A proscenium theater is a theater space whose primary feature is a large archway (the proscenium arch) at or near the front of the stage, through which the audience views the play. The audience directly faces the stage, which is typically raised several feet above front row audience level. This design was by far the most common for theater spaces in the 18th, 19th, and 20th centuries in Western theater. The main stage is the space behind the proscenium arch, often marked by a curtain which can be lowered or drawn closed. The space in front of the curtain is called the "apron." Proscenium stages range in size from small enclosures to several stories tall.

In general practice, a theater space is referred to as a "proscenium" any time the audience directly faces the stage, with no audience on any other side, even if there is not a formal archway over the stage. The side of the stage that faces the audience is referred to as the "fourth wall".

The phrase "breaking the proscenium" is also known as breaking the fourth wall: when the actor addresses the audience directly as part of the dramatic production.

In German, this type of stage is referred to as Guckkastenbühne.

Alternative theater layouts include the thrust stage and arena stage.