Abstract Background Youths' online problematic behaviors, such as smartphone or social network sites (SNS) addiction, gained increasing attention nowadays, due to their impact on concurrent and later adjustment, such as emotional and/or behavioral problems, academic impairments, or relational issues. Aims This study aims to evaluate the effectiveness of a pilot school‐based intervention to contrast online addictive behaviors while fostering adolescents' self‐regulative abilities. Materials & Methods The intervention started in January 2022 in an Italian junior high school located in Rome , and consisted of four meetings with students. A total sample of 462 15‐year‐old adolescents ( M age = 15.2; SD = 0.50; 41% females; N control = 214; N intervention = 248) was considered. Within the latent difference score framework, we examined short‐term changes from the pre‐to‐the‐postintervention levels of SNS and smartphone addiction, and self‐regulatory self‐efficacy (SRSE) beliefs as a possible booster of the intervention's effectiveness. Results Results showed a significant decrease in both online addictions ( SNS and smartphone addiction ), controlling for age, gender, sexual orientation, and socioeconomic status, because of the short‐term efficacy of the project. The buffering effect of SRSE beliefs was further supported. Conclusion These findings emphasized the usefulness of promoting youths' self‐regulative beliefs to contrast problematic tendencies, according to a Positive Youth Development perspective which focused on resources rather than only on the prevention of negative outcomes for youths' adjustment.