Adolescence is a critical stage in which gender expectations become deeply internalized and influence behavioral and emotional expression. Traditional social norms may conflict with one’s sense of self-expression, potentially increasing the risk of aggressive behavior. Boys are more likely to exhibit physical aggression, while girls tend to suppress anger. These differences arise not only from biological factors but also from social–psychological expectations and gender role conflict. This study aimed to examine how gender role conflict affects adolescents’ aggressive behavior and whether traditional masculinity and femininity mediate this relationship. A total of 398 adolescents aged 12–18 years (199 boys and 199 girls) participated in the study. The Gender Role Conflict Scale, the Traditional Masculinity–Femininity Scale, and the Buss–Perry Aggression Questionnaire were used for data collection. Results indicated that male students scored higher in both physical aggression and gender role conflict. Physical aggression increased with higher masculinity and decreased with higher femininity. Gender role conflict did not directly influence aggression but showed an indirect effect through masculinity traits. Overall, the findings reveal that the inconsistency between traditional gender expectations and self-expression influences adolescent aggression through traditional gender roles. Reducing rigid gender expectations and fostering more flexible gender attitudes may support healthier emotional expression and help prevent aggressive behaviors among adolescents.