While bivariate associations between motivation and academic achievement have been soundly established, only a few studies have documented evidence for its incremental predictive role above and beyond other student features related to student achievement, such as intelligence and personality. Moreover, it is not yet clear which motivational processes are most essential for academic achievement. The current study considered how an array of motivational processes related to academic achievement, controlling for intelligence and personality, in a large sample of Flemish seventh graders. Students' intelligence and need for cognition, that is, the personal preference to engage in cognitively demanding work, proved to be strong predictors of academic achievement. Yet, even after controlling for intelligence and personality, several motivational processes explained additional, unique variance in achievement, totaling about a quarter of the explained variance in school results. In particular, academic self-concept proved to relate most strongly to achievement, while achievement goals, achievement motivation, autonomous motivation, and effort beliefs additionally explained a unique portion of the variance in academic achievement. Although these findings were largely consistent across different operationalizations of achievement, motivational constructs explained more variance when achievement was measured with school grades instead of standardized tests. Given the more malleable nature of motivational dynamics compared with intelligence and personality differences, the unique, positive associations between motivation and achievement suggest that improving student motivation is a worthwhile undertaking.