Improvisational theater (improv) is a form of theater where dialog, characters, and story are created spontaneously by its actors on stage. In the last years, different improv techniques have gained increasing popularity and spread into fields beyond comedy and performing art, e.g., business organizations and educational programs. However, the beneficial impact of improv on psychological variables has barely been investigated. In this study, we aim to fill this gap and contribute to a scientific investigation of improvisational theater on various variables that measure creativity, acceptance, and psychological well-being. In a controlled trial, 30 participants in the intervention group and 28 in the waiting control group completed six different tests and questionnaires prior and post to a 6-week improv intervention or waiting time, respectively. We found significant improvement in participants’ creativity and psychological well-being due to the intervention but no evidence for enhanced acceptance.