Mystery Science Theater 3000 (abbreviated as MST3K) is an American science fiction comedy television series created by Joel Hodgson. The show premiered on KTMA-TV (now WUCW) in Saint Paul, Minnesota, on November 24, 1988. It then moved to nationwide broadcast, first on The Comedy Channel for two seasons, then Comedy Central for five seasons until its cancellation in 1996. Thereafter, it was picked up by The Sci-Fi Channel and aired for three more seasons until another cancellation in August 1999. A 60-episode syndication package titled The Mystery Science Theater Hour was produced in 1993 and broadcast on Comedy Central and syndicated to TV stations in 1995.

In 2015, Hodgson led a crowdfunded revival of the series with 14 episodes in its eleventh season, first released on Netflix on April 14, 2017, with another six-episode season following on November 22, 2018. A second successful crowdfunding effort in 2021 produced 13 additional episodes shown on the Gizmoplex, an online platform that Hodgson developed which launched in March 2022. As of 2023^[update], 230 episodes and a feature film had been produced as well as four live tours.

The show initially starred Hodgson as Joel Robinson, a janitor trapped by two mad scientists ("The Mads") on the Earth-orbiting Satellite of Love and forced to watch a series of B movies to monitor his reaction to them. To keep his sanity, Joel crafts sentient robot companions, including Tom Servo, Crow T. Robot, Cambot and Gypsy, to keep him company and help him humorously comment on each movie as it plays, a process known as riffing. Each two-hour episode would feature a single movie (often edited for time constraints), sometimes preceded by various old shorts and educational films, with Joel, Tom, and Crow watching in silhouette from a row of theater seats at the bottom of the screen. These "theater segments" were framed with interstitial sketches called "host segments". The show's cast changed over its duration; most notably, the character of Joel was replaced by Mike Nelson (played by Michael J. Nelson) halfway through the show's fifth season. Other cast members, most of whom were also writers for the show, include Trace Beaulieu, Josh Weinstein, Jim Mallon, Kevin Murphy, Frank Conniff, Mary Jo Pehl, Bill Corbett, Paul Chaplin, and Bridget Jones Nelson. The 2017 revival features a primarily new cast, including Jonah Ray who plays the new human test subject Jonah Heston, along with Felicia Day and Patton Oswalt as "The Mads" and Baron Vaughn, Hampton Yount, and Rebecca Hanson voicing Tom Servo, Crow T. Robot, and Gypsy, respectively. Season 13 brought back this cast while integrating Emily Connor, played by Emily Marsh, and others from the live tours in this era.

Initially MST3K did not garner high viewership numbers, but the show's popularity spread through online word-of-mouth by its fans known as "MSTies" or "Mysties" (who would remind others to "Keep circulating the tapes"), frequent repeats, syndication, and home media offerings produced by Rhino Entertainment. Currently, this popularity continues through Shout! Studios, who along with Hodgson, now own the rights to the show and supported the revived series. MST3K was listed as one of Time magazine's "100 Best TV Shows of All-TIME" in 2007, and TV Guide has noted MST3K as one of the top cult television shows. The show won a Peabody Award in 1993, was also nominated for two Emmy Awards in 1994 and 1995, and for the CableACE Award from 1992 to 1997. The show was considered highly influential, contributing towards the practice of social television, and former cast members launched projects similarly on riffing on films, including The Film Crew, RiffTrax, and Cinematic Titanic. MST3K also brought to light several older movies that had fallen into obscurity or had received little or no public attention when originally released. Many of these films were subsequently identified as among the worst movies ever made, most notably Manos: The Hands of Fate.

While the cast of MST3K changed throughout its history, the basic premise of the show remains consistent: a human test subject—first Joel Robinson (Joel Hodgson), then Mike Nelson (Michael J. Nelson), and most recently Jonah Heston (Jonah Ray) and Emily Connor (Emily Marsh)—has been imprisoned aboard the spacecraft Satellite of Love by mad scientists (collectively called "The Mads") and is forced to watch a series of bad movies in order to find one that will drive the test subject insane.

In an attempt to keep his sanity, Joel built sentient robots ("the bots") from parts aboard the Satellite of Love, and they subsequently remained aboard with Joel's successors as test subjects. The Bots include Tom Servo, Crow T. Robot, GPC (formerly Gypsy) who is in charge of satellite operations, and Cambot, the silent recorder of the experiments. Crow and Servo join the human test subject in watching the film in the satellite's theater. To keep from going mad, the trio frequently comment and wisecrack during the movie, a process known as "riffing". At regular intervals throughout the movie, the hosts leave the theater and return to the bridge of the satellite to perform sketches (commonly called "host segments") that often satirize the film being watched.

The general format of an MST3K episode has remained the same throughout the series' run. Episodes are approximately 90 minutes in running time (excluding commercial breaks) and begin with a short introductory segment in which the human host and the 'bots interact with the Mads before being sent the movie. During Joel Hodgson and Jonah Ray's tenures as hosts (and for a brief period at the start of the Mike Nelson era), the hosts and the Mads engage in an "invention exchange" in which they each show off their latest inventions. Sirens and flashing lights ("Movie Sign") then signal the characters to enter the theater.

In the theater, the human host and 'bots Tom and Crow sit in a row of theater seats, shown in silhouette along the bottom of the screen, an approach Hodgson called "Shadowramma". The three then riff on the film (which is sometimes accompanied by one or more shorts) as it plays for both them and the audience. Occasionally, the silhouette format is used as a source of humor, or as a means of creating unobtrusive censor bars for scenes containing nudity. The show transitions into and out of the theater via a "door sequence", a series of six doors that open or close as the camera (presumably Cambot) passes through them.