All Musicals Are Adaptations

""So start off on the right foot and select a story that is all prepared for you. The translation of that story to musical form is quite complex enough. Within that frame you will find more than adequate challenge to your originality and enough on which to experiment.""

- Alan Jay Lerner, Advice to Young Musical Writers

Many musicals - arguably most - are adaptations. There are two major reasons for this tendency:

a) Dramaturgy. Many musicals will have separate artists working on each aspect of the text - book, music and lyrics. Some musicals will have more than one person working on each aspect, and then you have the influence of directors, choreographers and producers. It's hard enough to write a good story as it is, so adapting an existing and proven story provides everybody working on the show with a touchstone.

b) Commerciality. Primarily, musical theatre has always been a commercial medium that tries to appeal to as wide an audience as possible. Moreover, as the sheer costs of staging a Broadway or West End musical continue to skyrocket, producers are under increasing pressure to guarantee their shows will be smash hits. Audiences are more likely to come see a musical (or play, or film ...) based on a property with which they are already familiar, so adaptations are a safer bet than original works, though of course they're not sure hits (as proven by the line of unsuccessful musical adaptations of Cyrano De Bergerac stretching back to 1899).

This trope is common enough that it would be more useful to list exceptions and parodies than straight examples, and it is often said "great musicals are not written, they are re-written". Note, however, that it can be difficult to define what counts as an "adaptation". Whilst many musicals draw their narrative structure directly from the movie, novel, stage play, comic book, short story, ancient Greek myth etc. on which they were based, many other musicals take their inspiration from a variety of unusual sources - a historical figure or event, a painting, a concept - but provide an original narrative. Going back to the roots of musical theater, a large number of operas are also adaptations of older material... and a fair number of them have been turned into musicals.

Incidentally, this is why so many so many musicals are subtitled The Musical!

Aversions

 * The American Astronaut
 * Annie Get Your Gun
 * Stephen Sondheim & Arthur Laurents' Anyone Can Whistle.
 * Avenue Q isn't an adaptation, though it is largely a Pastiche of Sesame Street.
 * The Beautiful Game and its rewrite The Boys in the Photograph.
 * The Book of Mormon
 * Brooklyn
 * Bye Bye Birdie, though it was inspired by the real-life drafting of Elvis Presley.
 * Caroline, Or Change
 * Chess
 * A Chorus Line
 * City of Angels is not an adaptation, though its Show Within a Show is a Film Noir adapted from a novel.
 * Company is a borderline example: it was based on a cycle of seven short plays, which however went unproduced.
 * Curtains
 * The Drowsy Chaperone
 * Follies
 * Grease
 * Hair (theatre)
 * Hedwig and The Angry Inch
 * In the Heights
 * Jukebox Musicals (Mamma Mia, Across the Universe, etc.) may or may not count. They usually have original stories, but not original songs.
 * Kiss Me Kate is an original musical... about the cast of a musical adaptation of Shakespeare's The Taming of the Shrew.
 * Labyrinth
 * Lady in the Dark
 * The Last Five Years
 * Linie 1 a german musical famous for almost completely taking place in a subway train
 * The Music Man, though it does have something in common with Meredith Willson's memoirs of his childhood, And There I Stood With My Piccolo.
 * Next to Normal
 * Of Thee I Sing
 * Paint Your Wagon, despite the author's preface (quoted above) to the published libretto advising musical writers against trying to write original stories.
 * Passing Strange
 * Pippin
 * The Rocky Horror Show First a musical, then the movie.
 * Singin in The Rain, although it was written to utilize a bunch of existing songs the studio already owned, is actually a double aversion: a movie musical that is neither based on an existing story or adapted from a Broadway musical. What's largely forgotten is that this was actually common practice for film musicals of the era, and had been for a good ten years; this is simply the most famous example.
 * A significant chunk of Don Bluth's work—namely, An American Tail, All Dogs Go to Heaven, A Troll in Central Park, and The Pebble and the Penguin.
 * Songs for A New World
 * Starlight Express but only because Andrew Lloyd Webber wanted to do The Railway Series but wouldn't have had the creative control he wanted.
 * Sunday in The Park With George: Not based on a previous film, play, or novel, but on a painting. Well, sort of.
 * Tabaluga und Lilli
 * Tabaluga und das verschenkte Glück
 * 13
 * tick, tick... Boom!
 * [title of show]
 * Lampshaded in "An Original Musical"
 * Top Hat
 * Urinetown
 * ...And everything by Gilbert and Sullivan, except Princess Ida, based on Tennyson's poem "The Princess"; and The Yeomen of the Guard which is based on a much older story.
 * Some musicals, such as 1776, The Civil War, Floyd Collins, Titanic(which coincidentally was produced the same year as the film of the same name), Elisabeth and Parade are not based on any literary source, per se, but rather on historical event. Though The Civil War does include a few direct quotes from speeches, etc., what these musicals get from history is their plots and many/most of their characters.
 * The same is somewhat true of Assassins as well, which takes historical figures and events, and mashes them all together into one timeless vacuum... type... thing.
 * Newsies is another example, this time for film.
 * Similarly, biographical musicals, such as Annie Get Your Gun, Evita and I Am Star Trek (Gene Roddenberry).
 * Rodgers and Hammerstein's Allegro and Me and Juliet were originals. All other musicals they wrote (including movie and TV musicals) were adaptations.
 * Zombie Prom is an original musical, which was adapted into a much-abridged film.

Parodies
"Mr. Burns: A musical about a doll? Why not a musical about the common cat? Or the King of Siam?"
 * The Simpsons has done this several times, often parodying the concept by having its musicals draw from bizarre or inappropriate sources. Marge starred in a musical adaptation of A Streetcar Named Desire (opposite Ned Flanders) whose cheery closing song managed to completely miss the point of the original source. Then there's Stop the Planet of the Apes: I Want to Get Off!, which featured breakdancing chimps and spontaneous piano solos. Smithers also wrote a musical inspired by Malibu Stacey dolls, providing an example of a musical based on a pre-existing concept while not being a direct adaptation.

"Bruce: ...you hate me, don't you."
 * When Andre Previn made A Streetcar Named Desire into an opera, he explicitly cited the "Simpsons scenario" ("Stella, Stella, can't you hear me yella?") as an instance of what he tried to avoid. In fact, the libretto simply sets the original text of the play to music.
 * And who could forget "Kickin' It: A Musical Journey Through the Betty Ford Center"?
 * And the musical adaption of Itchy and Scratchy, parodying the style of the Broadway version of The Lion King.
 * The Producers notes a musical adaptation of Hamlet, called Funny Boy; it isn't depicted, but its audience informs us "It's the worst show in town!" in the first scene of the stage version, which takes place on its opening and closing night.
 * The Fairly Odd Parents is another twofer: Waterworld: The Musical and, as a Continuity Nod to the episode parodying action movies, Loose Cannon Cop Who Doesn't Play by the Rules: The Musical.
 * MAD Magazine had "Keep on Trekkin'", a Star Trek musical that addressed the post-cancellation success of TOS in reruns in The Seventies. It ends with the cast turning down a network executive's offer of a Revival because they're making so much money already—it was written before the movie franchise was established in 1979.
 * "Coming Musicals" in MAD #41 suggested that, when Broadway starts running out of likely source material, new musicals could be based on telephone directories, railroad timetables, and cook books, producing song hits like "The Bell-Box Of My Heart" and "Oh, Your Lips Say Central Standard."
 * MAD #100 did an article conceiving musical versions of Moby Dick, Julius Caesar, A Tale of Two Cities and Tarzan.
 * Later issues had musical versions of Star Wars ("The Force and I") and The Lord of the Rings ("The Ring and I"). Note that Moby Dick, A Tale of Two Cities and The Lord of the Rings have since been adapted into serious stage musicals, and Disney's Tarzan received a Screen to Stage Adaptation. (Lord of the Rings has also since been... rather less seriously adapted.)
 * MAD TV did a skit in the late 1990s spoofing how campy the Batman movie franchise had become by having the 5th one done as a Broadway musical. In fact, Warner Bros. actually had Jim Steinman and David Ives working on a Batman musical for several years, but it didn't pan out.
 * Don't forget Erin Brockovich: "I may dress like a cheap table dancer / but give me a call if you think you've got cancer.."
 * The only thing preventing a Star Wars musical is George Lucas's dignity (dear God, we're screwed). But it has been adapted into an opera.
 * Except that it exists. And is brilliant.
 * Twice!
 * A Tale of Two Cities is also staged as a musical (Two Cities) in the Martin Short comedy A Simple Wish.
 * In the movie The Tall Guy Jeff Goldblum's character, trying to get into serious drama, finds himself starring in Elephant!, a musical version of The Elephant Man.
 * On Thirty Rock, Jenna has been in musical versions of Con Air and Mystic Pizza.
 * A cutaway reveals Peter once performed in Red Dawn - The Musical on Family Guy. "I'm a Wolverine/And my hatred keeps me warm..."
 * Batman Beyond had a scene featuring Terry taking Bruce Wayne to a musical about...Batman. The sad part is that the idea for it came from the fact that someone actually proposed a Batman musical in real life.

""Y'know Ferb, one of the best times we ever had was when we built that rollercoaster. We should do it again! This time, as a musical! Whadya say? We'll do all the same things, except we'll break into spontaneous singing and choreography with no discernable music source!""
 * Fans once had a character perform in a musical adaptation of the Book of Leviticus.
 * Sluggy Freelance had Zoe and Kent go see "The Cylon King," a Broadway musical based on Battlestar Galactica. Kent remarks they should have gotten tickets to "Thoroughly Merciless Ming" instead.
 * Gilligan's Island had the castaways staging a musical of version of Hamlet to try to persuade a producer to return to civilisation and take them with him. He steals their idea, returns to civilisation and leaves them stranded.
 * In a Monty Python's Flying Circus sketch, E. Henry Thripshaw announces that he hopes to turn his next disease into a musical (after his first disease became an In Name Only film).
 * The web series "The Battery's Down" parodies this with Ferris Buellers Day Off The Musical and Home Alone The Musical.
 * An episode of The Critic features Jay and Doris going to Andrew Lloyd Webber's newest musical, Hunch!, an adaptation of The Hunchback of Notre Dame. The sequence takes swipes at the commercialism ("Brought to you by Toyota: the hatchback fit for a hunchback!") and strange staging common to ALW's musicals. Note that this episode predated the Disney adaptation of Hunchback—which had its own problems trying to make the story a musical that could also move merchandise—by two years, making this Hilarious in Hindsight.
 * Also, Notre Dame de Paris premiered in 1998.
 * Rugrats has Reptar On Ice, an Ice Capades-like musical show based on a Godzilla-like action film franchise.
 * In Andrew Lippa's version of The Wild Party, the brothers d'Armano write a musical called Good Heavens, based on the Bible.
 * In a recent episode of The Venture Brothers, Rusty wants to make a musical about his life (a Johnny Quest boy-adventurer sort of childhood with its own cartoon show), though this never gets off the ground. He does get a duet with the in-universe version of Spiderman, the Brown Widow, which might be a parody of Spider Man Turn Off the Dark.
 * One episode of Phineas and Ferb revisits one of their early adventures: