This study aimed to explore the effectiveness of the virtual Hero program on the promotion of empathy (emotional contagion, emotional recognition, and perspective taking) and prosocial behavior toward strangers. Moreover, we studied whether promoting empathy strengthened the program’s impact on adolescents’ prosocial behavior. The study included 431 adolescents in the intervention group ( Mage = 13.64, 47% of participants identified as cisgender men, and 53% of participants identified as cisgender women) and 325 participants in the waitlist control group ( Mage = 13.22, 47% of participants identified as cisgender men, and 53% of participants identified as cisgender women). The adolescents lived in the urban zone of Buenos Aires, Argentina. Participants completed measures of prosocial behavior and empathy. The intervention included seven weekly online sessions (a pretest evaluation, five intervention sessions, and a posttest evaluation) of approximately 40 minutes each, while the control group completed only the pretest and posttest evaluations. The research findings show that the Hero virtual program was effective in promoting prosociality directly (when not mediated by emotional contagion and emotional recognition) and indirectly through emotional contagion and emotional recognition. In addition, the Hero program promoted emotional contagion and emotional recognition, but it was not effective at promoting perspective taking. In conclusion, internet-based interventions could play a prominent role in promoting positive behavior among young adolescents.