Gender inequality and restrictive norms are often reinforced and internalized during adolescence, influencing pivotal life choices. This paper presents results from a randomly-assigned gender norms intervention for young adolescents in Somalia that led to greater support for gender equality in reported attitudes among both girls and boys. In a novel lab-in-the-field experiment designed to observe social group dynamics, treated adolescents were also found to be less likely to succumb to peer pressure to conform when stating their gender attitudes in public. Perceptions of gender norms appears to shift for boys, leading to a greater public expression of gender egalitarian ideals. Furthermore, the findings show improved adolescent mental health, increased caring behavior towards siblings of the opposite sex, and a higher likelihood of involvement in household chores by boys. A complementary gender norms intervention for parents had limited marginal impact on the attitudes and behaviors of adolescents. The results suggest that gender norms interventions can be effective in influencing the attitudes and public discourse around gender equality, even in early adolescence.