Mainstage theatre is that which falls between studio theatre and large-scale events. It is usually performed in a proscenium theatre or on a thrust stage.

In the nineteenth and early twentieth century almost all theatres were built on the proscenium model. With the growth of studio theatres from the 1920s and their increasing adoption by traditional theatres as an anciliary space for smaller productions, theatrical management began to differentiate between 'the main theatre' and 'the studio theatre'. The concept of the 'main' theatre became unattractive to those members of the profession working on large-scale events and others who felt that the 'main' theatre was a diminishing part of modern theatre. The phrase 'main theatre' also lacked significance for those institutions that only had a single traditional stage. By the end of the twentieth century the name 'mainstage' was well-established as a description of traditional western theatres and the productions performed in them.

'Mainstage' is also a common contraction of 'main stage' used to describe the performance space with the largest audience capacity at a performing arts festival.

Mainstage bingo is the traditional version of the game played with books or cards (as opposed to mechanised boards).