第四面墙是一面在传统三壁镜框式舞台中虚构的「墙」，透过这面「墙」观众可以看到戏剧设置的世界中的情节发展。 即在大多数的写实和自然主义戏剧中演员假装观众不存在、自己演自己的；观众死板地坐在观众席观看演出。台上台下没有任何交互（或说不能有交互）。第四面墙的概念由德尼·狄德罗阐明，随着戏剧现实主义的发展，此概念在19世纪剧场当中流传开去，延伸了虚构作品和阅听者之间的虚构界限的这一想法。

第四面墙的在场，是现代现实主义剧的既定惯例，使得很多艺术家会着眼于把观众的注意力吸引到墙上，从而在界限「破开」的时候达致戏剧性和喜剧效果，例如，舞台上的演员直接对观众说话。

The fourth wall is the imaginary "wall" at the front of the stage in a traditional three-walled box set in a proscenium theatre, through which the audience sees the action in the world of the play. The concept is usually attributed to the philosopher, critic and dramatist Denis Diderot. The term itself was used by Molière. The fourth wall illusion is often associated with naturalist theatre of the mid 19th-century, and especially with the innovations of the French director André Antoine.