Individuals’ theories about emotions—the beliefs about the nature of emotions and the ability to influence them—have been linked to well-being. However, their causal role is not clear. To address this issue, we delivered a randomized controlled intervention to 1,645 middle school students that targeted their theories of emotion through interactive online modules. Students were taught that they could modify their emotions, get better at modifying their emotions with practice, and use strategies to improve their well-being. One month later, we found that students assigned to the intervention condition (compared to an active control condition) reported more adaptive theories of emotion and greater emotional well-being in school, although well-being outside of school was unchanged. Secondary analyses showed that these effects were present regardless of students’ race, gender, or grade level. These findings suggest that theories of emotion may be a promising target for improving adolescent well-being.