Do adolescents become more conscientious to compensate for a lack of intelligence, or does conscientiousness change independent of intelligence? The reasons for personality changes in adolescence have rarely been studied thus far, and in particular, the possible role of intelligence in that process has been neglected. The present study aims to close this research gap. In a large nationwide longitudinal sample of German adolescents (NEPS SC3; N = 7122), changes in conscientiousness and the question of whether fluid intelligence influences these changes were investigated. Latent change models indicated that conscientiousness increased in late adolescence, while the trajectories were less clear in early adolescence. Reasoning had a negative effect on changes in conscientiousness in early adolescence, which was more pronounced in boys than in girls.