Sealab 2021 is an American adult animated television series created by Adam Reed and Matt Thompson for Cartoon Network's late-night programming block, Adult Swim. Cartoon Network aired the show's first three episodes in December 2000 before the official inception of the Adult Swim block on September 2, 2001, with the final episode airing on April 24, 2005. Sealab 2021 is one of the only four original Williams Street series that premiered in 2000 before Adult Swim officially launched, the others being Aqua Teen Hunger Force, The Brak Show, and Harvey Birdman, Attorney at Law.

Much like Adult Swim's Space Ghost Coast to Coast, the animation used stock footage from a 1970s Hanna-Barbera cartoon, in this case the short-lived, environmentally-themed Sealab 2020, along with original animation. The show was a satirical parody of both the original Sealab series and the general conventions of the 1970s animated children's series. While there was initial resistance from several of the original series' creators to the reuse of their characters, production moved forward on the series. Sealab 2021 was produced by 70/30 Productions.