Swamp Thing is a superhero and antihero appearing in American comic books published by DC Comics. Created by writer Len Wein and artist Bernie Wrightson, the Swamp Thing has had several different incarnations throughout his publication. The character first appeared in House of Secrets #92 (July 1971) in a stand-alone horror story set in the early 20th century. The character found perhaps its greatest popularity during the original 1970s Wein/Wrightson run and in the mid-late 1980s during a highly acclaimed run under Alan Moore, Stephen Bissette, and John Totleben. Swamp Thing would also go on to become one of the staples of the Justice League Dark, a team featuring magical superheroes.

The character is often depicted as a swamp monster that resembles an anthropomorphic mound of vegetable matter seeking to protect nature and humanity from threats of both scientific and supernatural origin. These duties are often an expression of his designation as the Avatar of the Green, an illustrious title depicted as synonymous with both Swamp Thing and makes the character the embodiment of the cosmic energies that gives life to all plant life in the known universe, often dubbed "The Green". Several incarnations arise from the consciousness of other beings who are selected as the champion of the Parliament of Trees, the guiding and collective consciousness of all plant life, which includes past incarnations of Swamp Thing. Swamp Thing is also often in an elemental conflict with both rivals within the Green (i.e Floronic Man), rival elemental forces, such as "The Red" (embodies all animal life, including humanity), and most notably "The Rot" or "The Black" (embodies death), with their arch nemesis being Anton Arcane.

The original version of the character is Alexander Olsen, a scientist who was killed by his assistant vying for the affections of his wife. Returning as a swamp creature after his body is dumped, he takes revenge on his killer, but his wife runs off, unable to recognize him. He later becomes a local legend in Louisiana. His successor, Alec Holland, is the second and most well-regarded version of the character. A chemist working on a compound to enable plant growth in hostile environments, Holland is seemingly transformed by his own creation after his death at the hands of criminal elements. Stories vary in his being, sometimes a plant creature believing himself to be Alec possessing his memories while later stories make him the genuine Alec who transforms into the Swamp Thing. This version is also a reluctant ally of John Constantine and a later member of the Justice League Dark, considered a powerhouse among their ranks.

In 2021, a new incarnation of Swamp Thing was created. This version is Levi Kamei, a young Indian scientist chosen as the new Swamp Thing at a young age. Descended from a tribunal connected to the Kaziranga wetlands, his powers awaken following an altercation between the community and employers, which also resulted in his death and reincarnation. Following his awakening, various factions seeking to control the new Swamp Thing for their own nefarious agenda. Kamei is guided by Alec's spirit and fellow scientist Jennifer Reece in his new role. Existing concurrently with the Alec version, this Swamp Thing is instead more prominently a member of the Titans.

The character has been adapted from the comics into several forms of media, including feature films, television series, and video games. The character made his live-action debut in the film Swamp Thing (1982), with Dick Durock playing the Swamp Thing, while Ray Wise played Alec Holland. Durock played both Swamp Thing and Holland in the sequel film The Return of Swamp Thing (1989). Durock reprised the role again in the television series Swamp Thing (1990). The Swamp Thing was played by Derek Mears with Andy Bean playing Alec Holland in the television series Swamp Thing (2019). Another live-action film adaptation, titled Swamp Thing, is in development as an installment of the DC Universe (DCU) media franchise. IGN ranked him 28th in the Top 100 Comic Book Heroes list.

In 1971, DC Comics editor Joe Orlando contacted writer Len Wein for a last-minute eight-page backup story for House of Mystery. Wein came up with the idea for the character while riding a subway in Queens. He later recalled: "I didn't have a title for it, so I kept referring to it as 'that swamp thing I'm working on'. And that's how it got its name!" According to publisher Jenette Kahn, Orlando added the story's twist ending. Bernie Wrightson designed the character's visual image, using a rough sketch by Wein as a guideline. Wrightson completed the eight-page story in one weekend with the assistance of Michael Kaluta (who also modeled for the story's villain) and Jeff Jones, who inked two of the pages.

Wein and Wrightson developed a specific visual style for the stories, including yellow thought balloons and orange speech balloons, since "it was such an extraordinary effort for him to speak...whenever a caption referred to him, it was drippy like a swamp."

Len Wein was the writer for the first 13 issues, before David Michelinie and Gerry Conway finished up the series. Burgeoning horror artist Bernie Wrightson drew the first 10 issues of the series, while Nestor Redondo drew a further 13 issues, the last issue being drawn by Fred Carrillo. The original creative team worked closely together; Wrightson recalled that during story conferences, Wein would walk around the office acting out all of the parts. The Swamp Thing fought against evil as he sought the men who murdered his wife and caused his monstrous transformation, as well as searching for a means to transform back into his human form.