Theatre-based interventions have been used in health promotion with young people to address HIV and sexual health. In this study, we explored the experience of undergraduate student performers participating in a theatre-based HIV prevention and sexual health education intervention for high school students in the USA. Undergraduate students enrolled in a credit-bearing course to learn about HIV and sexual health, participatory theatre and health education techniques. We analysed students' reflective essays written throughout the semester to identify any changes and the intervention processes that promoted these changes. Students experienced five interrelated forms of transformation: (1) increased knowledge about HIV and sexual health; (2) changes in attitudes and communication about sex; (3) artistic growth; (4) emotional growth; and (5) clarification of career goals and future plans. Intervention processes that contributed to these transformations included improvisation, guided writing exercises, the creation of a close-knit cohesive group and interactions with a group of HIV-positive speakers. Theatre-based, peer-led sexual health programmes can provide a transformative experience for undergraduate student performers. These transformative effects are linked to specific activities and processes of the intervention and require examination in future research.