Parasyte (Japanese: 寄生獣, Hepburn: Kiseijū; lit. 'Parasitic Beasts') is a Japanese science fiction horror manga series written and illustrated by Hitoshi Iwaaki. It was published in Kodansha's Morning Open Zōkan (1989) and Monthly Afternoon (1989 to 1994). The manga was published in North America first by Tokyopop, then Del Rey, and finally Kodansha USA. The series follows Shinichi Izumi, a high school senior who is the victim of a failed attempt by a parasitic organism to take over his brain. The parasite, Migi, instead infects and takes over his arm, and both are forced in a peculiar partnership to fight other parasites.

The manga has been adapted into two live-action films in Japan, in 2014 and 2015. An anime television series adaptation produced by Madhouse, titled Parasyte -the maxim-, aired in Japan between October 2014 and March 2015. The English-language dub aired on Adult Swim's Toonami programming block in the United States between October 2015 and April 2016. A South-Korean live-action series spin-off, titled Parasyte: The Grey, premiered on Netflix in April 2024.

By August 2022, the manga had over 25 million copies in circulation, making it one of the best-selling manga series of all time. In 1993, Parasyte received the 17th Kodansha Manga Award for the general category, as well as the 27th Seiun Award for the best manga in 1996.

Parasyte centers on a 17-year-old male high school student named Shinichi Izumi, who lives with his mother and father in a quiet neighborhood in Fukuyama, Hiroshima, Japan. One night, tiny worm-like aliens with drill-like heads called Parasites arrive on Earth, taking over the brains of their hosts by entering through their ears or noses. One Parasite attempts to crawl into Shinichi's nose while he sleeps, but fails as Shinichi wakes up, and enters his body by burrowing into his arm instead. In the Japanese version, it takes over his right hand and is named Migi (ミギー), after the Japanese word for "right".

Because Shinichi was able to prevent Migi from traveling further up into his brain, both beings retain their separate intellects and personalities. As the duo encounters other Parasites, they capitalize on their strange situation and gradually form a strong bond, working together to survive. This gives them an edge in battling other Parasites who frequently attack the pair upon realization that Shinichi's human brain is still intact. Shinichi feels compelled to fight other Parasites, who devour other members of the species they infect as food, while enlisting Migi's help.

The series explores philosophical and psychological questions such as the meaning of humanity, humans' relationship to the environment and other species, the role of instinct and love, and the inherent anthropocentrism of morality. Shinichi's experience with Migi causes him to question if humanity has any right to claim moral superiority to the Parasites, while Migi and Reiko Tamura's experiences with humans cause them to take on more human traits, such as love and sacrifice.

Iwaaki chose a high school setting due to a scene he had thought of. When considering a scene where Migi turns his shape into a penis in front of Satomi Murano, Iwaaki believed that the scene would work best in that setting.

Written and illustrated by Hitoshi Iwaaki, Parasyte was first published for three chapters in the special issue of Kodansha's Morning, Morning Open Zōkan, from the F to H issues (August 1–October 3, 1989, issues). The series was later transferred to the publisher's Monthly Afternoon on November 25, 1989, and finished on December 24, 1994. Kodansha collected its 64 chapters in ten tankōbon volumes, released from July 23, 1990, to March 23, 1995. It was later republished in eight kanzenban volumes from January 23 to June 23, 2003.