In secondary school mathematics, females often display lower levels of motivational outcomes than males, which can lead to gender gaps in future study or career choice. To reduce these gaps, it is crucial to evaluate which aspects of classroom teaching quality might be involved. Teacher constructive support is one especially promising aspect, as it strongly relates to student motivational outcomes. The present pre-registered study investigates how student gender is related to self-concept, self-efficacy, and interest in secondary school mathematics lessons on the quadratic equation and examines the moderating role of teacher constructive support for this relationship. Using questionnaire data from the Teaching and Learning International Survey Video Study Germany (n = 1,116 secondary school students), we applied latent moderated structural equation models to examine the direct effects as well as the interaction of student gender and constructive support on student motivational outcomes in mathematics lessons. Female gender was negatively associated with self-concept and self-efficacy, but not with interest. Various facets of constructive support were positively associated with motivational outcomes, but no interaction effects with gender were found. These findings are discussed with regard to constructive support and the persisting gender gap in mathematics. Directions for future research are suggested.