A theatre organ is an organ installed in a movie theatre, most often modelled after the style originally devised by Robert Hope-Jones , which he called a "unit orchestra". Such instruments were typically built to provide the greatest possible variety of timbres with the fewest possible pipes, and often had pianos and other percussion instruments built in, as well as a variety of sound effects.

Many organ builders supplied instruments to theatres. The Wurlitzer company, to whom Hope-Jones licenced his name, was the most famous manufacturer, and the phrase Mighty Wurlitzer is to many synonymous with the theatre organ. Other manufacturers included Page, Marr and Colton, Compton, Moller, Morton, Conacher, Hilsdon, Kimball, Barton, Christie, and Hill Norman and Beard. These last two were both brand names for the same company, which specialised at the time in standardised extension organs with electro-pneumatic action, ideal for the theatre and then promoted as convenient and cost-effective for churches. In general the Christie brand was used for theatre organs, which came with contemporary styled consoles, while the firm's own name Hill Norman and Beard appeared on similar and sometimes identical pipes and actions supplied to customers seen as less frivolous, controlled by a traditional drawbar-stop console. Their standardised pipe, relay and blower packages were called unit organs, and for theatre use were augmented with percussion and other additional effects.

During the silent movie era, theatre organs were built in large numbers, in a variety of sizes, and filled the gap between a simple piano accompaniment and a full orchestra. Indeed, even when theatre owners hired orchestras to accompany silent movies, they usually also used an organ to provide relief to the orchestra, and to play for less-expensive showings.

After the development of sound movies, theatre organs were still occasionally retained to provide live music between features; many others were scrapped, or sold to churches, private homes, museums, ice rinks, rollatoriums, and restaurants (especially pizzerias). In that era, many of the tonal characteristics of theatre organs became somewhat more exaggerated than they had been in the silent movie era.