Understanding the factors that influence achievement among gifted students has been a longstanding interest of researchers in the field of gifted education. To that end, this study investigated the individual difference antecedents of achievement as a means to identify dispositions and motivational processes that inform the design of interventions to improve student performance. More specifically, we report the results of a mediation analysis in which the association between personality traits and academic achievement is explained by autonomous motivation in a sample of gifted students (N = 161). All Big Five personality traits were found to be significant predictors of achievement measured by ACT or ACT Explore scores. Agreeableness, neuroticism, and extraversion had negative direct associations with achievement. The positive associations of conscientiousness and openness with achievement were partially mediated by autonomous motivation. Results are discussed in terms of adapting educational practices to the specific personality traits and motivational orientations of gifted students.