranid army in the Warhammer 40,000 game. They were in fact the first part of the Tyranid army to make an appearance when they first appeared in the original Space Hulk game.