A libretto (Italian for "booklet") is the text used in, or intended for, an extended musical work such as an opera, operetta, masque, oratorio, cantata or musical. The term libretto is also sometimes used to refer to the text of major liturgical works, such as the Mass, requiem and sacred cantata, or the story line of a ballet.

Libretto (pronounced [liˈbretto]; plural libretti [liˈbretti]), from Italian, is the diminutive of the word libro ("book"). Sometimes other-language equivalents are used for libretti in that language, livret for French works, Textbuch for German and libreto for Spanish. A libretto is distinct from a synopsis or scenario of the plot, in that the libretto contains all the words and stage directions, while a synopsis summarizes the plot. Some ballet historians also use the word libretto to refer to the 15 to 40 page books which were on sale to 19th century ballet audiences in Paris and contained a very detailed description of the ballet's story, scene by scene.

The relationship of the librettist (that is, the writer of a libretto) to the composer in the creation of a musical work has varied over the centuries, as have the sources and the writing techniques employed.

In the context of a modern English language musical theatre piece, the libretto is often referred to as the book of the work, though this usage typically excludes sung lyrics.