The present study investigated direct as well as indirect relations between children's emotion regulation (anger- and avoidant-oriented strategies), behavior regulation, and mathematics achievement in a longitudinal study during the transition from primary to secondary school in Germany. Participants were 76 primary school students attending grade four at Time 1, of whom 51 participated again as secondary school students attending grade five at Time 2, as well as children's mothers and teachers. Anger- and avoidant-oriented emotion regulation strategies at Time 1 were negatively associated to behavior regulation at Time 2. Behavior regulation at Time 2 was positively related to mathematics achievement at Time 2. There were no direct relations between emotion regulation (anger- and avoidant-oriented strategies) at Time 1 and mathematics achievement at Time 2. However, we found significant indirect negative effects between emotion regulation (anger- and avoidant-oriented strategies) at Time 1 and mathematics achievement at Time 2 via behavior regulation at Time 1. Thus, anger- and avoidant-oriented emotion regulation at Time 1 indirectly affected mathematics achievement negatively at Time 2 via behavior regulation at Time 2. This study confirms the complex structure of self-regulatory processes and underlines the importance of specific emotion regulation strategies for mathematics achievement.