Objective Although optimism's beneficial role for various life areas is well documented, previous findings regarding its significance for students' achievement at school are inconclusive. This study examined the relation between optimism and academic achievement in early adolescents. It investigated the functional form of this relation, addressed whether the initial achievement level moderates this association, and compared this with effects on self‐esteem. Method We used a large German sample (N = 6,010; 53.2% females; baseline Mage = 14.1) with two measurement points over a period of 5 months (middle and end of 7th grade). Estimating LOESS curves and latent change‐regression models revealed three main findings. Results (a) Optimism showed a nonlinear association with subsequent changes in academic achievement: Optimism promoted academic achievement, but this positive association reached a plateau in above‐average optimism ranges and a minimum value in below‐average ranges of optimism. (b) The achievement level at t1 moderated this relation so that high optimism exerted a more positive effect for high‐achieving adolescents. (c) By contrast, optimism had an overall positive effect on self‐esteem. Conclusions The results therefore broaden the evidence on benefits of optimism by linking optimism to academic success in early adolescents but indicate only small and nonlinear associations.