Objectives: To investigate social functioning and mental wellbeing in 13-15 year-old adolescents in Iran and Finland, in order to explore potential cultural and gender-based differences during early adolescence. Methods: One thousand and one (1001) adolescents from Iran and 2205 adolescents from Finland (age range 13-15 years) filled in a questionnaire consisting of the following scales: the Mini Direct and Indirect Aggression Scale (Mini-DIA), the Self-Efficacy Questionnaire for Children (SEQ-C), the School Burnout Inventory (SBI), and the Multidimensional Scale for Perceived Social Support Assessment (MSPSSA). Results: Iranian boys scored highest on Aggression, Victimization, and School Burnout, and lowest on Social Support, Academic Self-Efficacy, and Interpersonal Self-Efficacy. The Finnish boys scored highest on Interpersonal Self-Efficacy and Emotional Self-Efficacy, and lowest on School Burnout and Victimization. Finnish girls had the highest scores on Social Support and Academic Self-Efficacy, but the lowest on Aggression and Emotional Self- Efficacy. The Iranian girls did not have any highest or lowest scores in this sample. Conclusion: Clear differences due to culture and gender were found. It appears that Iranian boys, despite their higher societal status than girls, experience their school environment as more stressful than Iranian girls do, and than Finnish boys do as well. The findings are discussed.