The Mystery Motivator intervention has been shown to increase prosocial behavior among school-age children by capitalizing on the anticipation afforded by reward concealment or randomization. Although numerous evaluations have supported the effectiveness of the Mystery Motivator, the overall status of this intervention is still unknown. The purpose of the current meta-analysis was to quantify the effects of the Mystery Motivator for improving behavior outcomes in school-age children and determine if participant, setting, and intervention components moderate intervention effects. A systematic review of academic databases was conducted that yielded a total of 24 studies. These studies were evaluated for methodological rigor and analyzed for effects using Tau-U. Results indicate a strong overall effect for this intervention (Tau-U = 0.77 (CI[subscript 95] [0.73 to 0.80]). The results discussed and practical issues are considered for future research on the use of the Mystery Motivator to promote the academic and social-emotional development of children and adolescents.