ABSTRACT In partnership with a sport-based Experiential Philanthropy Intervention – The Play Better Program – we conducted a pre-registered, longitudinal experiment examining whether repeatedly reflecting on prosocial activity could boost adolescents’ objective generosity. Adolescents (N = 114; aged 9–16) practiced charitable giving throughout their 2-month sports season and were randomly assigned to repeatedly reflect on the importance of their prosocial activity (Reflection condition) or to write about their everyday activities (Control condition). Adolescents completed an objective measure of generosity at pre- and post-intervention and self-reported measures of prosocial character. Across conditions, adolescents donated objectively more at post- vs. pre-intervention. However, adolescents in the Reflection (vs. Control) condition were no more generous and did not report greater prosocial character at post-intervention. Overall, these findings highlight the malleability of human prosociality and the need for additional scholar-practitioner collaborations to uncover whether and how Experiential Philanthropy Interventions boost long-term generosity among the next generation of givers.