An oratorio (Italian pronunciation: [oraˈtɔːrjo]) is a musical composition with dramatic or narrative text for choir, soloists and orchestra or other ensemble.

Like most operas, an oratorio includes the use of a choir, soloists, an instrumental ensemble, various distinguishable characters (e.g. soloists), and arias. However, opera is musical theatre, and typically involves significant theatrical spectacle, including sets, props, and costuming, as well as staged interactions between characters. In oratorio, there is generally minimal staging, with the chorus often assuming a more central dramatic role, and the work is typically presented as a concert piece – though oratorios are sometimes staged as operas, and operas are not infrequently presented in concert form.

A particularly important difference between opera and oratorio is in the typical subject matter of the text. An opera libretto may deal with any conceivable dramatic subject (e.g. history, mythology, Richard Nixon, Anna Nicole Smith and the Bible); the text of an oratorio often deals with sacred subjects, making it appropriate for performance in the church, which remains an important performance context for the genre. Catholic composers looked to the lives of saints and stories from the Bible. Protestant composers often looked to Biblical topics, but sometimes looked to the lives of notable religious figures, such as Carl Loewe's "Jan Hus", an oratorio about the early reformer, Jan Hus. Oratorios became extremely popular in early 17th-century Italy partly because of the success of opera and the Catholic Church's prohibition of spectacles during Lent. Oratorios became the main choice of music during that annual period for opera audiences.

Conventionally, oratorio implies the sincere religious treatment of sacred subjects, such that non-sacred oratorio is generally qualified as 'secular oratorio': a piece of terminology that would, in some historical contexts, have been regarded as oxymoronic, or at least paradoxical, and viewed with a degree of scare-quoted skepticism. Despite this enduring and implicit context, oratorio on secular subjects has been written from the genre's origins.