This study explored the relationships between aggression and happiness in adolescents. The study was carried out with middle school students in a Central Anatolian city during the first semester of the 2019-2020 academic year. A total of 539 students, 281 females and 258 males, participated in the study. Pearson correlation coefficient and hierarchical regression analysis were employed in the analysis of the data obtained. According to the correlation analysis, a negative significant relationship between the four sub-dimensions of the Aggression Questionnaire (physical aggression, hostility, anger and verbal aggression) and adolescents’ happiness levels was determined. According to the regression analysis results, physical aggression, hostility, anger and verbal aggression were negative predictors of happiness among adolescents. In addition, the hostility and physical aggression level among adolescents had a moderator effect in the relationship between verbal aggression and anger, and happiness among adolescents. These results indicate that taking control of aggressive thoughts, emotions and behaviors by adolescents for healthy development and increased happiness level is vital during adolescence, which is a critical period in terms of personality development. Study findings were discussed within the framework of the literature, and recommendations were offered for researchers and practitioners.