Longform improvisation, often referred to simply as longform, is an approach to improvisational theatre and improvisational comedy consisting of one or more scenes which are connected by a narrative thread or theme.

In longform, audiences are often asked for one or more suggestions or inputs which drive the narrative of the performance. This may take the form of a specific existing type of theater, for example a full-length improvised play or Broadway-style musical. Or, the scenes may be largely unrelated with the exception of a single point of inspiration. Unlike shortform, no further inputs are solicited. The performance is created by the performers without breaking the narrative thread. Longform improvisation is the predominant form of improvisational performance in Chicago, considered the birthplace of contemporary improvisational theatre. However, longform improvisation is performed around the world. Perhaps the best known longform structure is The Harold, developed by ImprovOlympic cofounder Del Close.

The longform improvisational approach is contrasted with shortform improvisation, or shortform. The superiority or inferiority of one approach versus another is a continuing subject of discussion in the improv community.