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Improvisational theatre, often called improvisation, improv or impro in British English, is the form of theatre, often comedy, in which most or all of what is performed is unplanned or unscripted, created spontaneously by the performers. In its purest form, the dialogue, action, story, and characters are created collaboratively by the players as the improvisation unfolds in present time, without use of an already prepared, written script.

Improvisational theatre exists in performance as a range of styles of improvisational comedy as well as some non-comedic theatrical performances. It is sometimes used in film and television, both to develop characters and scripts and occasionally as part of the final product.

Improvisational techniques are often used extensively in drama programs to train actors for stage, film, and television and can be an important part of the rehearsal process. However, the skills and processes of improvisation are also used outside the context of performing arts. This practice, known as applied improvisation, is used in classrooms as an educational tool and in businesses as a way to develop communication skills, creative problem solving, and supportive team-work abilities that are used by improvisational, ensemble players. It is sometimes used in psychotherapy as a tool to gain insight into a person's thoughts, feelings, and relationships.