Narcissism is a personality trait characterized by a sense of being more important and entitled than others. Narcissism is high in adolescence and puts adolescents at risk of psychopathology and problematic social relationships. Why is narcissism persistent in adolescence? Bridging insights from developmental, clinical, social, and personality psychology, we examined whether adolescents (ages 11–15) high in narcissism maintain narcissism through downward social comparisons (e.g., “I am better than my classmates”), not downward temporal comparisons (e.g., “I am better now than when I was younger”). A cross-sectional study (N = 382, 97% Dutch) showed that adolescents higher in narcissism made more downward social and temporal comparisons. In a longitudinal study (N = 389, 99% Dutch), we assessed adolescents’ narcissism levels at the beginning of the school year and at 3-month follow-up. In-between, we captured adolescents’ comparisons through daily diary assessments. Adolescents higher in narcissism made more downward social and temporal comparisons. Downward social – but not temporal – comparisons partially mediated the 3-month stability of narcissism. In both studies, self-esteem was unrelated to downward comparisons. Thus, downward social – but not temporal – comparisons contribute to the maintenance of adolescent narcissism, and these comparisons constitute a potentially malleable developmental mechanism to curtail narcissism.