Sex differences in mathematics achievement have been a controversial topic in educational psychology for a long time. This study sheds light on the developmental aspects of sex differences in math achievement and domain-specific motivational variables such as self-concept and interest. Using a Reciprocal Effects Model (REM), we analyzed 2,342 German fifth to seventh grade students who participated in a large-scale longitudinal study. Math self-concept was validated as a consistent predictor of subsequent achievement and interest for both sexes, supporting the self-enhancement part of the REM using test-scores and teacher-assigned grades. However, math achievement affects subsequent self-concept inconsistently (i.e., the skill development part). Although the bivariate relationships between the constructs were homogeneous across sex and over time, there were large sex differences in the motivational constructs, but not in the achievement measure regardless of achievement measures. The present findings underline the importance of considering both the mean and the covariance structure when describing sex differences in academic achievement. In addition, they also stress the impact of motivational constructs on educational achievement, which also have implications for sex-specific intervention programs in general.