<![CDATA[Background: Adolescent mental health is a critical issue that is gaining global attention. Bullying in school has been shown to have negative impact on self-concept, academic achievement, and increasing the risk of depression and anxiety. Peer support-based interventions are considered potential because adolescents are more open with their peers than adults, but research on their effectiveness in Indonesia is still limited Purpose: To assess the effectiveness of peer support interventions in improving mental health, self-concept, and preventing bullying behavior among adolescents in school environments. Methods: The study used a quasi-experimental design with a pretest-posttest control group design. The sample consisted of 60 junior high school students selected using purposive sampling, but the final number analyzed was 49 students (23 control, 26 intervention).The peer support intervention was implemented for 8 weeks through weekly sessions that included mental health literacy, communication skills, and strategies for dealing with bullying through discussion and role-play. The research instruments included the Strengths and Difficulties Questionnaire (SDQ) to measure mental health, the Olweus Bullying Questionnaire to measure bullying, and the Rosenberg Self-Esteem Scale to measure self-concept. Data analysis used t-tests and Wilcoxon tests with a significance level of p<0.05. Result: The results showed that in the intervention group, the average SDQ score decreased from 15.2 to 11.9 (Δ = –3.3 ± 6.1; p=0.017), indicating an improvement in mental health. Self-concept scores increased from 27.8 to 29.7 (Δ = +1.9 ± 13.0; p=0.115), while perpetrator bullying scores (Δ = –0.4 ± 8.2; p=0.736) and victim bullying scores (Δ = –0.2 ± 5.5; p=0.881) decreased, although not significantly. In the control group, SDQ scores decreased from 14.8 to 12.0 (Δ = –2.8 ± 6.3; p=0.015) and self-concept scores increased significantly from 28.1 to 31.7 (Δ = +3.6 ± 12.5; p=0.029). However, changes in perpetrators (Δ = +1.0 ± 8.6) and victims (Δ = –0.9 ± 5.6) of bullying were not significant (p>0.05). Intergroup analysis showed no significant differences (p>0.05) even though the intervention group showed greater mental health improvement (ΔSDQ –3.3 vs –2.8). This indicates that peer support has the potential to improve adolescents' psychological well-being, although it is not yet statistically significant during this intervention period. Implication: These findings indicate that peer support interventions is effective in improving mental health, but has not yet had a significant impact on self-concept and bullying behavior. Further research with longer duration, larger sample sizes, and teacher and parent involvement is recommended to strengthen the intervention's impact and support the implementation of mental health-friendly school policies. Keywords: Bullying; Mental health; Self-concept; Peer support; Adolescents]]>
