Gas-s-s-s (on-screen title: Gas! -Or- It Became Necessary to Destroy the World in Order to Save It.) is a 1970 American post-apocalyptic black comedy film directed by Roger Corman, written by George Armitage, and produced and released by American International Pictures. The plot follows survivors of an accidental military gas leak involving an experimental agent that kills everyone on Earth over the age of 25 (a cartoon title sequence shows a John Wayne-esque Army General announcing — and denouncing — the "accident"; the story picks up as the last of the victims are dying with social commentary on Medicare and Medicaid). The subtitle alludes to the 1968 quote "it became necessary to destroy the town to save it" attributed to a U.S. Army officer after the Battle of Bến Tre in Vietnam.

Robert Corff and Elaine Giftos star, with Ben Vereen, Cindy Williams, Bud Cort, and Talia Shire (credited as "Tally Coppola") in early roles. Country Joe McDonald, who provides the music, makes an appearance as a spokesman for "AM Radio".

It was Corman's final film for AIP as director, after a long association (although he would subsequently produce a few more films for the studio). He was unhappy as AIP made several cuts without his approval, including the removal of the final shot in which God comments on the action — a shot Corman regarded as one of the greatest he had made in his life.