A proscenium stage is a type of theatre stage that has an architectural frame, known as the proscenium arch, which separates the stage from the auditorium. The arch creates a "window" around the scenery and performers, giving everyone in the audience a good view because the performers need only focus on one direction rather than continually moving around the stage to give a good view from all sides. The proscenium arch also simplifies the hiding and obscuring of objects from the audiences view, such as sets, performers not currently performing, and theatre technology.

Proscenium stages are structurally different from thrust stages or arena stages. The stage is deep and sometimes raked, meaning the stage is gently sloped rising away from the audience. Sometimes the front of the stage extends past the proscenium into the auditorium, which is known as an apron or forestage. Theatres containing proscenium stages are known as proscenium arch theatres and often include an orchestra pit for live music and a fly tower for the movement of scenery and lighting.

The proscenium stage was first installed in a permanent theatre in 1618-19 at the Farnese Theatre built in Parma, Italy. Since then, it has become the most common form of theatre building in the 18th, 19th, and 20th centuries. The proscenium arch has been used to frame the action of a play, and it has been expanded to enclose the lower side of the stage, creating a "picture frame" or an imaginary fourth wall through which the audience experiences the illusion of spying on characters behaving exactly as if they were unobserved.