"Quagmire" is the twenty-second episode of the third season of the science fiction television series The X-Files. It premiered on the Fox network in the United States on May 3, 1996. It was written by Kim Newton and directed by Kim Manners. The episode is a "Monster of the Week" story, unconnected to the series' wider mythology. "Quagmire" earned a Nielsen household rating of 10.2, being watched by 16 million viewers in its initial broadcast. The episode received mostly positive reviews from television critics.

The show centers on FBI special agents Fox Mulder (David Duchovny) and Dana Scully (Gillian Anderson) who work on cases linked to the paranormal, called X-Files. Mulder is a believer in the paranormal, while the skeptical Scully has been assigned to debunk his work. In this episode, Mulder and Scully investigate a series of deaths at a lake in Georgia that Mulder believes were caused by a "sea" monster that the locals have affectionately named Big Blue.

Although the episode was written by Newton, noted writer Darin Morgan provided assistance on the script. Because of this, the episode contains several references to previous Morgan-penned episodes, like "Clyde Bruckman's Final Repose" and "War of the Coprophages". It is the last episode to feature Scully's dog, Queequeg, and the only one to mention the dog's name. One specific dialogue scene featuring Mulder and Scully stranded on a small rock was praised by critics and included approximately 10 pages of dialogue. Series co-star Gillian Anderson later recalled that she "loved" the scene.